Content uploaded by Brenda M Murdoch
Author content
All content in this area was uploaded by Brenda M Murdoch
Content may be subject to copyright.
Altered Cohesin Gene Dosage Affects Mammalian
Meiotic Chromosome Structure and Behavior
Brenda Murdoch
1.
, Nichole Owen
1.
, Michelle Stevense
2
, Helen Smith
1
, So Nagaoka
1
, Terry Hassold
1
,
Michael McKay
3
, Huiling Xu
4
, Jun Fu
5
, Ekaterina Revenkova
6
, Rolf Jessberger
2
, Patricia Hunt
1
*
1School of Molecular Biosciences, Washington State University, Pullman, Washington, United States of America, 2Institute of Physiological Chemistry, Technische
Universita
¨t Dresden, Dresden, Germany, 3Sydney University and the North Coast Cancer Institute, Lismore, New South Wales, Australia, 4Divisions of Research and
Radiation Oncology, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia, 5Genomics, BioTec, Technische Universita
¨t Dresden, Dresden, Germany,
6Department of Developmental and Regenerative Biology, Mount Sinai School of Medicine, New York, New York, United States of America
Abstract
Based on studies in mice and humans, cohesin loss from chromosomes during the period of protracted meiotic arrest
appears to play a major role in chromosome segregation errors during female meiosis. In mice, mutations in meiosis-specific
cohesin genes cause meiotic disturbances and infertility. However, the more clinically relevant situation, heterozygosity for
mutations in these genes, has not been evaluated. We report here evidence from the mouse that partial loss of gene
function for either Smc1b or Rec8 causes perturbations in the formation of the synaptonemal complex (SC) and affects both
synapsis and recombination between homologs during meiotic prophase. Importantly, these defects increase the frequency
of chromosomally abnormal eggs in the adult female. These findings have important implications for humans: they suggest
that women who carry mutations or variants that affect cohesin function have an elevated risk of aneuploid pregnancies
and may even be at increased risk of transmitting structural chromosome abnormalities.
Citation: Murdoch B, Owen N, Stevense M, Smith H, Nagaoka S, et al. (2013) Altered Cohesin Gene Dosage Affects Mammalian Meiotic Chromosome Structure
and Behavior. PLoS Genet 9(2): e1003241. doi:10.1371/journal.pgen.1003241
Editor: R. Scott Hawley, Stowers Institute for Medical Research, United States of America
Received September 4, 2012; Accepted November 28, 2012; Published February 7, 2013
Copyright: ß2013 Murdoch et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Work in the authors’ laboratories was supported by NIH grants HD37502 (PH), ES013527 (PH), HD21341 (TH), and GM062517 (RJ and ER) and by a grant
from the DFG to RJ (SPP1384, JE150/-2). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the
manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: pathunt@vetmed.wsu.edu
.These authors contributed equally to this work.
Introduction
In humans, the likelihood of an aneuploid conception is
extremely high due to errors in chromosome segregation that
occur during the meiotic divisions (reviewed in: [1–4]). Although
errors occur during both spermatogenesis and oogenesis, over
90% of aneuploidy arises in the oocyte, and the incidence of errors
is strongly enhanced by maternal age [1]. Recently, studies from
several different laboratories have provided direct evidence that
perturbations in cohesin proteins affect the orderly segregation of
homologs at meiosis I (MI) and of sister chromatids at the second
meiotic division (MII) [5–8] and deterioration of cohesion has
been postulated to be a major mechanism of human age-related
aneuploidy [7,9,10].
The trimeric core complex of cohesin is a heterodimer of SMC3
and SMC1 proteins that forms a two-sided triangle closed by a
kleisin protein (reviewed in: [11–14]). Although vertebrates have a
single SMC3 protein, there are two SMC1 variants (aand b) and
one (SMC1b) is specific to meiocytes. Meiotic cells also have three
kleisins (RAD21, RAD21L and REC8) that differ in their
spatiotemporal features. The final cohesin component is a stromal
antigen (SA) protein; two SA proteins are found in somatic cells
(SA1 and SA2), a third, SA3 (STAG3), is present in meiocytes.
Different combinations of core component proteins create a
variety of cohesin complexes in vertebrate meiocytes [15,16], but
the particular functions of individual complexes remain poorly
understood.
Mice deficient for either REC8 or SMC1breveal essential
meiotic roles for these cohesins. In both Rec8
2/2
and Smc1b
2/2
males, synaptonemal complexes (SC) are shortened, synapsis
between homologous chromosomes is impaired, and spermato-
cytes die in early/mid pachytene [17–19]. In females, similar
synaptic defects are evident in REC8-deficient oocytes, and cell
death occurs around the time of dicytate arrest [17,19]. In females
deficient for SMC1b, however, mature oocytes are produced
(albeit at reduced numbers), levels of recombination are reduced
and, importantly, sister chromatid cohesion (SCC) is poorly
maintained and connections between homologs and sister
centromeres are lost prematurely [18]. The report that these
cohesion defects were remarkably elevated in 2- and 4- by
comparison with 1-month old females provided the first evidence
of an age-related weakening of cohesion in female mice [5].
More recently, a link between cohesins and age-related
aneuploidy in normal female mice has been provided [7,8],
leading to the provocative hypothesis that deterioration of cohesins
is the cause of the maternal age effect on aneuploidy [7]. However,
although the human data suggest that loss of cohesin is a major
factor, the available evidence suggests that multiple factors
contribute to the age-related increase in segregation errors during
human female meiosis (reviewed in: [4]).
PLOS Genetics | www.plosgenetics.org 1 February 2013 | Volume 9 | Issue 2 | e1003241
The loss of cohesin hypothesis presupposes no or insufficient
turnover of proteins in the cohesin complex after they are loaded
onto chromosomes during prophase in the fetal ovary. Consistent
with this idea, although meiosis-specific cohesins are transcribed
during oocyte growth in the adult ovary [20,21], there is no
evidence that functional proteins are produced. Further, two lines
of evidence suggest that the protein complex established during
fetal development is both necessary and sufficient. First, if
transcription of Smc1b is prevented in growing oocytes, chromo-
some segregation occurs normally [20], indicating that cohesin
loaded during fetal development is sufficient for proper chromo-
some disjunction. Second, loss of cohesion induced by destruction
of REC8 protein could not be rescued by ectopic expression of a
Rec8 transgene during oocyte growth [22].
