Non-homologous End-joining Genes are not Inactivated in Human Radiation-induced Sarcomas with Genomic Instability

Abstract

DNA double-strand break (DSB) repair pathways are implicated in the maintenance of genomic stability. However the alterations of these pathways, as may occur in human tumor cells with strong genomic instability, remain poorly characterized. We analyzed the loss of heterozygosity (LOH) and the presence of mutations for a series of genes implicated in DSB repair by non-homologous end-joining in five radiation-induced sarcomas devoid of both active Tp53 and Rb1. LOH was recurrently observed for 8 of the 9 studied genes (KU70, KU80, XRCC4, LIG4, Artemis, MRE11, RAD50, NBS1) but not for DNA-PKcs. No mutation was found in the remaining allele of the genes with LOH and the mRNA expression did not correlate with the allelic status. Our findings suggest that non-homologous end-joining repair pathway alteration is unlikely to be involved in the high genomic instability observed in these tumors.

Full text

J. Radiat. Res., Vol. 46, No. 2 (2005); http://jrr.jstage.jst.go.jp
Regular Paper
J. Radiat. Res., 46, 223–231 (2005)
Non-homologous End-joining Genes are not Inactivated in Human
Radiation-induced Sarcomas with Genomic Instability
Sandrine H. LEFÈVRE
1
**
, Arnaud COQUELLE
1
**
, Nathalie GONIN-LAURENT
1
,
Andrej CÖR
2
, Nicolas VOGT
1
, Laurent CHAUVEINC
3
, Philippe ANRACT
4
,
Bernard DUTRILLAUX
1
, Sylvie CHEVILLARD
5
and Bernard MALFOY
1
*
Radiation-induced tumor/Sarcoma/Non-homologous end-joining/Genomic instability/Mutations.
DNA double-strand break (DSB) repair pathways are implicated in the maintenance of genomic sta-
bility. However the alterations of these pathways, as may occur in human tumor cells with strong genomic
instability, remain poorly characterized. We analyzed the loss of heterozygosity (LOH) and the presence
of mutations for a series of genes implicated in DSB repair by non-homologous end-joining in five radia-
tion-induced sarcomas devoid of both active Tp53 and Rb1. LOH was recurrently observed for 8 of the 9
studied genes (
KU70, KU80, XRCC4, LIG4, Artemis, MRE11, RAD50, NBS1
) but not for
DNA-PKcs
. No
mutation was found in the remaining allele of the genes with LOH and the mRNA expression did not cor-
relate with the allelic status. Our findings suggest that non-homologous end-joining repair pathway alter-
ation is unlikely to be involved in the high genomic instability observed in these tumors.
INTRODUCTION
It is generally considered that genomic instability, at least
partially, may result from DNA double strand breaks (DSBs)
and incomplete/incorrect repair of them.
1)
Two major path-
ways exist in mammalian cells for DSBs repair: homologous
recombination (HR) and non-homologous end-joining
(NHEJ).
2,3,4)
NHEJ is the predominant DSB-repair mecha-
nism in mammalian cells and is also involved in V(D)J
recombination. NHEJ involves the DNA end-binding het-
erodimer Ku70/Ku80, the catalytic subunit of the DNA-PK
complex, the XRCC4 gene product, Lig4 and Artemis. Three
other genes,
MRE11
,
RAD50
and
NBS1,
otherwise involved
in HR, could be implicated in the processing which must
take place at the DSB generated by mutagenic agents before
NHEJ can ensue (review in
5)
).
In transgenic mice, biallelic inactivation of either
Ku70,
Ku80, Lig4
or
Xrcc4
induces genomic instability.
6,7,8)
In dou-
ble knocked-out mice
DNA-PKcs
–/–
/p53
–/–
, Ku80
–/–
/p53
–/–
,
Lig4
–/–
/p53
–/–
or
Xrcc4
–/–
/p53
–/–
,
genomic instability was
associated with early B-cell tumor development, which was
not observed in mice in which only one of the NHEJ genes
was knocked-out.
7–13)
Furthermore, when the RB1 pathway
is altered in INK4a/ARF knock out mice and wild type for
TP53, the development of soft-tissue sarcomas with clonal
amplifications, deletions and translocations is observed after
the inactivation of single allele of the
Lig4
gene.
