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Single gene mutations causing SGA
Marie J.E. Walenkamp*MD, PhD
Doctor
Jan M. Wit MD, PhD
Professor
Department of Paediatrics, J6-S, Leiden University Medical Centre,
PO Box 9600, 2300 RC Leiden, The Netherlands
The growth hormone–insulin-like growth factor-I (GH–IGF-I) axis plays a key role in intra-uter-
ine growth and development. This review will describe the consequences of genetic defects in
various components of the GH–IGF-I axis on intra-uterine growth and development. Animal
knockout experiments have provided evidence for the GH-independent secretion of IGF-I
and its effect in utero. Reports of patients with a deletion or mutation of the IGF-I and
IGF1R genes have provided insight into the role of intra-uterine IGF-I in the human. Homozy-
gous defects of the IGF-I gene have dramatic effects on intra-uterine growth and development,
whereas heterozygous defects of the IGF1R gene have a more variable clinical presentation. The
phenotype in relation to the genotype of the different disorders will be reviewed in this
chapter.
Key words: IGF-I; growth hormone; insulin; receptor; mutation; IUGR; development.
Human growth is a complex process, comprising a continuum that starts at concep-
tion and ends in young adulthood. Intra-uterine growth probably involves the most
dynamic period of this continuum, characterized by organogenesis in the first trimes-
ter, major cellular hyperplasia in the second trimester, and maturation of the organs
and further body growth in the third trimester. Normal prenatal growth is dependent
on an optimal intra-uterine environment, determined by maternal factors, placental
function and fetal conditions. In addition, the growth hormone (GH)–insulin-like
growth factor (IGF)-I axis plays a key role in intra-uterine growth and development.
This review focuses on the consequences of disruptions in the various components
of the GH–IGF-I axis on intra-uterine growth and development.
* Corresponding author. Tel.: þ31 715262824; Fax: þ31 715248198.
E-mail address:m.walenkamp@lumc.nl (M.J.E. Walenkamp).
1521-690X/$ - see front matter ª2008 Elsevier Ltd. All rights reserved.
Best Practice & Research Clinical Endocrinology & Metabolism
Vol. 22, No. 3, pp. 433–446, 2008
doi:10.1016/j.beem.2008.02.001
available online at http://www.sciencedirect.com
THE GH–IGF-I AXIS
The pulsatile secretion of GH is regulated by the hypothalamic factors GH-releasing
hormone and somatostatin, which are controlled by a wide range of neurotransmitters
and neuropeptides.
1
The biological actions of GH are mediated by the transmembrane
GH receptor (GHR). Binding of GH to its receptor results in activation of the JAK-
STAT signal transduction pathway and the transcription of target genes, including
IGF-I and IGF binding protein (IGFBP)-3.
2
IGF-I is a single-chain, 70-amino-acid peptide with a molecular weight of 7.65 kDa,
organized into four domains: the A and B domain with a similar structure to the A and
B chain of insulin (49% sequence identity); the C domain, similar in structure to the C-
peptide of pro-insulin; and the D domain.
3
The IGF-I prohormone also contains a C-
terminal E-peptide that is cleaved in the Golgi apparatus before secretion.
4
Although
IGF-I is mainly produced in the liver, this growth factor is synthesized in all cells of the
body.
2
IGF-II is a 67-amino-acid peptide with a molecular weight of 7.47 kDa.
The IGFs are associated with six soluble high-affinity IGFBPs. Changes in expression
of the IGFBPs and the IGFBP proteases play an important role in modulating the ac-
tions of the IGFs. In the circulation, the IGFs form a ternary complex with IGFBP-3
or IGFBP-5 and a large protein named ‘acid labile subunit’ (ALS).
5
These three mole-
cules form a 150-kDa complex, restricting the IGFs to the circulation and prolonging
their half-life.
5
The biological functions of IGF-I are primarily mediated through the type 1 IGF-I
receptor (IGF1R). The IGF1R gene is located on the distal long arm of chromosome
15 (15q26.3) and has a similar organization compared with the insulin receptor (IR)
gene, with sequence homology varying from 41% to 84% depending on the domain.