The combined data from these recent studies in mice not only
suggest that loss of cohesin plays a major role in meiotic errors,
they imply that certain levels of cohesin must be maintained for
proper chromosome segregation in oocytes. This, coupled with
data from studies in Drosophila where reduction of the Smc1
cohesin protein was used to increase nondisjunction in experi-
mentally aged oocytes [23], caused us to wonder whether
haploinsufficiency for meiosis-specific cohesin genes might induce
an age-independent meiotic phenotype in mice. We report here
evidence from studies using several different mouse models that
partial loss of gene function for either Smc1b or Rec8 results in
perturbations in the formation of the synaptonemal complex (SC)
that affect both synapsis and recombination between homologs
during meiotic prophase. Importantly, these subtle prophase
defects increase the frequency of eggs with chromosome abnor-
malities in the adult female. These findings have important clinical
implications since they suggest that women carrying mutations or
variants in meiosis-specific cohesin genes that affect cohesin dosage
may be at increased risk of producing children with chromosome
abnormalities.
Results
Cohesin heterozygotes have increased synaptic defects
and decreased recombination levels
In initial studies, we examined pachytene cells from females
heterozygous for mutations in either Smc1b or Rec8. We analyzed
the incidence of synaptic defects and recombination levels for each
of the two cohesins, and observed significantly increased levels of
defects in the heterozygotes. For the analysis of synapsis, we
defined two broad categories of defects – minor or major –
depending on the type and extent of the abnormality (see
Materials and Methods and Figure 1). Minor defects were
significantly increased in both Smc1b (35.7% of oocytes by
comparison with 17.0% in sibling controls; x
2
1df
= 37.7;
p,0.0001) and Rec8 heterozygotes (43.1% of oocytes by compar-
ison with 15.2% in controls; x
2
1df
= 64.7; p,0.0001) (Figure 1A).
The most common minor defects observed in both Smc1b and Rec8
heterozygotes were forks at the ends of the SC (Figure 1B, top
panel). Major defects included pachytene oocytes with partial or
complete asynapsis of at least one bivalent (Figure 1C, 1D). For
Smc1b these defects were observed in 9.4% of oocytes from
heterozygotes and only 0.3% from controls (x
21df
= 36.1;
p,0.0001); for Rec8, 6.4% of oocytes from heterozygotes had
major defects compared to only 1.2% from controls (x
2
1df
= 17.9;
p,0.0001).
To determine if recombination levels were affected, we analyzed
MLH1 foci, since this mismatch repair protein localizes to sites of
future crossovers [24]. The mean number of foci per cell was
significantly reduced in both Smc1b and Rec8 heterozygotes
(Figure 2): for Smc1b, means 6standard errors were 27.960.25
and 25.060.25 for controls and heterozygotes, respectively
(t = 8.2; p,0.0001) and, for Rec8, 28.760.28 and 26.660.26 for
controls and heterozygotes, respectively (t = 5.3; p,0.0001).
Because recombination failure is a well-known correlate of meiotic
nondisjunction [2,4], we examined the frequency of SCs lacking
an MLH1 focus. As shown in Table 1, a striking difference in the
proportion of SCs with 0, 1, 2 or 3 or more MLH1 foci was
evident in both Smc1b and Rec8 heterozygotes, with the frequency
of ‘‘MLH1-less’’ SCs highly elevated in both (x
2
1df
= 12.6,
p,0.001 and x
2
1df
= 16.6, p,0.001, respectively). In addition to
comparing the total number of SCs involved, we compared the
incidence of cells with one or more SC lacking a focus and found a
significant increase in such cells in both heterozygotes: 55 of the
113 cells (49%) examined from Smc1b heterozygous females by
comparison with 36 of the 126 cells (29%) in wild-type controls
(x
21df
= 4.1; p,0.05); and 58 of the 181 cells (32%) examined from
Rec8 heterozygotes by comparison with 12 of the 123 cells (10%) in
controls (x
21df
= 12.4; p,0.001).
Although the focus of our studies was on oogenesis, we
conducted an analysis of pachytene cells in males to determine if
the phenotypic consequences of cohesin heterozygosity extend to
spermatogenesis. Similar effects on recombination were evident in
male carriers of both cohesin mutations (Table 2); e.g., mean
MLH1 values per cell were significantly decreased for both Smc1b
(t = 7.3; p,0.0001) and Rec8 (t = 6.8; p,0.0001) heterozygotes by
comparison with wild-type males.
Synaptonemal complex morphology is altered in cohesin
heterozygotes
Because the number of MLH1 foci is correlated with SC length
[25], we measured the SCs in pachytene stage cells (Figure 2C,
2D). The mean total SC length was significantly reduced in
oocytes from both Smc1b (186.761.18 mm and 211.761.05 mm,
for heterozygotes and controls, respectively; t = 7.2; p,0.0001) and
Rec8 heterozygotes (181.160.96 mm and 194.361.14 mm for
heterozygotes and controls, respectively; t = 3.1; p,0.001). Im-
portantly, the overall reduction in genome-wide SC length (10% in
Smc1b and 6% in Rec8 heterozygotes) was proportional to the
decrease in mean MLH1 counts. Further, we noted a difference in
SC morphology in oocytes from heterozygotes by comparison with
wild-type siblings. Specifically, dual staining with antibodies to
SYCP3, which detects the axial/lateral elements, and SYCP1,
which detects the transverse filament of the SC, typically yields a
merged yellow signal, indicating co-localization of the two
Author Summary
Chromosome segregation errors during meiosis are the
leading cause of birth defects and miscarriages in humans.
While the basis for these errors is unknown, recent studies
suggest that defective sister chromatid cohesion may be
an important contributor. Accordingly, we tested the
hypothesis that partial loss of gene function for either of
two meiosis-specific cohesins, Smc1b or Rec8, might
adversely affect synapsis or recombination between
homologs during meiotic prophase. Our analyses of
different mouse models demonstrate cohesin dosage
effects on meiosis in both males and females. Importantly,
reduced gene function led to an increase the frequency of
chromosomally abnormal eggs in the adult female,
suggesting that, in humans, women carrying cohesin
mutations may be at an increased risk of chromosomally
abnormal pregnancies.
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 2 February 2013 | Volume 9 | Issue 2 | e1003241
proteins. However, in both Smc1b and Rec8 heterozygotes, SCs
exhibited a harlequin appearance, with regions of discontinuous
red and green signals as well as merged yellow signals. This was
particularly evident in telomeric regions (Figure 3A, 3B).
Synaptic and recombination defects are also evident in
homozygous carriers of an Smc1b hypomorphic
mutation
We recently used homologous recombination to generate a
hypomorphic allele of Smc1b (see Materials and Methods). An
analysis of synapsis and recombination in oocytes from heterozy-
gous and homozygous carriers of this mutation revealed a gene
dosage-specific increase in major synaptic defects, with approxi-
mately ten-fold higher levels in homozygous carriers of the
hypomorphic allele (Figure 4A; x
22df
= 35.7; p,0.0001). Similarly,
MLH1 levels varied significantly among the three genotypes
(F = 118.2; p,0.0001) but the effect was entirely due to
homozygotes (Figure 4B).