14)
Germ line mutations in DSB-repair genes have been
observed in cancer-predisposing human diseases character-
ized by genomic instability:
NBS1
in the Nijmegen-break-
age-syndrome
15)
or
MRE11
in the ataxia telangiectasia-like
disorder (ATLD).
16)
Mutations in
LIG4
are also suspected to
predispose to cancer.
17,18)
However, in cancers from non-pre-
disposed patients, mutations in NHEJ genes have rarely been
found.
19,20)
Altogether these data indicate that NHEJ genes could be
involved in the induction of genomic instability by two dif-
ferent mechanisms: the alteration of a tumor suppressor gene
function by inactivation of both alleles or reduced expres-
sion associated with the loss of one allele. The consequences
of NHEJ gene inactivation may be strengthened by biallelic
co-inactivation of Tp53 or Rb1.
We previously published the cytogenetic and molecular
analysis of a series of radiation-induced tumors developing
in the field of irradiation in patients treated by radiotherapy
*Corresponding author: Phone: 33 1 42 34 66 85,
Fax: 33 1 42 34 66 74,
E-mail: bernard.malfoy@curie.fr
1
Institut Curie - CNRS - UPMC UMR7147, CEA LRC38, 26 rue d'Ulm, 75248
Paris, France;
2
Institut for Histology and Embryology, Medical Faculty, Koryt-
kova 2, 1000 Ljubljana, Slovenia;
3
Institut Curie, Service de Radiothérapie, 26
rue d'Ulm, 75248 Paris, France;
4
Hôpital Cochin. Service de Chirurgie Ortho-
pédique et Oncologique. 27 rue du Fbg St Jacques. Paris. France;
5
CEA, DSV
DRR, 60 avenue du Général Leclerc 92265 Fontenay-aux-Roses, France.
** These authors contributed equally to the work.

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Table 1.
LOH pattern of NHEJ genes in the radiation-induced tumors.
Locus case 1 case 3 case 4 case 5 case 6 Location
D22S423 + + – + 38,706,686
D22S279 – 39,347,314
D22S276 + + – + 40,336,790
KU70 40,342,854
D22S1157 + – – + 40,564,359
D22S1178 + + – – 40,715,192
D22S1166 + – + 41,377,998
D22S270 + + – + 41,378,029
D22S1179 – – + 41,917,371
D2S334 + – + 214,457,660
D2S2319 – – + 214,752,101
D2S2361 + + – 216,303,933
D2S137 + – + + 216,644,805
KU80 261,874,071
D2S382 – + – 216,874,071
D2S301 +–+–+217,712,579
D2S2248 – + 217,764,456
D2S164 – – – 217,785,698
D5S626 + – + – 81,658,815
D5S2083 + – + – 81,921,800
D5S641 – 82,038,983
D5S1347 + – – – 82,310,607
XRCC4 82,038,983
D5S1959 + – + 82,623,326
D5S1948 + – + – 82,948,355
D5S459 – – + – 83,402,144
D5S107 + – – + 83,635,324
D13S173 + + + 106,604,890
D13S1258 + – 106,401,474
D13S796 ++++–106,687,142
D13S778 + + + – 107,274,555
LIG4 107,657,792
D13S1265 + + + 108,126,458
D13S895 + + + – 108,386,267
D13S1315 + + – 109,143,179
D10S1707 – – 13,871,659
D10S223 + + – 13,871,274
D10S506 + – + – 13,832,822
D10S1725 – + – – 13,641,414
D10S1664 – + – – 14,343,869