6
Both are heterotetrameric (a
2
b
2
) transmembrane glycoproteins, synthesized as
a single-chain preproreceptor, and consisting of an a-subunit that is mainly involved
in ligand binding and a b-subunit containing the tyrosine kinase domain. Ligand bind-
ing to the tyrosine kinase receptor results in receptor autophosphorylation on in-
tracellular tyrosine residues, and activation of the receptor’s intrinsic tyrosine
kinase, initiating distinct intracellular signalling pathways.
7
The main signalling path-
ways are the PI3 kinase/Akt and MAP kinase ERK 1/2 pathways, resulting in protein
synthesis, glucose transport and nuclear targets involved in cell proliferation and
apoptosis.
3
EXPRESSION OF IGFS AND IGFBPS DURING
INTRA-UTERINE DEVELOPMENT
In many species, the IGF-I and IGF-II genes are expressed in fetal tissues from the ear-
liest stage of pre-implantation development to the final phase of tissue maturation just
before birth.
8
In chicken embryos, IGF1R mRNA is expressed as early as Day 0 and
IGF-I is present from Day 2.
9
In pre-implantation mouse embryos, IGF-II ligand and re-
ceptor gene activity is detectable as early as at the two-cell stage; the time when tran-
scription from the embryonic genome is activated. Receptors for insulin and IGF-I are
detectable from the eight-cell stage. Transcripts for insulin or IGF-I are not detectable
in pre-implantation mouse embryos.
10
In the humans, transcripts of the IGF1R are
present in oocytes and pre-implantation embryos. Of the ligands, only IGF-II tran-
scripts are present, consistent with the findings in mice.
11
These data indicate an important role for the IGF system during early development.
One of the mechanisms by which IGF-I exerts its role at this stage appears to be
434 M. J. E. Walenkamp and J. M. Wit
prevention of apoptosis. This is supported by studies in porcine embryos in which IGF-
I increased the rate of blastocyst formation and total cell number, and decreased the
incidence of apoptosis by regulating the expression of pro- and anti-apoptosis-related
genes.
12
Also, in humans, IGF-I significantly increases the development of embryos to
the blastocyst stage. In addition, the proportion of apoptotic nuclei is decreased by
IGF-I in the human embryo. This suggests that IGF-I rescues embryos in vitro which
would otherwise arrest, and acts as a survival factor during pre-implantation human
development.
13
All human fetal tissues express IGF-I/IGF-II mRNA. Differential patterns of IGF ex-
pression are observed in different fetal tissues and vary with gestational age.
8
GENETIC DEFECTS IN THE GH–IGF-I AXIS
In the last few years, reports of patients with genetic defects in various components of
the GH–IGF-I axis, in addition to animal knockout experiments, have increased our
knowledge of the role of the GH–IGF-I axis in intra-uterine growth and development.
The established genetic defects in the GH–IGF-I axis are described below, focusing on
the intra-uterine effects.
Pituitary GH secretion
Classical GH deficiency can be the result of a mutation in the GH releasing hormone
receptor (GHRH-R) gene. Little mice, with a naturally occurring missense mutation of
the extracellular part of the GHRH-R, have a normal birth weight.
14
Also, humans
with a GHRH-R mutation are born with a normal birth weight.
15
GH deficiency
due to mutations in genes playing a role in the ontogenesis of GH-producing cells in
the anterior pituitary does not affect intra-uterine growth. This is demonstrated by
the normal birth weights of mice with a Prop1 mutation (Ames dwarf) or a Pit1 muta-
tion (Snell and Jackson dwarf).
14
Also, infants with Prop1 mutations have birth weight
and length within the normal range.
16
As illustrated by the normal birth weight and
length in two families with a GH-1 gene mutation [mean birth weight 0.9 standard
deviation scores (SDS), mean birth length 0.7 SDS], GH deficiency does not appear
to affect intra-uterine growth.
17
Children with congenital GH deficiency have a birth
length in the lower normal range.
18
These cases illustrate that GH is not determinative for intra-uterine growth,
although some impact of GH deficiency on growth in the late third trimester may ex-
plain why birth size is in the lower normal range in some cases.
GHR and GH signalling
The biological effects of GH can only be reached in the presence of a normal function-
ing GHR and an intact post-receptor signalling pathway. The earliest and most com-
monly described genetic abnormalities of the GHR gene include deletions and
mutations in the extracellular domain of the GHR, resulting in reduced GH binding.
19
Birth weight of these patients is normal and birth length is in the lower normal
range.