Haploinsufficiency for SMC1bor REC8 increases the
incidence of chromosomally abnormal eggs
Because alterations in the number and location of the sites of
recombination are associated with meiotic nondisjunction [2] and
the frequency of SCs lacking an MLH1 focus was increased in
heterozygous carriers of both mutations, we analyzed metaphase II
-arrested eggs to determine if defects induced during meiotic
prophase affect the genetic quality of the eggs produced by
heterozygous females. No difference was evident in the rate of
Figure 1. Synaptic errors are increased in cohesin heterozygotes. (A–D) Proportion of pachytene stage cells exhibiting minor and major
synaptic defects. SCs were visualized using an antibody against SYCP3 to detect the axial/lateral elements of the synaptonemal complex. For Smc1b,
n = 350 for heterozygous and n = 300 for wild-type siblings; for Rec8, n = 450 for heterozygous and n = 250 for wild-type siblings. (A) Proportion of
cells (6SE) exhibiting minor defects. (B) Representative images of SCs with minor defects; SC with a fork (top), SC with an internal bubble (bottom).
(C) Proportion of cells (6SE) exhibiting major synaptic defects. (D) Representative images of SCs with major defects; partial asynapsis (top), complete
asynapsis (bottom).
doi:10.1371/journal.pgen.1003241.g001
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 3 February 2013 | Volume 9 | Issue 2 | e1003241
germinal vesicle breakdown (meiotic resumption) or polar body
extrusion (data not shown). However, the overall incidence of
chromosomally abnormal eggs was 2–4 fold greater in heterozy-
gotes than in wild-type controls (Table 3); for Smc1b heterozygotes,
the increase reached statistical significance (x
2
= 4.0; p,0.05).
Because of the relatively small series of eggs that we were able to
analyze, it was not possible to determine whether statistically
meaningful differences existed between heterozygotes and wild
type controls for specific categories of abnormality. Nevertheless, it
is notable that the increases in chromosomally abnormal eggs in
Figure 2. Recombination levels are reduced in cohesion heterozygotes. (A) The number of MLH1 foci in pachytene cells from Smc1b
heterozygotes was significantly decreased by comparison with wild-type controls (mean MLH1 foci/cell 6SE = 25.060.25 and 27.960.25,
respectively; t = 8.2; p,0.0001). Data represent 126 cells from 5 heterozygous females (grey bars) and 113 cells from 5 wild-type siblings (black bars).
(B) A similar reduction was evident in Rec8 heterozygotes (mean MLH1 foci/cell 6SE = 26.660.26 and 28.760.28, respectively; t = 5.3; p,0.0001). Data
represent 181 cells from 7 heterozygotes (grey bars) and 124 cells from 5 sibling controls (black bars). (C, D) SC length was also significantly decreased
for both (C) Smc1b (mean 6SE = 186.761.18 mm for 126 cells from 5 heterozygotes (grey bars) and 211.761.05 mm for 113 cells from 5 controls
(black bars); t = 7.2; p,0.0001) and (D) Rec8 heterozygotes (181.160.96 mm for 179 cells from 7 heterozygotes (grey bars) and 194.361.14 mm for 123
cells from 5 controls (black bars); t = 3.1; p,0.001).
doi:10.1371/journal.pgen.1003241.g002
Table 1. Comparison of MLH1 foci distribution in oocytes
from cohesin heterozygotes and sibling controls.
Genotype Foci per SC Total
SCs
01 2
$
3
Smc1b +/+36 (1.4%) 1512 (60.0%) 913 (36.2%) 59 (2.3%) 2520
+/267 (3.0%) 1590 (70.4%) 574 (25.4%) 29 (1.3%) 2260
Rec8 +/+13 (1.0%) 1436 (58.4%) 971 (39.5%) 50 (2.0%) 2460
+/270 (2.0%) 2323 (64.2%) 1186 (32.8%) 41 (1.1%) 3620
doi:10.1371/journal.pgen.1003241.t001
Table 2. Mean MLH1 foci/cell in male heterozygotes and
wild-type siblings.*
Genotype Number of cells Average MLH1
Smc1b +/+Rec8 +/+87 22.360.24
Smc1b +/295 20.360.17
Rec8 +/256 20.460.20
*Matings of compound heterozygotes were used to generate males for these
studies.
doi:10.1371/journal.pgen.1003241.t002
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 4 February 2013 | Volume 9 | Issue 2 | e1003241
heterozygotes resulted primarily from premature sister chromatid
separation (PSCS) (Figure 5A). For Smc1b heterozygotes, 22.2%
(10 of 45 eggs) exhibited PSCS by comparison with 3.7% (1 of 27
eggs) in wild-type siblings; similarly, in Rec8 heterozygotes, 10% (9
of 90 eggs) exhibited PSCS by comparison with 6% (2 of 33 eggs)
in wild-type siblings (Table 3).
Unexpectedly, our analysis also suggested an increased incidence
of chromatid breaks and anaphase bridges in heterozygotes: 4.4% (2
of 45) of eggs from Smc1b and 2.2% (2 of 90) of eggs from Rec8
heterozygotes (Figure 5, Table 3). By comparison, structural
abnormalities were not detected in eggs from wild-type controls.
Subsequently we analyzed oocytes at the diakinesis/MI stage to
determine if structural aberrations were evident before the onset of
anaphase I. Indeed, both Smc1b and Rec8 heterozygotes exhibited
low but elevated levels of structurally abnormal chromosomes by
comparison with controls: 5.6% (4 of 72) of oocytes from Smc1b and
4.2% (2 of 48) of oocytes from Rec8 heterozygotes exhibited
aberrations (Table 3). All but one of these aberrations involved a
break in a chromatid or a whole chromosome, and the remaining
aberration was an end-to-end fusion between nonhomologous
bivalents. Similar aberrations were not observed in 94 oocytes
analyzed from control females. Although structural abnormalities
were only observed in oocytes and eggs from heterozygotes, the
difference did not reach significance for either mutation. However,
because it is inherently difficult to obtain analyzable chromosome
preparations from single oocytes and eggs, this presumably reflects
the small number of cells analyzed.
To determine if numerical or structural chromosomal aberra-
tions were also a feature of male meiosis, we analyzed MI and MII
stage spermatocytes from heterozygotes. An increased frequency
of univalents was evident at MI in both Smc1b and Rec8
heterozygotes; for Smc1b, 8.5% (6 of 71) in heterozygotes and
0% (0 of 42) in wild-type males (x
21df
= 3.7, p,0.05), and for Rec8
14.7% (5 of 34) in heterozygotes and 0% (0 of 42) in controls (x
21
df
= 6.6, p,0.02). However, a corresponding increase in hyper-
ploid cells was not observed among MII spermatocytes (Table 3).