D10S191 + – + 14,599,641
Artemis 14,989,884

Non-homologous End joining and Genomic Instability 225
J. Radiat. Res., Vol. 46, No. 2 (2005); http://jrr.jstage.jst.go.jp
Locus case 1 case 3 case 4 case 5 case 6 Location
D10S1653 – + – – 15,717,838
D10S1763 + – – 16,034,297
D10S1477 + – 16,622,034
D8S255 – – – 40,004,275
D8S268 – – – 41,366,650
centromere
D8S1460 – – – 47,154,380
DNA-PKcs 48,848,222
D8S531 – – – 49,186,410
D8S519 – – – – 49,248,030
GATA9A01 – – 49,334,873
D8S1716 – – – 49,888,078
D8S1745 – – – 50,382,536
D5S666 + – 130,555,525
D5S2110 + – + 130,885,313
D5S2057 – – 130,896,705
RAD50 131,920,529
D5S2002 + – + 132,404,305
D5S1876 + + – 132,730,097
D5S2117 +–+–133,065,027
D5S2053 + + – – 133,145,049
D8S273 – – + 88,980,544
D8S1800 + – – – 89,763,957
D8S88 + – – – + 90,917,944
NBS1 91,014,890
D8S1811 + – – 91,301,671
D8S1724 – + 91,488,969
D8S1476 + – – + 92,501,400
D8S270 + – – + 93,089,546
D11S1995 – – + 91,981,378
D11S4118 – – + 92,998,467
D11S1311 – – + 92,987,553
D11S4176 – – + + 93,711,810
MRE11 93,790,121
D11S1757 – + + 94,338,979
D11S1788 – – 94,467,663
D11S991 – + 94,475,790
D8S1333 – – + + 94,523,216
For each microsatellite the name, the position in the chromosome sequence and the LOH status is given. Microsatellites were
selected using the human genome sequence (released may 2004) available at the UCSC Genome Bioinformatics site, (http://
genome.ucsc.edu/). Location: position of the first nucleotide on the chromosome sequence; normal types: forward primer of the
microsatellite; bold types: beginning of the gene. For the DNA-PKcs gene, located on chromosome 8 long arm near the cen-
tromere, the LOH status of sub-centromeric microsatellites was also analysed on the short arm. LOH: +; no LOH: –; all the blank
is non-informative.

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Table 2.
Primers used for NHEJ gene sequencing
Gene Forward Primer Reverse Primer
KU70 1 5’ CGCTTCCCTGCGCCAAAG 3’ 5’ ACCCACAGCACTTCACTGAG 3’
2 5’ GCCACGGATCTGACTACTCA 3’ 5’ TATTGGAGGAGGCTTGAGAG 3’
3 5’ AGTGATCTCTGTGGGCATTA 3’ 5’ GTCATCCAACTCTTCTTCCT 3’
4 5’ TATTTTGTGGCTTTGGT 3’ 5’ TGTTCCGGCTCCATCAAATC 3’
5 5’ TTTGAGAACCCCGTGCTG 3’ 5’ TTAACTGGCTGAGGACAAGG 3’
KU80 1 5’ GACCAAAGCGCCTGAGGA 3’ 5’ ATTGCAGGGAGATGTCACA 3’
2 5’ GCAAAAGTCAGCTGGATA 3’ 5’ CATCATCATCATTTAAGCAA 3’
3 5’ GATATACAAAAAGAAACAGT 3’ 5’ TTACTGTTTTTCAAGGATG 3’
4 5’ TGCAGCTGCCTTTCATGG 3’ 5’ TCAGCAGGATTCACACTT
5 5’ CCACTTCAGCGTCTCCAG 3’ 5’ GCGATGGCAGCTCTCTTA
XRCC4 1 5’ AAGTGGGGCTGCCTCTTTAA 3’ 5’ ACATAGTCTAGTGAACATCAA 3’
2 5’ TGCAGAAAGAAAATGAAAGG 3’ 5’ CAGTACTCTCATCATAGACT 3’
LIG4 1 5’ TAAGAACCACAAAGATGTCA 3’ 5’ CTGAGTTCCTACAGAAGGATC 3’
2 5’ AAAAGTCTGTAGGCAACTGA 3’ 5’ TGCATCAATTACTTCATTG 3’
3 5’ TGCAGAAAACACAAGCTCAT 3’ 5’ CTCCTTGTCATCTCTTATCT 3’
