Evidence against GRB10 as the gene responsible for Silver-Russell syndrome

Imprinted Long Non-Coding RNAs in Mammalian Development and Disease

Imprinted genes play diverse roles in mammalian development, homeostasis, and disease. Most imprinted chromosomal domains express one or more long non-coding RNAs (lncRNAs). Several of these lncRNAs are strictly nuclear and their mono-allelic expression controls in cis the expression of protein-coding genes, often developmentally regulated. Some imprinted lncRNAs act in trans as well, controlling target gene expression elsewhere in the genome. The regulation of imprinted gene expression-including that of imprinted lncRNAs-is susceptible to stochastic and environmentally triggered epigenetic changes in the early embryo. These aberrant changes persist during subsequent development and have long-term phenotypic consequences. This review focuses on the expression and the cis- and trans-regulatory roles of imprinted lncRNAs and describes human disease syndromes associated with their perturbed expression.

Parent of origin gene expression in the bumblebee, Bombus terrestris, supports Haig's kinship theory for the evolution of genomic imprinting

Genomic imprinting is the differential expression alleles in diploid individuals, with the expression being dependent on the sex of the parent from which it was inherited. Haig's kinship theory hypothesizes that genomic imprinting is due to an evolutionary conflict of interest between alleles from the mother and father. In social insects, it has been suggested that genomic imprinting should be widespread. One recent study identified parent-of-origin expression in honey bees and found evidence supporting the kinship theory. However, little is known about genomic imprinting in insects and multiple theoretical predictions must be tested to avoid single-study confirmation bias. We, therefore, tested for parent-of-origin expression in a primitively eusocial bee. We found equal numbers of maternally and paternally biased expressed genes. The most highly biased genes were maternally expressed, offering support for the kinship theory. We also found low conservation of potentially imprinted genes with the honey bee, suggesting rapid evolution of genomic imprinting in Hymenoptera.

Imprinted genes in clinical exome sequencing: Review of 538 cases and exploration of mouse-human conservation in the identification of novel human disease loci

Human imprinting disorders cause a range of dysmorphic and neurocognitive phenotypes, and they may elude traditional molecular diagnosis such exome sequencing. The discovery of novel disorders related to imprinted genes has lagged behind traditional Mendelian disorders because current diagnostic technology, especially unbiased testing, has limited utility in their discovery. To identify novel imprinting disorders, we reviewed data for every human gene hypothesized to be imprinted, identified each mouse ortholog, determined its imprinting status in the mouse, and analyzed its function in humans and mice. We identified 17 human genes that are imprinted in both humans and mice, and have functional data in mice or humans to suggest that dysregulated expression would lead to an abnormal phenotype in humans. These 17 genes, along with known imprinted genes, were preferentially flagged 538 clinical exome sequencing tests. The identified genes were: DIRAS3 [1p31.3], TP73 [1p36.32], SLC22A3 [6q25.3], GRB10 [7p12.1], DDC [7p12.2], MAGI2 [7q21.11], PEG10 [7q21.3], PPP1R9A [7q21.3], CALCR [7q21.3], DLGAP2 [8p23.3], GLIS3 [9p24.2], INPP5F [10q26.11], ANO1 [11q13.3], SLC38A4 [12q13.11], GATM [15q21.1], PEG3 [19q13.43], and NLRP2 [19q13.42]. In the 538 clinical cases, eight cases (1.7%) reported variants in a causative known imprinted gene. There were 367/758 variants (48.4%) in imprinted genes that were not known to cause disease, but none of those variants met the criteria for clinical reporting. Imprinted disorders play a significant role in human disease, and additional human imprinted disorders remain to be discovered. Therefore, evolutionary conservation is a potential tool to identify novel genes involved in human imprinting disorders and to identify them in clinical testing.

[Epigenomic variations manifesting as a loss of heterozygosity affecting imprinted genes represent a molecular mechanism of autism spectrum disorders and intellectual disability in children]

AIM

Long continuous stretches of homozygosity (LCSH) are regularly detected in studies using molecular karyotyping (SNP array). Despite this type of variation being able to provide meaningful data on the parents' kinship, uniparental disomy and chromosome rearrangements, LCSH are rarely considered as a possible epigenetic cause of neurodevelopmental disorders. Despite their direct relationship to imprinting, LCSH in imprinted loci have not been considered in terms of pathogenicity. The present work is aimed at studying LCSH in chromosomal regions containing imprinted genes previously associated with disease in children with idiopathic intellectual disability, autism, congenital malformations and/or epilepsy.

MATERIAL AND METHODS

Five hundred and four patients with autism spectrum disorders and intellectual disability were examined.

RESULTS

LCSH affecting imprinted loci associated with various diseases were identified in 40 (7.9%) individuals. Chromosomal region 7q21.3 was affected in twenty three cases, 15q11.2 in twelve, 11p15.5 in five, 7q32.2 in four. Four patients had 2 LCSH affecting imprinted loci. Besides one LCSH in 7q31.33q32.3 (~4 Mbp) region, all LCSH were 1-1.6 Mbp. Clinically, these cases resembled the corresponding imprinting diseases (e.g. Silver-Russell, Beckwith-Wiedemann, Prader-Willi, Angelman syndromes). Parental kinship was identified in 8 cases (1.59%), which were not affected by LCSH at imprinted loci.

CONCLUSION

The present study shows that LCSH affecting chromosomal regions 7q21.3, 7q32.2, 11p15.5 and 15p11.2 occur in about 7.9% of children with intellectual disability, autism, congenital malformations and/or epilepsy. Consequently, this type of epigenetic mutations is obviously common in a group of children with neurodevelopmental disorders. LCSH less than 2.5-10 Mbp are usually ignored in molecular karyotyping (SNP array) studies and, therefore, an important epigenetic cause of intellectual disability, autism or epilepsy with high probability remains without attention.