In addition, out of a total of 180 MI and MII cells scored, only a
single cell with a structural abnormality was observed. This
suggests that, although chromosome abnormalities also occur in
male heterozygotes, most are eliminated during the meiotic
divisions.
SMC1bprotein levels also determine meiotic phenotypes
during spermatogenesis
To generate an additional model for varying levels of cohesin
subunits during meiosis, we took advantage of mice carrying an
Smc1b-Localization and Affinity Purification ‘‘LAP’’ BAC con-
struct on an Smc1b
2/2
background [20]. The abundant and easily
accessible material available from males provided the most
efficient means of comparing protein levels in different individuals,
and we screened multiple Smc1b
2/2
males carrying the construct
and compared the meiotic phenotype of males with high and low
protein levels (Figure 6A). Importantly, tagged protein localizes to
chromosomes during meiotic prophase, suggesting it can fulfill its
normal physiological role (Figure 6B). In Smc1b null males, cells
become arrested in early/mid-pachytene, stage IV [18], and males
with low SMC1b-LAP protein levels exhibited an indistinguish-
able meiotic phenotype. However, in males with higher protein
Figure 3. Synaptonemal complex formation is disturbed in cohesin heterozygotes. (A, B) Representative images of pachytene stage
oocytes from control and Rec8 heterozygous females. SCs were visualized using antibodies against SYCP1, to detect the transverse filament of the
central element (green), and SYCP3, to detect the axial/lateral elements (red). (A) Pachytene cell from wild-type female; arrow denotes single SC
shown in enlarged images below showing SYCP3 signal (left panel), SYCP1 signal (middle panel) and merged signals (right panel). (B) Pachytene cell
from a Rec8 heterozygote showing non-uniform SC staining, with red staining at the ends of most SCs; arrow denotes single SC shown in enlarged
images below showing SYCP3 (left panel), SYCP1 (middle panel) and merged (right panel) signals.
doi:10.1371/journal.pgen.1003241.g003
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 5 February 2013 | Volume 9 | Issue 2 | e1003241
levels (although still somewhat lower than wild-type), the Smc1b
null phenotype was rescued, with cells completing meiosis as
evidenced by the presence of elongated spermatids. To assess
homolog synapsis, we examined the staining patterns of c-H2A.X
and HORMAD1. In wild-type males, c-H2A.X typically forms a
single distinct focus of staining around the predominately
unsynapsed X and Y chromosomes, but also localizes to
unsynapsed areas of autosomes, if present. In high SMC1b-
LAP expressing cells, a strong single focus of c-H2A.X sex body
staining was evident, with proportions similar to wild-type cells
(Figure 6B). In contrast, low expressing cells exhibited multiple foci
per nucleus in an irregular pattern, similar to that observed in
Smc1b null males [18]. HORMAD1, which localizes to unsynapsed
regions [26], was indistinguishable from wild-type in males with
high levels of SMC1b-LAP protein, suggesting normal synapsis.
However, in males with low protein levels, HORMAD1 was
consistently found on multiple chromosomes per nucleus, indicat-
ing the presence of asynaptic regions and mirroring the phenotype
of Smc1b
2/2
cells. Although these studies suggest that rescue of the
null phenotype depends on the levels of SMC1b-LAP protein
produced, subtle meiotic differences may exist between these
males and wild-type mice, as well as between the SMC1b-LAP
low expressers and Smc1b
2/2
cells.
Discussion
Recent data from a variety of sources support the hypothesis
that, in the mammalian oocyte, meiotic cohesion weakens over
time, and loss of cohesion contributes to meiotic errors that cause
aneuploidy [10]. This prompted us to ask whether heterozygosity
for mutations in meiosis-specific cohesin genes might also elicit
meiotic effects in mammals. The data presented here using four
different mouse models provide evidence that altered gene dosage
for either of the meiosis-specific cohesins, Rec8 or Smc1b adversely
impacts the events of meiotic prophase during both oogenesis and
spermatogenesis. These findings are consistent with a recent report
on a somatic chromosome instability phenotype in mice hetero-
zygous for the centromeric cohesin protector Shugoshin-1 (Sgol1)
[27]. Enhanced tumorigenesis following carcinogen exposure and
increased chromosome mis-segregation in mouse embryonic
fibroblasts were observed. Given the data presented in our report
it seems likely that the role of Sgol1 in cohesin protection and not
some other, unknown function of shugoshin caused these
phenotypes.
Our analyses of heterozygous carriers of mutations in either
Rec8 or Smc1b and of homozygous carriers of a hypomorphic allele
of Smc1b demonstrated defects in both synapsis and recombina-
tion. These findings suggest that appropriate dosage of each
cohesin subunit is essential for these events. Consistent with this
interpretation, analyses of Smc1b null males carrying either a
‘‘high’’ or ‘‘low’’ expressing SMC1btransgene revealed strikingly
different meiotic phenotypes. Specifically, carriers of the low
expressing transgene exhibited a meiotic phenotype virtually
identical to the SMC1b‘‘knockout’’ [18], with no cells progressing
beyond mid-pachytene. In contrast, in males carrying the high
expressing transgene, normal synapsis was restored, the sex body
was indistinguishable from wild-type, and elongating spermatids
were present, indicating cells were able to progress beyond
pachytene. Taken together, our data provide compelling evidence
that the correct dosage of meiotic cohesins is essential for normal
meiotic prophase.
Importantly, the subtle prophase changes induced by altered
cohesion gene dosage have a significant impact on gamete quality.
As reported for other meiotic mutations [28–30] and for
environmental effects that alter synapsis and recombination [31],
the synaptic defects and alterations in the number and placement
of recombination events observed at prophase increased the
likelihood of meiotic chromosome segregation errors. However,
unlike other meiotic mutations, our results suggest that reductions
Figure 4. An
Smc1b
hypomorphic mutation affects synapsis
and recombination. (A) Proportion of cells (6SE) with apparently
normal synapsis, minor, or major defects. Although levels of synaptic
defects were similar in wild-type and heterozygous females (+/hy), a
marked increase in major defects was evident in homozygotes for the
Smc1b hypomorphic allele (hy/hy); this produced a highly significant
among-group difference in the frequency of synaptic defects (x
22
df
= 35.7; p,0.0001). (B) Similarly, mean genome-wide MLH1 values 6
SE were markedly decreased in homozygotes (20.7.960.21) by
comparison with controls and heterozygotes (27.360.32 and
26.560.33, respectively) (F = 118.2; p,0.0001). These data represent
101 cells from 4 hypomorphic females (hashed bars), 100 cells from 5
hypomorphic heterozygotes (grey bars) and 100 cells from 4 wild-type
siblings (black bars).
doi:10.1371/journal.pgen.1003241.g004
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 6 February 2013 | Volume 9 | Issue 2 | e1003241
in cohesin components may increase the likelihood of structural, as
well as numerical, chromosome abnormalities.