4 5’ CATTGTTCAGATTAAAGCAG 3’ 5’ AAATAACTATCACCATAGCA 3’
5 5’ ATTTTGCCCGTGAATATGAT 3’ 5’ GTATTTTATCATTACCACCT 3’
Artemis 1 5’ TTTTGGGGTCCCGGACTCT 3’ 5’ ACACTCCTCCCGACTTGGAAT 3’
2 5’ GGGCAGAGTCAAAGACATCCA 3’ 5’ GGAAGACCGGCATAAAGGCT 3’
3 5’ TGTCCTGTGAACGCATATCCA 3’ 5’ TGGGAGGAGATGTGAGTTGAT 3’
4 5’ ATCTCAGTCACCAAAGCTTTTC 3’ 5’ TGACTGTCATCTCTGTGCAGG 3’
RAD50 1 5’ TGGAATAGAGGACAAAGATAA 3’ 5’ CTGATCATTTCTCGGTCAAT 3’
2 5’ TGGTGAAAAGGTCAGTCTCGA 3’ 5’ CTTTCGGCTATCCAAGGCT 3’
3 5’AATCTCTCTAAAATAATGAAAC 3’ 5’ CATCAGTTGGTTGGCAG 3’
4 5’ TTGTGAGAGAGAGACAAGA 3’ 5’ TTCTGATTTGTTCATCTTTG 3’
5 5’ GATGCTGACCAAAGACAAA 3’ 5’ TTGTAACTCAGCCTCTGTC 3’
6 5’ GTCAGAGAGTTTTTCAGACA 3’ 5’ TGTTTCTCCTGGTTGACTTG 3’
7 5’ CTAAGCTACAAGGAATAGAC 3’ 5’ TCACTTAGTTGAGCTATTACT 3’
8 5’ AGCAAAAAGAAACTGAACTT 3’ 5’ TCATTATTGCTTGGTCAAGA 3’
9 5’ ATGAGGACAACAGAACTTGT 3’ 5’ ACGCTGCTGTGAGCGACTTT 3’
10 5’ CTCTTGCACATGCTCTGGTT 3’ 5’ TGAGGACCTACATTTCTATG 3’
NBS1 1 5’ ACGTCGGCCCCAGCCCTGA 3’ 5’ CAGTAAATCCTCCAAGTTGC 3’
2 5’ TAGATGTCTCTGGAAAACT 3’ 5’ GTATCAACAACACACGTTCC 3’
3 5’ AGAAGAGAATGAAGAAGAAC 3’ 5’ ACAGTGGGTGCGTCTTCTGA 3’
4 5’ GGGATTTGAGTGAAAGGCCA 3’ 5’ TCTGTATCTGTATTTTTCCAC 3’
5 5’AAAAGGGAAAGGGATGAAGAA3’ 5’ AGTCTTCCTTGAGTTCACGT 3’
6 5’ GCAGTACCAGAAAGTAGCAA 3’ 5’ GCCTGAAGTAGATGCTTAC 3’
MRE11 1 5’ ATCGAGTGCATTTTCTGAC 3’ 5’ TGAAATGTTGAGGTTGCC 3’
2 5’ CATGGGTGAACTATCAAGAT 3’ 5’ CTGGGGAAAGAGAAGTAACC 3’
3 5’ TCACAACCTGGAAGCTCA 3’ 5’ GTACTGTTTTACAAGATCTT 3’
4 5’ ACTTTGGGAAACTTATCACA 3’ 5’ TTCCTTTGATGGTTGCTG 3’
5 5’ GCAGAACAGATGGCTAAT 3’ 5’ TTCATTTTTCCTGTATCTTG 3’

Non-homologous End joining and Genomic Instability 227
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for bilateral hereditary retinoblastoma.
21)
These tumors
displayed numerous rearranged chromosomes and a high
degree of karyotype instability. In these tumors, both RB1
and TP53 genes were biallelically altered. In order to better
understand the mechanisms underlying the high genomic
instability observed in these tumors, we investigated the pos-
sible implication of NHEJ genes.
MATERIAL AND METHODS
Biological material
The study was performed on 5 radiation-induced sarco-
mas: 3 osteosarcomas (cases 1, 5 and 6), 1 malignant periph-
eral sheath nerve tumor (case 3) and 1 leiomyosarcoma (case
4) collected at the Institut Curie. Five sporadic osteosarco-
mas collected at the Hopital Cochin were used as control
(cases 10–14). Informed consent from patients or parents
was obtained for all cases. Three tumors were grown as
xenografts in athymic mice (cases 3, 4 and 6), while both
fresh and xenografted tumors were available for case 3.
Xenografted tumors were analyzed at passage 3. Animal
care and housing were in accordance with institutional
guidelines as stipulated by the Ministère de l'Agriculture,
Direction de la Santé et de la Protection Animale.