20–22
The GHR uses JAK-STAT proteins as the signal transduction pathway. STAT5b is the
preferred STAT protein. Upon activation, STAT5b regulates the expression of a variety
of target genes, including IGF-I. Several patients with a STAT5b mutation have been
Single gene mutations causing SGA 435
described recently. Although postnatal growth is extremely retarded, birth weight is
normal in all cases except one, and birth length is normal.
19
IGF-I
Targeted mutagenesis of the genes encoding IGF-I, IGF-II and IGF1R in mice have
provided insight in the role of the IGF-I system in prenatal growth and development.
IGF-I /mice have a birth weight of 60% of their wild-type littermates.
23,24
This is in
contrast to the normal birth weight found in mice with GH deficiency or GH resis-
tance, and strongly suggests that, in prenatal mice, IGF-I is secreted independently
of GH and that disruption of the IGF-I gene results in severely compromised intra-uterine
growth. The proposed underlying mechanism for growth retardation in IGF-I knock-
out mice is an elongation of the cell cycle time, resulting in fewer proliferative events
and generation of fewer cells compared with wild-type mice. In this way, IGF-I is an
essential factor for the maintenance of growth at a normal rate.
24
The first human IGF-I gene defect was described in 1996 by Woods et al.
25
This pa-
tient was born at 37 weeks of gestation. The parents were first cousins once removed.
A caesarean section was performed because of poor fetal growth. The pregnancy had
been uneventful. At birth, the infant had symmetrical growth retardation, with a birth
weight of 1.4 kg (3.9 SDS), length 37.8 cm (5.4 SDS) and head circumference of
27 cm (4.9 SDS). Placental weight was 350 g (1.3 SDS). Throughout infancy and
childhood, bilateral sensorineural deafness was found and psychomotor development
was delayed. Biochemical analysis revealed undetectable levels of IGF-I, with no re-
sponse to the administration of GH at a dose of 0.1 U/kg for 4 days. Spontaneous
12-h GH secretion showed abnormally high peaks and an elevated baseline between
peaks. ALS and IGFBP-3 values were within the normal range. Molecular analysis
showed a homozygous deletion of exons 4 and 5 of the IGF-I gene. This deletion re-
sults in a mature truncated IGF-I peptide of 25 amino acids instead of 70, followed by
an out-of-frame nonsense sequence and a premature stop codon.
The second patient with a genetic defect of the IGF-I gene was first described in
1969 by van Gemund et al.
26
, but the defect was elucidated in 2005. This patient
was the first child of a second marriage of the mother. Four healthy children were
born in the first non-consanguineous marriage of the mother. The second marriage
was consanguineous; the grandfathers of the patient were brothers. The patient was
born after 8 months of gestation with a birth weight of 1420 g (3.9 SDS) and length
of 39 cm (4.3 SDS). The youngest son of the second marriage was born at term with
a birth weight of 1900 g (4.5 SDS). Both brothers showed bilateral hearing loss, mi-
crocephaly and severe mental retardation. Biochemical evaluation revealed elevated
basal GH levels (20 mU/L in the oldest brother and 4.5 mU/L in the youngest brother)
and an excessive response of GH to insulin-induced hypoglycaemia (191 mU/L and
309 mU/L, respectively). The authors concluded that familial unresponsiveness to
the somatotropic effects of GH was combined with intact glucoregulation and lipolysis.
At 55 years of age, the oldest brother was re-evaluated.
27
The youngest brother died
at 32 years of age of aspiration pneumonia. As the phenotype of the patient was strik-
ingly similar to that of the patient with the IGF-I deletion, the authors were surprised
to find extremely elevated IGF-I levels (þ7.3 SDS). IGFBP-3 was normal. An IGF1R
defect was excluded by demonstrating normal
3
H thymidin incorporation in cultured
fibroblasts of the patient, upon stimulation with wild-type IGF-I. Subsequently, the IGF-I
gene was sequenced. This analysis identified a homozygous G >A nucleotide substitu-
tion at position 274, changing valine at position 44 of the mature IGF-I protein to
436 M. J. E. Walenkamp and J. M. Wit
methionine (Val
44
Met). Receptor binding assays showed that recombinant Val
44
Met
has an approximately 90-fold lower affinity for the IGF1R compared with wild-type
IGF-I. Competition binding studies revealed no significant contribution of Val
44
Met
to the displacement of tracer IGF-I from IGF1R expressing membranes of bovine pla-
centa, despite the increased levels of IGF-I in serum. These data supported the inacti-
vating nature of the mutation. Remarkably, the distribution of Val
44
Met over the
various molecular weight classes in serum was normal, suggesting normal binding of
Val
44
Met to the IGFBPs. Structural analysis by nuclear magnetic resonance imaging
confirmed retention of near-native structure with only local side-chain disruptions de-
spite the significant loss of function.