The harlequin appearance of SCs, particularly near the
chromosome ends, and the occurrence of anaphase bridges in
oocytes are consistent with the previous report of an increase in
telomere defects in SMC1bknockout animals [32]. However, our
results extend the hypothesis that SMC1bplays an essential role in
the protection of meiotic telomeres, suggesting a more general
effect of cohesin dosage. That is, in the majority of oocytes in
which structural abnormalities were identified, the breaks occurred
interstitially or in pericentromeric regions, not at telomeres (e.g.,
Figure 5C right and 5D left). This suggests that cohesins play a role
in protecting not only telomeres, but all of the DNA within the
context of the synaptonemal complex. The most obvious
explanation is that the observed breaks represent sites of
unrepaired DNA damage, due either to a delay in repair imposed
by defects in synapsis or to impediments in DNA repair as recently
reported for Rad21 [33]. However, in addition to single chromatid
breaks that would be predicted to result from failure to complete
repair at the site of a double strand break, we also observed breaks
involving both chromatids in a small number of cells. Thus, it is
possible that the DNA held within the altered SC scaffold may
somehow be rendered vulnerable to damage during late prophase,
after the repair of programmed double strand breaks is complete.
This raises obvious questions for future studies; e.g., does altered
gene dosage for meiosis-specific cohesins sensitize meiocytes to
DNA damaging agents and do allelic variants play a role in the
generation of human structural chromosome abnormalities?
Further, our findings in mice have important implications for
humans. The incidence of aneuploidy in humans is astonishingly
high and the magnitude of the effect of advancing maternal age
makes it difficult to discern effects of other, more subtle, causal
agents. Nevertheless, it has been postulated that some individuals
may be prone to nondisjunction because they carry mutations in
meiotic genes [34]. Our studies in mice suggest that this is the case
for carriers of mutations or variants that affect meiosis-specific
cohesin gene dosage. Importantly, our data suggest that the
increases might not be limited to numerical abnormalities but
could also include structural rearrangements, and that this would
be evident even at young maternal ages.
Our analyses suggest cohesin dosage effects in both male and
female mice. Does this mean that, in humans, the likelihood of
producing chromosomally abnormal gametes may be increased for
both men and women who carry mutant or variant alleles of
cohesin genes? Weakened cohesion would be expected to lead to
both premature loss of the cohesion that holds chiasmata in place
– yielding univalent chromosomes at MI - and to premature
separation of sister centromeres – yielding single chromatids at
Table 3. Chromosome abnormalities from cohesion heterozygotes and wild-type siblings.
Female
Smc1b
+
/
+
Smc1b
+
/
2
Rec8
+
/
+
Rec8
+
/
2
Diakinesis/MI
Total Cells 37 72 57 48
Normal 37 (100.0%) 68 (94.4%) 57 (100.0%) 46 (95.8%)
Univalents 0 1 (1.4%) 0 0
St. Abnormal*0 3 (4.2%)* 0 2 (4.2%)
MII Arrested eggs
Total Cells 27 45 33 90
Normal 25 (92.6%) 33 (73.3%) 31 (93.9%) 78 (86.6%)
Hyperploid 1 (3.7%) 0 0 1 (1.1%)
PSCS** 1 (3.7%) 10 (22.2%) 2 (6.0%) 9 (10.0%)
St. Abnormal 0 2 (4.4%) 0 2 (2.2%)
Male
Smc1b
+
/
+
Smc1b
+
/
2
Rec8
+
/
+
Rec8
+
/
2
Diakinesis/MI
Total Cells 42 71 42 34
Normal 42 (100.0%) 65 (91.5%) 42 (100.0%) 29 (85.3%)
Univalents 0 6 (8.5%) 0 5 (14.7%)
St. Abnormal 0 0 0 0
MII
Total Cells 25 51 25 24
Normal 25 (100.0%) 51 (100.0%) 25 (100.0%) 23 (95.8%)
Hyperploid 0 0 0 0
PSCS 0 0 0 0
St. Abnormal 0 0 0 1 (4.2%)
*Structural abnormalities (MI abnormalities include 3 cells with chromatid breaks, 1 cell with a chromosome break, and 1 cell with an end-to-end fusion between two
bivalents. All MII abnormalities were chromatid breaks).
**Premature sister chromatid separation.
doi:10.1371/journal.pgen.1003241.t003
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 7 February 2013 | Volume 9 | Issue 2 | e1003241
either MI or MII. Our analysis of Rec8 and Smc1b heterozygotes
confirm the occurrence of both univalents and single chromatids
(Table 3, Figure 5A). Further, previous studies by us and others
(e.g., [7,35,36]) demonstrate a straightforward correlation between
the presence of univalents and/or single chromatids and the
production of aneuploid eggs. Thus, abnormalities in cohesion are
attractive candidates for female-derived aneuploidies. However,
although univalents and single chromatids are frequently able to
evade meiotic spindle assembly checkpoint control mechanisms in
mammalian females [37], in males the checkpoint is extremely
stringent [38]. Thus, in the male, aberrant chromosomes caused
by cohesion deficiencies would be expected to cause metaphase
arrest and cell death, possibly reducing sperm numbers. However,
they would not be expected to appreciably increase the frequency
of aneuploid sperm. This expectation is confirmed by the results of
our studies of male heterozygotes: the frequency of univalents at
MI was significantly increased in both Rec8 and Smc1b heterozy-
gotes, but a corresponding increase in aneuploidy was not evident
at MII (Table 3), suggesting effective elimination of cells with
univalent at MI.
Our analyses also suggested an increase in chromosome breaks
prior to, or at the onset of, anaphase I in females. We observed a
range of different structural aberrations in oocytes and eggs from
female heterozygotes, including anaphase bridges, breaks near
centromeric regions, nonhomologous fusions, and different types
of interstitial breaks (Figure 5). These findings suggest that carriers
might be at increased risk of transmitting different types of
aberrations, including Robertsonian fusions and other reciprocal
translocations, deletions, and duplications. Whether structural
aberrations would also be transmitted by male carriers almost
certainly depends upon their origin. That is, based on our
understanding of sex-specific differences in cell cycle control, the
expectation is that spermatocytes with unrepaired double strand
breaks would be effectively eliminated during prophase [38]. In
contrast, breaks induced just prior to or during the meiotic
divisions would not be expected to trigger cell death, since the
presence of sticky chromosome ends or fragments at metaphase
should not interfere with spindle attachment. Similarly, since
anaphase bridges are manifested after cell division is initiated, they
do not trigger cell cycle arrest [39]. Thus, the fact that structural
aberrations were comparatively rare in spermatocytes at MI and
MII by comparison with comparable stages in the female supports
the hypothesis that most breaks result from the presence of a small
number of unrepaired double strand breaks.