DNA and RNA preparation
DNA and RNA were extracted from frozen samples using
Qiagen kits (Qiagen S.A. Courtaboeuf, France). Preparation
integrity was checked by capillary electrophoresis (Agilent
Tech, Massy, France). Reverse transcription was performed
using Superscript II (Invitrogen S.A. Cergy Pontoise,
France) and random primers.
Loss of heterozygosity
LOH was analyzed after PCR amplification of microsat-
ellite loci (Table 1). 15 ng of template DNA were amplified
with 15 pmole each of 5’(6-FAM)-labeled sense and non-
labeled antisense primers. Others experimental conditions
were previously described.
21)
Microsatellites were selected
both sides of each gene using the human genome sequence
(released may 2004) available at the UCSC Genome Bioin-
formatics site, (http://genome.ucsc.edu/).
DNA fragments
were run on an ABI PRISM 3100 Genetic Analyzer (Applied
Biosystem). Allelic size and intensities were determined
using the GeneScan Analysis Program. One allele was con-
sidered to be lost when all the informative microsatellites
both sides of the gene displayed LOH.
Sequencing
Published sequences (Genebank) for
KU70
(XM-
0010020),
KU80
(J044977),
XRCC4
(U40622),
Artemis
(NM-022487),
LIG4
(X83441),
RAD50
(NM-005732),
NBS1
(NM-002485) and
MRE11
(NM-005590) were used to
design specific primer pairs for RT-PCR amplification of
overlapping fragments for each cDNA (Table 2). RT-PCR
fragments were directly sequenced by using Big Dye Termi-
nator Sequencing Kits (Applied Biosystems, Courtaboeuf,
France).
Real-time fluorescent detection quantitative RT-PCR
(RT-Q-PCR)
cDNAs were amplified by using the GeneAmp 5700
sequence detection system and SYBR Green PCR Kits
(Applied Biosystems, Courtaboeuf, France). PCR conditions
were as described in.
22)
All measurements were performed at
least in triplicate using 100ng of cDNA, presented values are
the mean of the experimental data. Variations were of 30 –
40%. Primers were selected by using the PrimerExpress pro-
gram (Applied Biosystems, Courtaboeuf, France). Only
primer pairs with efficiency of > 90% were retained for fur-
ther experiments (Table 3). Expression levels were calculat-
ed using a standard curve constructed with serial dilutions of
Table 3.
Primers used for RT-Q-PCR
Gene Forward Primer Reverse Primer
KU70 5’ ACAAGTACAGGCGGTTTGCTTC 3’ 5’ GGCCTCAGGTAATGGTGTTTCT 3’
KU80 5’ CTGTGTATGGACGTGGGCTTTA 3’ 5’ GTGCCATCTGTACCAAACAGGA 3’
XRCC4 5’ TGGGACAGAACCTAAAATGGCT 3’ 5’ GTCTTCTGGGCTGCTGTTTCTC 3’
LIG4 5’ CTTGCCCGAGGCCAGTTA 3’ 5’ GCAAAAGGAACGTGAGATGCA 3’
Artemis 5’ TAAAGCCTTTATGCCGGTCTTC 3’ 5’ TGGCAGAGGATCATCAAAGAGA 3’
DNA-PKcs 5’ CCAGCTCTCACGCTCTGATATG 3’ 5’ GCGTGTACCATGATGCTGTACA 3’
RAD50 5’ ATACGTGACCTGTGGCGAAGTA 3’ 5’ TCGGCATCAGACCGTATTTCTA 3’
NBS1 5’ CAGCAGACCAACTCCATCAGAA 3’ 5’ TGAGGGTGTAGCAGGTTGTGTT 3’
MRE11 5’ CACAAAGCCTTCAGAAGGAACA 3’ 5’ AAACGACGTACCTCCTCATCGA 3’
HPRT 5’ CGTTTCCTTGGTCAGGCAGTATAAT 3’ 5’ AAGGGCATACCTACAACAAACTTG 3’

S. H. Lefèvre
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Fig. 1.