28
The third patient with an IGF-I gene defect was described by Bonapace et al.
29
This
patient was born after 39 weeks of gestation and an uneventful pregnancy. His birth
weight was 1480 g (4 SDS), length was 41 cm (6.5 SDS) and head circumference
was 26.5 cm (<5th percentile). Other phenotypical features included sensorineural
deafness and delayed psychomotor development. His parents were first cousins. Bio-
chemical features consisted of low serum IGF-I levels (1 ng/mL, normal range 3.7–
152 ng/mL) and elevated GH concentrations upon stimulation with arginine (18 ng/
mL, normal range 10–12 ng/mL). IGFBP-3 levels were normal. After administration
of 0.6 IU GH/day intramuscularly for 7 days, IGF-I levels did not change. Sequencing
of the IGF-I gene revealed a homozygous T >A transversion in exon 6 of the IGF-I
gene, resulting in the expression of a smaller exon 6 (340 bp instead of 450 bp) and
altering the E domain of the IGF-I prohormone. Functional studies were not per-
formed. However, a later study showed that this mutation cannot explain the clinical
and biochemical phenotype, as it was found in homozygous and heterozygous states in
controls of normal height, corresponding to 4% of studied alleles.
30
The fourth patient, a boy born from a consanguineous marriage after 40 weeks of
gestation, was presented in 2006. His birth weight was 2350 g (2.5 SDS), length was
44 cm (3.7 SDS) and head circumference was 32 cm (3 SDS).
31
Hearing tests were
normal and he showed mild developmental delay. Biochemically, IGF-I levels were re-
duced, with normal levels of IGFBP-3 and ALS. IGF-I gene analysis revealed a homozy-
gous missense mutation resulting in the change of a highly conserved arginine located
in the C domain of the protein into a glutamine. Affinity for the IGF1R was decreased
two- to three-fold, resulting in decreased IGF1R autophosphorylation. The authors
concluded that partially diminished IGF-I activity has dramatic consequences on fetal
growth and development.
To the present authors’ knowledge, these four patients are the only documented
patients with a genetic defect in the IGF-I gene. A common feature in these patients
is severe intra-uterine growth retardation (IUGR), which is in line with the findings
in mice and in contrast with the normal birth size generally found in patients with
GH deficiency or GH resistance, implicating that in humans, IGF-I secretion before
birth is independent of GH. Apparently, IGF-II, which is not affected in these patients,
is unable to compensate for the loss of IGF-I activity in these patients in utero.
Besides these reports on rare genetic defects, there is also more indirect evidence
suggestive of a role of IGF-I in intra-uterine growth. In numerous studies in normal
term infants, using cord blood or fetal blood across a range of birth weights, a positive
relationship has been found between cord blood IGF-I and birth weight.
32
In addition,
in pregnancies complicated by IUGR, umbilical cord blood IGF-I is reduced compared
with pregnancies with normal fetal growth.
32
Finally, the finding that genetically deter-
mined low IGF-I levels, due to polymorphisms in the IGF-I gene promoter region, result
in a reduced birth weight and length supports the role of IGF-I in fetal growth.
33,34
Single gene mutations causing SGA 437
There are also indications of an IGF-I dose effect on intra-uterine growth. Hetero-
zygous IGF-I mice are found to be smaller than their wild-type littermates, with 10–
20% smaller organs.
35
These mice were otherwise healthy. In humans, there also
appears to be an effect of IGF-I heterozygosity. The parents of the first patient had
heights in the lower normal range (father 1.8 SDS, mother 1.4 SDS) with low se-
rum IGF-I levels (118 ng/mL and 101 ng/mL in the father and mother, respectively, nor-
mal adult range 126–369 ng/mL) and normal levels of IGF-II and IGFBP-3.
25
The
present authors had the opportunity to study 24 family members of the patient
with the inactivating mutation of the IGF-I gene, and nine of them carried the mutation.