In summary, our results suggest that proper orchestration of
meiotic prophase in mammals requires accurate dosage of meiosis-
specific cohesin genes and that women who are asymptomatic
carriers of mutations or polymorphic variants in meiosis-specific
cohesins may be at increased risk of producing chromosomally
abnormal gametes. Specifically, we hypothesize that women who
carry a functionally impaired allele have an increased risk of
aneuploid pregnancies even at young maternal ages and may even
be at increased risk of transmitting structural abnormalities. For
males, however, our findings suggest that, because of stringent
meiotic cell cycle control mechanisms, most numerical errors and
structural rearrangements would be prevented from contributing
to the population of viable gametes.
Materials and Methods
Ethics statement
All animal experiments were approved by the WSU Institu-
tional Animal Care and Committee and conducted in accordance
with the Guide for Care and Use of Laboratory Animals.
Animals
The Smc1b and Rec8 mutants used in this study have been
described previously [18,19]. The Smc1b mutation removes exon
10, which codes for approximately 40% of the hinge domain
necessary for the formation of the SMC heterodimer that is
essential for formation of the cohesin complex function. In
Smc1b
2/2
mice, Smc1b gene expression is dramatically reduced,
Figure 5. Chromosome abnormalities are increased in cohesin
heterozygotes. Representative images of numerical and structural
defects in cohesin heterozygotes. (A) Premature sister chromatid
separation in MII eggs; arrows denote the unattached single
chromatids. (B) Anaphase bridges between MII eggs and first polar
bodies. (C) Chromatid defects in MII eggs; arrows denote abnormal
chromatid. (Left) acentric fragment; (middle) deletion of almost an
entire chromatid; (right) proximal chromatid break; note that acentric
fragment remains attached to telomere of intact sister chromatid. (D)
Defects in diakinesis stage oocytes: (left) bivalent with a chromatid
break in one chromosome; (middle) nonhomologous end-to-end
chromosome fusion that appears to involve both chromatids of two
bivalents; (right) proximal break in a single chromatid.
doi:10.1371/journal.pgen.1003241.g005
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 8 February 2013 | Volume 9 | Issue 2 | e1003241
and SMC1bprotein is not detectable by either immunoblotting or
immunofluorescence [18]. The Rec8 mutation has removed 19 of
the 20 coding exons of the gene and functional transcripts are not
detectable in Rec8
2/2
mice [19]. To identify wildtype and
heterozygous females for these studies the genotypes were
determined by PCR amplification of DNA as described previously
[18,19].
The hypomorphic allele of Smc1b was created during the
construction of the loxP-SMC1bdescribed previously [20]. The
mutant allele retains the neo gene but in reverse orientation.
Genotyping was performed as described previously [20] using
primers 1, 2 and a new primer (CAC GCG TCA CCT TAA TAT
GC) designed for the reversed neo cassette with a product of
,520 bp for the hypomorphic allele. The mutation is on a mixed
Figure 6. The effect of variable levels of SMC1b-LAP expression during meiosis. (A) Protein extracts from wild-type mice and Smc1b
2/2
mice harboring the SMC1b-LAP construct. Samples were run on an SDS-PAGE gel and blotted for endogenous SMC1band the SMC1b-LAP protein
(using a GFP antibody). High and low expressers are indicated. Loading controls; SMC3 and Karyopherin B1. (B) Whole testis tissue was cyrosectioned,
fixed and stained using DAPI and antibodies against GFP, H2A.X and HORMAD1. Examples of high and low expressers are presented, with images
representing typical observations.
doi:10.1371/journal.pgen.1003241.g006
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 9 February 2013 | Volume 9 | Issue 2 | e1003241
genetic background, but experiments were done using animals
from second or third generation backcrosses to C57BL/6.
Homozygous hypomorphic males are sterile (2 males, 6 months)
and females are subfertile (3 females, 8 litters at ,2pups/litter, 6
months).
The Smc1b-LAP BAC construct was generated by recombineer-
ing [40] using a BAC carrying the genomic locus
(ENSG00000077935) and the R6Kamp-hNGFP plasmid to
generate an N-terminally tagged Smc1b gene within the natural
gene expression control elements. The Smc1b-LAP BAC was
injected into blastocysts, and five founders were obtained and
mated with animals carrying the Smc1b mutation.
Meiotic analyses
For analyses of synapsis and recombination, surface spread
preparations were made from 18 dpc fetal ovaries and testes from
8-week old males [41]. Slides were immunostained, examined on a
Zeiss epifluorescence microscope, imaged with a CCD camera,
and analyzed using Axiovision software. Pachytene stage cells were
identified on the basis of synaptonemal complex (SC) morphology
[42], and all cells were analyzed by two independent observers
who were blinded with regard to the genotype of the animals. To
calculate the frequency of synaptic defects, 50 pachytene cells per
animal were scored for the presence of minor or major defects. If
one or more SC exhibited a gap (identified as a small discontinuity
in localization of the SC protein SYCP3), a fork (a separation of
the telomeric region of the SC, comprising no more than one third
of the overall length of the SC), or a bubble (an internal separation
of the SC comprising no more than one third of the length of the
SC) the cell was categorized as having minor defects. The cell was
categorized as having a major defect if one-third or more of the
length of an SC was asynapsed. To analyze recombination, slides
were immunostained with antibodies to MLH1 (Calbiochem,
Millipore MA, USA) and SYCP3 (Santa Cruz Biotechnology, Inc.,
CA, USA). The total number of MLH1 foci per cell was
determined for at least 25 cells per individual and for at least
five animals of each genotype. MLH1 foci were scored only if the
signals were punctate in appearance, localized on the SC, and
separated from adjacent foci by at least one signal domain. For
studies of SC morphology, slides were immunostained with
antibodies to both SYCP1 (Santa Cruz Biotechnology, Inc., CA,
USA) and SYCP3 and the appearance of the SC was examined for
50 pachytene cells per genotype.
Cytogenetic analysis of diakinesis/MI and MII cells
Chromosome preparations were made from oocytes collected
from 28-day old females and matured in vitro as described
previously [43]. Briefly, oocytes were collected and cultured in
Waymouth medium (Gibco, Invitrogen Carlsbad, Ca, USA)
supplemented with 10% fetal bovine serum and 0.23 mM sodium
pyruvate. After two hours in culture, oocytes that remained at the
germinal vesicle stage were removed, and the remaining oocytes
were cultured for 4 hrs or overnight (,12–14 hrs) to obtain MI
and MII eggs, respectively. After culturing, eggs were treated in a
hypotonic solution (0.9% sodium citrate), fixed onto slides with 3:1
methanol:acetic acid, and stained with DAPI. Metaphase images
were captured with a CCD camera on a Zeiss epifluorescence
microscope and analyzed by two independent observers who were
blinded with respect to genotype. For the analysis of MI and MII
stage spermatocytes, air dried preparations were made as
described by Evans [44] and the same capturing and blind scoring
methodology used.