Expression levels of NHEJ genes analyzed by quantitative RT-Q-PCR. Cases 1, 3 – 6, radiatio-induced tumors;
cases with LOH are displayed with filled bars and those without LOH are displayed with open bars. Cases 10 – 14, sporadic
cases without LOH. Expression levels were calculated using a standard curve obtained from serial dilutions of case 4 and
normalized using hypoxanthine phosphoribosyltransferase 1 (HPRT)

Non-homologous End joining and Genomic Instability 229
J. Radiat. Res., Vol. 46, No. 2 (2005); http://jrr.jstage.jst.go.jp
case 4 cDNA. Normalization was performed using hypoxan-
thine phosphoribosyltransferase 1 (HPRT) expression, ana-
lyzed with the same procedure.
RESULTS
The status of
KU70, KU80, XRCC4, LIG4, DNA-PKcs,
Artemis, RAD50, NBS1
and
MRE11
genes was analyzed in
the 5 radiation-induced tumors (Table 1). Of the 70 studied
microsatellites, LOH was found in 45% of the informative
cases. For the genes of the KU70/KU80/XRCC4/LIG4/Arte-
mis/DNA-PKcs complex, one allele of 2 or 3 genes was lost
in each tumor except for case 1 where 5 genes were impli-
cated. No LOH was found for DNA-PKcs, whereas LIG4
was monoallelic in 4 of the 5 cases. For tumor 6, it was pre-
viously shown that the distal part of chromosome 13, which
harbors the
LIG4
gene, was biallelically retained.
21)
For the
genes of the MRE11/RAD50/NBS1 complex, LOH was
observed for 1 or 2 gene(s) in all cases. In case 3, the tumor
and the xenograft had the same LOH pattern for all studied
genes.
The presence of mutations in monoallelic genes was
investigated by sequencing the cDNAs. No mutation was
observed. An alternative splicing transcript of
Artemis
cor-
responding to the insertion of the exon 2b was observed.
23)
The expression of the full length and of the variant tran-
scripts was similar in all tumors and corresponding normal
tissues (not shown). The functional significance of such vari-
ants is presently unknown.
23)
The same microsatellites were
analyzed in the 5 sporadic osteosarcomas. No LOH was
observed, an indication of a higher genomic stability in these
sarcomas as compared with the radiation-induced sarcomas
(not shown).
The dosage of the gene expression by RT-Q-PCR was per-
formed for radiation-induced and sporadic sarcomas (Fig.
1). The NHEJ genes were expressed at various levels in all
tumors. However, the differences were not significant. In
particular, similar levels of expression were observed for
radiation-induced sarcomas with or without LOH as well as
for radiation-induced or sporadic sarcomas. Thus, no rela-
tionship was found between allelic status and gene expres-
sion.
DISCUSSION
In the radiation-induced sarcomas, with marked rear-
rangement of the genome and biallelic inactivation of
RB1
and
TP53,
none of the genes
KU70, KU80, DNA-PKcs,
XRCC4, Artemis, LIG4 IV, RAD50, MRE11
or
NBS1
was
biallelically inactivated. The co-inactivation of
TP53
and
RB1
pathways is frequently observed in tumors.
11)
One con-
sequence is the suppression of a major apoptotic pathway.
However several other functions, linked to cell cycle control
and genome stability, may also be altered (review in
24,25)
). In
tumors, the loss of the G1 checkpoints controlled by Rb1
and Tp53 is the basis for a tendency to endoduplication.
26)
However, it has been shown that, by itself, the inactivation
of Tp53 may be insufficient to drive instability in human
tumor cell lines.
27,28)
In mouse embryonic stem cells, a hap-
lo-insufficiency of
RB1
induces a measurable genomic insta-
bility which is strengthened when both alleles are lost.
29)
In
contrast, in brain tumors of trangenic mice, inactivation of
both Tp53 and Rb1 family proteins favors tumor progression
but most tumor cells remain diploid and do not exhibit
genetic instability.
30)
Thus, other genes rather than TP53 and
RB1 must be implicated in the strong genomic instability
observed in these tumors.
31)
Convincing data exist on the
implication of the biallelic inactivation of NHEJ genes and
on cooperation with the Tp53 pathway for the induction of
marked genomic instability in mice experimental systems.