These carriers had a significantly lower birth weight than the non-carriers (3048 g vs
3358 g) and a significantly lower height (1.0 vs 0.4 SDS). Total IGF-I levels in the
carriers were higher than in the non-carriers (þ0.6 SDS vs 0.3 SDS).
27
Effect of a genetic IGF-I defect on brain development
Central nervous system development begins in the embryo with the formation and
closure of the neural tube, followed by the rapid division of pluripotential cells, which
migrate to the periphery of the neural tube and differentiate into neural or glial cells.
During embryogenesis, IGF-I mRNA expression is detectable in many brain regions.
36
From IGF-I knockout experiments, a key role of IGF-I in the complex process of cen-
tral nervous system development has become evident, including rescuing neurons
from apoptosis.
37,38
Psychomotor development is normal in patients with GH deficiency or insensitiv-
ity.
39
However, in patients with a mutation or deletion of the IGF-I gene, microcephaly
at birth and severe mental retardation is found, implicating an essential role for IGF-I in
prenatal brain development. In heterozygous carriers of the inactivating IGF-I mutation,
head circumference was significantly lower than in the non-carriers (1.0 SDS vs þ0.5
SDS)
27
, implicating an effect of IGF-I haplo-insufficiency on the central nervous system.
Effect of a genetic IGF-I defect on auditory development
The role of IGF-I in auditory function in IGF-I knockout mice has been studied re-
cently. Auditory brainstem responses in IGF-I /mice showed bilateral sensorineu-
ral hearing loss, involving all frequencies.
40
At a cellular level, a significant decrease in
number and size of auditory neurons, increased apoptosis of cochlear neurons, and
reduced volume of the cochlea was found.
41
Apparently, IGF-I is a key factor in the
development and maturation of the inner ear. This was confirmed in patients with
a mutation or deletion of the IGF-I gene. Audiograms of these patients demonstrated
severe bilateral sensorineural deafness.
25,27,29
This is re-inforced by absent brainstem-
evoked potentials in one of these patients.
27
Hearing problems have not been re-
ported in patients with GH deficiency or GH insensitivity. Audiometry was performed
in heterozygous carriers of the IGF-I mutation, revealing hearing abnormalities in
seven individuals.
27
However, no statistical significant association with carriership
could be detected.
ALS
ALS expression occurs late in fetal life
42
and appears to have no effect on fetal growth,
as demonstrated in ALS knockout mice.
43
Recently, several patients with mutations in
438 M. J. E. Walenkamp and J. M. Wit
the ALS gene have been reported. However, birth data are limited
44–46
, so the role of
ALS in fetal growth remains to be established.
IGF1R
The phenotype of the IGF1R/and the IGF1R//IGF-I/knockout mice is indistin-
guishable, including a birth weight of 45% of normal. All of them died of respiratory
failure immediately after birth.
23
This is strong evidence that the IGF-I ligand does not
utilize any receptor other than the IGF1R. Homozygous IGF1R mutations in humans
have not been described, probably because this genetic defect is not compatible with life.
Mice with a heterozygous mutation of the IGF1R are phenotypically normal, regard-
less of paternal or maternal transmission of the mutated allele.
23
Several reports in
children with heterozygous defects of the IGF1R have demonstrated a variable pheno-
type. The first patients were described by Abuzzahab et al in 2003
47
, supplemented by
a detailed description of one of the families by Raile et al.
48
To date, four other families
have been described.
49–52
The auxological measurements at birth are summarized in
Ta b l e 1 . IGF-I levels are elevated in most cases.
The compound heterozygous mutation described in the first family resulted in re-
duced binding of IGF-I to the IGF1R in fibroblasts, and binding studies indicated that
IGF-I binding affinity was one-third of that in controls. In line with this observation,
receptor phosphorylation assays showed reduced receptor signalling.
47
In the second
family, a heterozygous point mutation in exon 2 of the IGF1R gene resulted in a reduced
number of receptors in fibroblasts compared with controls.
47
In addition, receptor
autophosphorylation and phosphorylation of downstream signalling proteins was de-
creased.
48
The mother and daughter described by Kawashima et al have a heterozygous
mutation at the cleavage site of the IGF1R precursor, resulting in failure of processing
Table 1. Birth data of the five families with a heterozygous insulin-like growth factor I receptor muta-
tion. Growth data are expressed as standard deviation scores.