Cryosectioning and Western analysis of testes from
Smc1b-LAP males
Whole testes were immersed in O.C.T Compound (Tissue-Tek
4583) in specimen molds (Tissue-Tek 4566 Cyromold
15 mm615 mm65 mm) and frozen at 280uC. 7 mm sections
were cut using a Leica CM1900 and placed on microscope slides
(StarFrost K078; 76626 mm). Sections were fixed using 4%
formaldehyde (Sigma F8775) in 16PBS for 15 mins at 22uC and
permeablised using 0.15% Triton-X100 (Servas 37240) for
10 mins at 22uC and washed twice in 16PBS. Slides were
blocked in 2% BSA (Sigma A2153) in 16PBS for 30 mins and
primary antibodies were incubated at 4uC for 16 hours. Primary
antibodies: anti-SYCP3 mouse monoclonal, hybridoma cell line
supernatant (1:1, kind gift from Christa Heyting), anti-c-H2A.X
phospho-ser139 (1:600, mouse monoclonal IgG
1
, Millipore 05-
636), anti-eGFP (1:500, goat, MPI Dresden), anti-Hormad1
(1:700, guinea-pig, kind gift from A. To´th, Dresden). Slides were
washed 36in 16PBS and secondary antibodies were incubated
for 1–2 hours at 22uC. Slides were washed 36in 16PBS and
mounted using VectaShield mounting media (Vecta Laboratories,
H-1000) plus 1 mg/ml DAPI and 24650 mm coverslips (Engel-
brecht, K12450, depth 0.13–0.17 mm). Testes sections were
imaged using a Leica Axiophot microscope at 1006or 406
magnification with oil of refractive index 1.518 (Zeiss, Immersol
518 F).
For protein extraction and Western blotting, the tunica
albuginea was removed from the testes and a single cell suspension
created using Dounce homogenisation (loose pestle) in Buffer B
(5 mM KCl, 2 mM DTT, 40 mM Tris.HCl (pH 7.5) 2 mM
EDTA, 0.5 mM spermidine and protease inhibitors) followed by
Dounce homogenisation (tight pestle). The cell suspension was
centrifuged at 8000 rpm for 3 mins, the nuclear pellet resus-
pended in Buffer C (5 mM KCl, 1 mM DTT, 15 mM Tris.HCl
(pH 7.5), 0.5 mM EDTA, 0.5 mM spermidine and protease
inhibitors), 250 mM ammonium sulphate (pH 7.4) was added
and incubated on ice for 30 mins. Samples were centrifuged at
45,000 rpm for 30 mins at 4uC. Supernatant was collected and
protein content measured by Bradford before being stored at
220uC in Laemmni buffer for Western analysis. 5 mg of protein
was run on a 6% SDS-PAGE gel, transferred to a nitrocellulose
membrane, and blocked in 5% milk in PBST (PBS plus 0.1%
Tween-20) for 1 hour at 22uC. Primary antibodies were added at
1mg/ml in 5% milk in PBST for 16 hours at 4uC. Primary
antibodies: anti-karyopherin b1 (H-300) (sc-11367 rabbit poly-
clonal IgG Santa Cruz), anti-eGFP (goat, MPI Dresden) and anti-
SMC3 (A300-060A, rabbit, Bethyl). Blots washed 36in PBST and
HRP-conjugated secondary antibodies were added for 1 hour at
22uC in 5% milk in PBST. Blots were washed 36in PBST and
developed using chemiluminescent HRP substrate (Millipore,
WBKLS) and imaged on a Kodak ImageStation 2000MM.
Author Contributions
Conceived and designed the experiments: TH PH RJ. Performed the
experiments: BM NO MS HS SN HX JF ER. Analyzed the data: BM NO
MS HS SN TH RJ PH. Contributed reagents/materials/analysis tools: MS
TH MM HX ER RJ PH. Wrote the paper: BM NO HS SN TH MM RJ
PH.
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 10 February 2013 | Volume 9 | Issue 2 | e1003241
References
1. Hassold T, Hunt P (2001) To err (meiotically) is human: the genesis of human
aneuploidy. Nat Rev Genet 2: 280–291.
2. Hassold T, Hall H, Hunt P (2007) The origin of human aneuploidy: where we
have been, where we are going. Hum Mol Genet 16 Spec No. 2: R203–208.
3. Homer H (2007) Ageing aneuploidy and meiosis: eggs in a race against time.:
Yearbook of Obstetrics and Gynaecology. pp. 139–158.
4. Nagaoka SI, Hassold TJ, Hunt PA (2012) Human aneuploidy: mechanisms and
new insights into an age-old problem. Nat Rev Genet 13: 493–504.
5. Hodges CA, Revenkov a E, Jessberger R, Hassold TJ, Hunt PA (2005)
SMC1beta-deficient female mice provide evidence that cohesins are a missing
link in age-related nondisjunction. Nat Genet 37: 1351–1355.
6. Liu L, Keefe DL (2008) Defective cohesin is associated with age-dependent
misaligned chromosomes in oocytes. Reprod Biomed Online 16: 103–112.
7. Chiang T, Duncan FE, Schindler K, Schultz RM, Lampson MA (2010)
Evidence that weakened centromere cohesion is a leading cause of age-related
aneuploidy in oocytes. Curr Biol 20: 1522–1528.
8. Lister LM, Kouznetsova A, Hyslop LA, Kalleas D, Pace SL, et al. (2010) Age-
related meiotic segregation errors in mammalian oocytes are preceded by
depletion of cohesin and Sgo2. Curr Biol 20: 1511–1521.
9. Chiang T, Schultz RM, Lampson MA (2012) Meiotic origins of maternal age-
related aneuploidy. Biol Reprod 86: 1–7.
10. Jessberger R (2010) Deterioration without replenishment–the misery of oocyte
cohesin. Genes Dev 24: 2587–2591.
11. Hirano T (2006) At the heart of the chromosome: SMC proteins in action. Nat
Rev Mol Cell Biol 7: 311–322.
12. Nasmyth K (2011) Cohesin: a catenase with separate entry and exit gates? Nat
Cell Biol 13: 1170–1177.
13. Nasmyth K, Haering CH (2009) Cohesin: its roles and mechanisms. Annu Rev
Genet 43: 525–558.
14. Onn I, Heidinger-Pauli JM, Guacci V, Unal E, Koshland DE (2008) Sister
chromatid cohesion: a simple concept with a complex reality. Annu Rev Cell
Dev Biol 24: 105–129.
15. Jessberger R (2011) Cohesin complexes get more complex: the novel kleisin
RAD21L. Cell Cycle 10: 2053–2054.