6–
11)
Such mechanisms do not seem to be operative in the pres-
ently studied radiation-induced human sarcomas where no
biallelic inactivation of NHEJ gene was observed. Differenc-
es between murine models and human tumors could be
linked to the nature of the implicated tissues, which may be
a determining factor for the phenotypic expression of these
mutations. In addition, the nature and the number of genes
cooperating in tumorigenesis may differ in mouse and in
human. Such differences have been underlined, for example,
in the predisposition to retinoblastoma, which requires the
co-inactivation of Rb1 and p107 in mouse and only Rb1 in
human.
32,33)
Finally, because their significant roles in cell
survival, it is possible that complete loss of the function of
NHEJ genes is not compatible with human tumor develop-
ment. For instance, data from murine and human
KU80
defi-
cient cells support the species differences that could exist in
the requirement for this protein. In mouse, inactivation of
one allele of
KU80
has no phenotypic consequences and
biallelic inactivation of the gene only induced a proliferation
defect and genomic instability.
6,7,34,35)
In contrast, human
HCT116 colon cancer cells, heterozygous for
KU80
but wild
type for
TP53
, undergo a reduction in proliferation and tend
to become polyploid cells. Biallelic inactivation of the gene
result in cells with poor viability which die by apoptosis
after a few cell divisions.
36)
Our results are more in line with
the limited available data on human tumors, where muta-
tions are rarely found in the NHEJ genes. In particular, genes
implicated in cancer-predisposing diseases do not seem to be
recurrently mutated in sporadic tumors.
19,20)
In fact, muta-
tions were observed at a high level only for
RAD50
or
MRE11
in the context of microsatellite instability.
37,38)
Since
our radiation-induced tumors were stable at microsatellites,
21)
such mutations, linked to defects in the mismatch-repair
systems, were not expected in our series.
In human, it has been shown that haplo-insufficiency for
KU80
36)
and possibly for
LIG4
18)
may influence genome sta-
bility. In transgenic mice deficient for the INK4a/ARF on
the RB1 pathway, haplo-insufficiency for LIG4 is sufficient

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to induce the formation of sarcoma with an unstable
genome.
14)
However, no alteration of the end-joining activity
was found in a series of human breast cancer cell lines where
an haplo-insufficiency could be suspected.
39)
All radiation-
induced tumors analyzed here had LOH in 2 to 5 of the core
NHEJ components (KU70, KU80, XRCC4, Artemis or
LIG4). In addition, 1 or 2 LOH were observed for the genes
of the MRE11/RAD50/NBS1 complex. This high rate of
LOH is likely to be related to the loss of numerous chromo-
somes and chromosome segments observed in these tumors.
21)
In contrast, no LOH was observed for the same genes in
the sporadic sarcomas. The level of expression of NHEJ
genes was very similar in the analyzed tumors, independent-
ly of their sporadic or radiation-induced origin. In the radi-
ation-induced sarcomas, LOH were not associated with a
measurable decrease of the mRNA expression of the genes
(Fig. 1). The uncertainty in the quantification by RT-Q-PCR
might make it difficult to completely exclude that, in some
cases, one or some of these genes were really underex-
pressed. However, the striking similarities in the expression
variations measured between sporadic sarcomas, without
LOH, and radiation-induced sarcomas, with numerous LOH,
argue against this hypothesis. Thus, presently, no experimen-
tal data support a role of a haplo-insufficiency of the DSB
repair pathway on the genome stability in these tumors.
However, perturbations in the function of others genes
controlling cell-cycle
40)
or spindle-assembly checkpoints,
41)
chromatin assembly
42)
or telomere length,
43)
could be
involve in the genome instability of these tumors in addition
to the biallelic inactivation of both TP53 and RB1.
Conclusion
The presently available data are not in favor of a major
role for a defect in the NHEJ pathway to account for the
genomic instability observed in the radiation-induced sarco-
mas studied here.
ACKNOWLEDGMENTS
S.L. and N.G-L were fellows of the Ministère de l'Educa-
tion Nationale et de la Recherche. A.C. was a fellow of Elec-
tricité de France (EDF). This work was supported by EDF
(RB 2005-10) and the Institut Curie-CEA “Programme Inci-
tatif et Coopératif Instabilité génétique et radiorésistance des
tumeurs”.
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Received on November 10, 2004
1st Revision received on March 22, 2005
Accepted on March 22, 2005