Subject Reference Mutation Birth weight Birth length Head circumference
Index case 47 Compound heterozygous
R108Q and K115N
3.5
Mother K115N 2
Father R108Q 2
Index case 47 R59 stop 3.5 5.8 4.6
Brother 48 R59 stop 2.7 2.1
Mother 48 R59 stop 2.4 1.6
Index case 50 R709Q 1.5 1.0
Mother R709Q 1.6
Index case 49 E1050K 3.3 4.2 5.6
Mother E1050K 2.1 0.3
Index case 52 R431L 1.8 3.2
Mother R431L 1.7
Index 51 R481Q 4.9 3.1
Aunt R481Q Final height: 5.0
Mother unknown Final height: 5.7
Single gene mutations causing SGA 439
of the IGF1R precursor protein to mature IGF1R.
50
The present authors described
a mother and daughter with a heterozygous mutation in the intracellular kinase do-
main, resulting in reduced autophosphorylation and activation of downstream signal-
ling cascades.
49
Finally, the mutation described by Inagaki et al resulted in an altered
domain within the a-subunit of the IGF1R, resulting in reduced phosphorylation and
decreased cell proliferation.
51
As demonstrated in Ta b l e 1 , the degree of IUGR varies, which can be explained by
the different nature of the various mutations, resulting in a different degree of remain-
ing signalling. Patients with an identified affected mother seem to be more severely
growth retarded at birth than patients with an unaffected mother. A possible explana-
tion is that maternal IGF-I resistance during pregnancy affects placental size and, as
a consequence, fetal growth. This is supported by a strongly positive correlation be-
tween the rate of IGF-I increase during pregnancy and placental weight
53,54
, and by
the observation that placentas from IUGR pregnancies are characterized by decreased
expression of IGF1R and signal transduction proteins.
55
IGF1R haplo-insufficiency is associated with a variable degree of IUGR (1.8 to
5.6 SDS), as is demonstrated in patients with a terminal deletion of chromosome
15q, which includes the IGF1R gene.
56–61
Reports of patients with three copies of
the IGF1R gene and macrosomia at birth support the concept that a gene dose effect
plays a role in intra-uterine growth.
62,63
IGF1R signalling
To the authors’ knowledge, no genetic defects in the IGF-I signalling pathway have been
described. However, in pregnancies complicated by IUGR, the IRS-2 and Akt pathway
are down-regulated in the placenta, implying a role for IGF-I signalling in fetal growth.
55
IGFBPs
The IGFBPs modulate the actions of the IGFs. In mice, overexpression of IGFBP-1 re-
sults in a modest and transient impairment of fetal growth, and maternal IGFBP-1 ex-
cess is associated with reduced fetal growth.
64
In humans, many reports have described
an inverse correlation between maternal and fetal IGF-I levels and fetal size.
8,65–67
It has
been suggested that in some cases of IUGR, IGFBP-1 inhibits the growth-promoting ef-
fect of IGF-I in utero by binding fetal IGF-I and inhibiting IGF activity. This is supported
by the observation that IGFBP-1 levels are markedly elevated in the umbilical cord
blood of babies with combined respiratory and metabolic acidosis as a result of pro-
found and prolonged hypoxia, which is considered to be a leading cause of IUGR.
68
The mechanism to restrict IGF-mediated growth in utero under conditions of chronic
hypoxia and limited substrate availability appears to be a conserved physiological mech-
anism. Knockdown of IGFBP-1 in zebrafish significantly alleviates hypoxia-induced
growth retardation, whereas overexpression of IGFBP-1 causes growth retardation un-
der normoxia. Furthermore, re-introduction of IGFBP-1 to the IGFBP-1 knocked-
down embryos restores the hypoxic effects on embryonic growth.
69
Insulin
Knowing the effects of IGF-I on intra-uterine growth and considering the homology
between insulin and IGF-I, the effects of a defect in insulin secretion cannot be left
440 M. J. E. Walenkamp and J. M. Wit
unmentioned. Babies with pancreas agenesis show severe IUGR.