16. Uhlmann F (2011) Cohesin subunit Rad21L, the new kid on the block has new
ideas. EMBO Rep 12: 183–184.
17. Bannister LA, Reinholdt LG, Munroe RJ, Schimenti JC (2004) Positional
cloning and characterization of mouse mei8, a disrupted allelle of the meiotic
cohesin Rec8. Genesis 40: 184–194.
18. Revenkova E, Eijpe M, Heyting C, Hodges CA, Hunt PA, et al. (200 4) Cohesin
SMC1 beta is required for meiotic chromosome dynamics, sister chromatid
cohesion and DNA recombination. Nat Cell Biol 6: 555–562.
19. Xu H, Beasley MD, Warren WD, van der Horst GT, McKay MJ (2005)
Absence of mouse REC8 cohesin promotes synapsis of sister chromatids in
meiosis. Dev Cell 8: 949–961.
20. Revenkova E, Herrmann K, Adelfalk C, Jessberger R (2010) Oocyte cohesin
expression restricted to predictyate stages provides full fertility and prevents
aneuploidy. Curr Biol 20: 1529–1533.
21. Revenkova E, Adelfalk C, and Jessberger R (2010) Cohesin in oocytes - tough
enough for mammalian meiosis? Genes 495–504.
22. Tachibana-Konwalski K, Godwin J, van der Weyden L, Champion L, Kudo
NR, et al. (2010) Rec8-containing cohesin maintains bivalents without turnover
during the growing phase of mouse oocytes. Genes Dev 24: 2505–2516.
23. Subramanian VV, Bickel SE (2008) Aging predisposes oocytes to meiotic
nondisjunction when the cohesin subunit SMC1 is reduced. PLoS Genet 4:
e1000263. doi:10.1371/journal.pgen.1000263
24. Anderson LK, Reeves A, Webb LM, Ashley T (1999) Distribution of crossing
over on mouse synaptonemal complexes using immunofluorescent localization of
MLH1 protein. Genetics 151: 1569–1579.
25. Lynn A, Koehler KE, Judis L, Chan ER, Cherry JP, et al. (2002) Covariation of
synaptonemal complex length and mammalian meiotic exchange rates. Science
296: 2222–2225.
26. Wojtasz L, Daniel K, Roig I, Bolcun-Fila s E, Xu H, et al. (2009) Mouse
HORMAD1 and HORMAD2, two conserved meiotic chromosomal proteins,
are depleted from synapsed chromosome axes with the help of TRIP13 AAA-
ATPase. PLoS Genet 5: e1000702. doi:10.1371/journal.pgen.1000702
27. Yamada HY, Yao Y, Wang X, Zhang Y, Huang Y, et al. (2012)
Haploinsufficiency of SGO1 results in deregulated centrosome dynamics,
enhanced chromosomal instability and colon tumorigenesis. Cell Cycle 11: 479–
488.
28. Shin YH, Choi Y, Erdin SU, Yatsenko SA, Kloc M, et al. (2010) Hormad1
mutation disr upts synaptonemal complex formation, recombination , and
chromosome segregation in mammalian meiosis. PLoS Genet 6: e1001190.
doi:10.1371/journal.pgen.1001190
29. Yuan L, Liu JG, Hoja MR, Wilbertz J, Nordqvist K, et al. (2002) Female germ
cell aneuploidy and embryo death in mice lacking the meiosis-specific protein
SCP3. Science 296: 1115–1118.
30. Kuznetsov S, Pellegrini M, Shuda K, Fernandez-Ca petillo O, Liu Y, et al. (2007)
RAD51C deficiency in mice results in early prophase I arrest in males and sister
chromatid separation at metaphase II in females. J Cell Biol 176: 581–592.
31. Susiarjo M, Hassold TJ, Freeman E, Hunt PA (2007) Bisphenol A exposure in
utero disrupts early oogenesis in the mouse. PLoS Genet 3: e5. doi:10.1371/
journal.pgen.0030005
32. Adelfalk C, Janschek J, Revenkova E, Blei C, Liebe B, et al. (2009) Cohesin
SMC1beta protects telomeres in meiocytes. J Cell Biol 187: 185–199.
33. Xu H, Balakrishnan K, Malaterre J, Beasley M, Yan Y, et al. (2010) Rad21-
cohesin haploinsufficiency impedes DNA repair and enhances gastrointestinal
radiosensitivity in mice. PLoS ONE 5: e12112. doi:10.1371/journal.-
pone.0012112
34. Alfi OS, Chang R, Azen SP (1980) Evidence for genetic control of
nondisjunction in man. Am J Hum Genet 32: 477–483.
35. Hunt P, LeMaire R, Embury P, Sheean L, Mroz K (1995) Analysis of
chromosome behavior in intact mammalian oocytes: monitoring the segregation
of a univalent chromosome during female meiosis. Hum Mol Genet 4: 2007–
2012.
36. Merriman JA, Jennings PC, McLaughlin EA, Jones KT (2012) Effect of aging on
superovulation efficiency, aneuploidy rates, and sister chromatid cohesion in
mice aged up to 15 months. Biol Reprod 86: 49.
37. Nagaoka SI, Hodges CA, Albertini DF, Hunt PA (2011) Oocyte-specific
differences in cell-cycle control create an innate susceptibility to meiotic errors.
Curr Biol 21: 651–657.
38. Burgoyne PS, Mahadevaiah SK, Turner JM (2009) The consequences of
asynapsis for mammalian meiosis. Nat Rev Genet 10: 207–216.
39. Koehler KE, Millie EA, Cherry JP, Burgo yne PS, Evans EP, et al. (2002) Sex-
specific differences in meiotic chromosome segregation revealed by dicentric
bridge resolution in mice. Genetics 162: 1367–1379.
40. Poser I, Sarov M, Hutchins JR, Heriche JK, Toyoda Y, et al. (2008) BAC
TransgeneOmics: a high-throughput method for exploration of protein function
in mammals. Nat Methods 5: 409–415.
41. Peters AH, Plug AW, van Vugt MJ, de Boer P (1997) A drying-down technique
for the spreading of mammalian meiocytes from the male and female germline.
Chromosome Res 5: 66–68.
42. Moses MJ (1980) New cytogenetic studies on mammalian meiosis. New York,
N.Y.: Raven Press.
43. Hodges CA, Ilagan A, Jennings D, Keri R, Nilson J, et al. (2002) Experimental
evidence that changes in oocyte growth influence meiotic chromosome
segregation. Human Reproduction 17: 1171–1180.
44. Evans EP, Breckon G, Ford CE (1964) An air-drying method for meiotic
preparations from mammalian testes. Cytogenetics 15: 289–294.
Cohesin Gene Dosage Affects Chromosome Structure
PLOS Genetics | www.plosgenetics.org 11 February 2013 | Volume 9 | Issue 2 | e1003241