70
Other evidence for
the role of insulin in fetal growth comes from children with mutations in the glucoki-
nase gene. Glucokinase catalyses the rate-determining step in glycolysis and plays
a role as pancreatic b-cell glucose sensor. Consequently, mutations in the gene result
in altered glucose sensing and decreased insulin secretion. Children with a mutation in
the glucokinase gene are approximately 500 g smaller than unaffected siblings, suggest-
ing that the reduced fetal insulin secretion in response to maternal blood glucose
levels impairs fetal growth.
71
Population genetics analysis showed that a common var-
iation of the glucokinase gene is associated with birth weight, also indicating the role of
insulin in fetal growth.
72
The effects of insulin are mediated by the IR. Mutations in the IR in humans cause
severe IUGR as seen in leprechaunism, or a various degree of IUGR as seen in Rabson-
Mendenhall syndrome, depending on remaining IR activity.
73,74
DIAGNOSTIC APPROACH
The number of patients with a genetic defect in the GH–IGF-I axis is small. However,
alertness of physicians evaluating children born SGA will identify more patients. Ge-
netic analysis of the GH–IGF-I axis should be considered in patients born SGA with
suggestive phenotypic features such as microcephaly, deafness or mental retardation.
For a detailed description of the diagnostic work-up of a patient with short stature and
born SGA, the reader is directed to a recent review on this topic.
75
In short, the dif-
ferential diagnosis in a patient with short stature, born SGA and with a small head cir-
cumference includes IGF-I deficiency or IGF-I insensitivity. IGF-I levels will determine
which gene should be analysed. When IGF-I levels are lower than 2 SDS, an IGF-I
deletion or an IGF-I mutation is considered and the IGF-I gene should be analysed. In
case of IGF-I levels above 0 SDS, an inactivating missense mutation of the IGF-I gene
or a heterozygous mutation of the IGF1R gene are possible defects.
Besides conventional genetic diagnostic tools such as sequence analysis and fluores-
cent in-situ hybridization, which are time-consuming techniques, a quick and inexpen-
sive technique was recently described to detect duplications and deletions in the
genome: multiplex ligation-dependent probe amplification (MLPA).
76
In short, oligonu-
cleotide probes of interesting exons of the gene are synthesized. If two adjacently-an-
nealing probes hybridize on a target sequence (DNA of the patient), ligation will occur.
The ligated products serve as a template for polymerase chain reaction amplification.
By using a fluorescently labelled primer, the products can be separated according to
size and quantified. Since the MLPA assay can be easily extended to include as many
as 40 different probe sets, this technique is ideally suited for the simultaneous evalu-
ation of copy number changes of multiple genomic regions, and is therefore expected
to contribute significantly to the elucidation of genetic causes of SGA.
SUMMARY
The GH–IGF-I axis plays a key role in growth and development. GH deficiency or GH
resistance has no influence on intra-uterine growth and development. In contrast, IGF-I
deficiency due to a deletion or mutation of the IGF-I gene has a major impact on intra-
uterine growth and development, resulting in severe IUGR, microcephaly, psychomo-
tor retardation and deafness. Important growth-promoting actions of IGF-I include
proliferation and differentiation, as well as prevention of apoptosis. IGF-I resistance
Single gene mutations causing SGA 441
due to a genetic defect of the IGF1R gene has a more variable impact on intra-uterine
growth and development, probably dependent on remaining signalling activity. Maternal
IGF1 resistance may also play a role. Genetic analysis of IGF-I or IGF1R is advised in
patients with SGA and suggestive symptoms such as microcephaly, retardation or deaf-
ness, in combination with extremely low or high IGF-I levels.
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Practice points
IGF-I is essential for normal intra-uterine growth and development
in children born SGA presenting with microcephaly and mental retardation,
a genetic defect of IGF-I or the IGF1R should be considered
a detailed description of phenotypic features in patients born SGA is needed to
perform a well-targeted genetic analysis
MLPA is a quick and inexpensive technique to detect deletions and duplications
in the genome
Research agenda
genome-wide genetic analysis in children born SGA is needed to find novel
genes and pathways involved in intra-uterine growth and development
more patients with genetic defects in the GH–IGF-I axis need to be identified
in order to reveal the exact mechanisms underlying the role of IGF-I in prenatal
life
screening of short and tall patients with new genetic techniques, such as oligo-
nucleotide micro-array hybridization, may be helpful to detect new defects in
the GH–IGF-I axis, for example in the IGF-I signalling pathway
442 M. J. E. Walenkamp and J. M. Wit
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