Maternal 5mCpG Imprints at the PARD6G-AS1 and GCSAML Differentially Methylated Regions Are Decoupled From Parent-of-Origin Expression Effects in Multiple Human Tissues

A hallmark of imprinted genes in mammals is the occurrence of parent-of-origin-dependent asymmetry of DNA cytosine methylation (5^mC) of alleles at CpG islands (CGIs) in their promoter regions. This 5^mCpG asymmetry between the parental alleles creates allele-specific imprinted differentially methylated regions (iDMRs). iDMRs are often coupled to the transcriptional repression of the methylated allele and the activation of the unmethylated allele in a tissue-specific, developmental-stage-specific and/or isoform-specific fashion. iDMRs function as regulatory platforms, built through the recruitment of chemical modifications to histones to achieve differential, parent-of-origin-dependent chromatin segmentation states. Here, we used a comparative computational data mining approach to identify 125 novel constitutive candidate iDMRs that integrate the maximal number of allele-specific methylation region records overlapping CGIs in human methylomes. Twenty-nine candidate iDMRs display gametic 5^mCpG asymmetry, and another 96 are candidate secondary iDMRs. We established the maternal origin of the 5^mCpG imprints of one gametic (PARD6G-AS1) and one secondary (GCSAML) iDMRs. We also found a constitutively hemimethylated, nonimprinted domain at the PWWP2AP1 promoter CGI with oocyte-derived methylation asymmetry. Given that the 5^mCpG level at the iDMRs is not a sufficient criterion to predict active or silent locus states and that iDMRs can regulate genes from a distance of more than 1 Mb, we used RNA-Seq experiments from the Genotype-Tissue Expression project and public archives to assess the transcriptional expression profiles of SNPs across 4.6 Mb spans around the novel maternal iDMRs. We showed that PARD6G-AS1 and GCSAML are expressed biallelically in multiple tissues. We found evidence of tissue-specific monoallelic expression of ZNF124 and OR2L13, located 363 kb upstream and 419 kb downstream, respectively, of the GCSAML iDMR. We hypothesize that the GCSAML iDMR regulates the tissue-specific, monoallelic expression of ZNF124 but not of OR2L13. We annotated the non-coding epigenomic marks in the two maternal iDMRs using data from the Roadmap Epigenomics project and showed that the PARD6G-AS1 and GCSAML iDMRs achieve contrasting activation and repression chromatin segmentations. Lastly, we found that the maternal 5^mCpG imprints are perturbed in several hematopoietic cancers. We conclude that the maternal 5^mCpG imprints at PARD6G-AS1 and GCSAML iDMRs are decoupled from parent-of-origin transcriptional expression effects in multiple tissues.

Long contiguous stretches of homozygosity spanning shortly the imprinted loci are associated with intellectual disability, autism and/or epilepsy

BACKGROUND

Long contiguous stretches of homozygosity (LCSH) (regions/runs of homozygosity) are repeatedly detected by single-nucleotide polymorphism (SNP) chromosomal microarrays. Providing important clues regarding parental relatedness (consanguinity), uniparental disomy, chromosomal recombination or rearrangements, LCSH are rarely considered as a possible epigenetic cause of neurodevelopmental disorders. Additionally, despite being relevant to imprinting, LCSH at imprinted loci have not been truly addressed in terms of pathogenicity. In this study, we examined LCSH in children with unexplained intellectual disability, autism, congenital malformations and/or epilepsy focusing on chromosomal regions which harbor imprinted disease genes.

RESULTS

Out of 267 cases, 14 (5.2 %) were found to have LCSH at imprinted loci associated with a clinical outcome. There were 5 cases of LCSH at 15p11.2, 4 cases of LCSH at 7q31.2, 3 cases of LCSH at 11p15.5, and 2 cases of LCSH at 7q21.3. Apart from a case of LCSH at 7q31.33q32.3 (~4 Mb in size), all causative LCSH were 1-1.5 Mb in size. Clinically, these cases were characterized by a weak resemblance to corresponding imprinting diseases (i.e., Silver-Russell, Beckwith-Wiedemann, and Prader-Willi/Angelman syndromes), exhibiting distinctive intellectual disability, autistic behavior, developmental delay, seizures and/or facial dysmorphisms. Parental consanguinity was detected in 8 cases (3 %), and these cases did not exhibit LCSH at imprinted loci.

CONCLUSIONS

This study demonstrates that shorter LCSH at chromosomes 7q21.3, 7q31.2, 11p15.5, and 15p11.2 occur with a frequency of about 5 % in the children with intellectual disability, autism, congenital malformations and/or epilepsy. Consequently, this type of epigenetic mutations appears to be the most common one among children with neurodevelopmental diseases. Finally, since LCSH less than 2.5-10 Mb in size are generally ignored in diagnostic SNP microarray studies, one can conclude that an important epigenetic cause of intellectual disability, autism or epilepsy is actually overlooked.

Growth receptor binding protein 10 inhibits glucose-stimulated insulin release from pancreatic β-cells associated with suppression of the insulin/insulin-like growth factor-1 signalling pathway

Growth receptor binding protein 10 (Grb10) is an adaptor protein that interacts with the insulin receptor and insulin-like growth factor (IGF)-1 receptor. Overexpression of Grb10 in muscle cells and adipocytes inhibits insulin signalling, and transgenic mice overexpressing Grb10 exhibit impaired glucose tolerance. However, the roles of Grb10 in β-cells remain unknown. The aim of the present study was to explore the effect of Grb10 on β-cell function. The effects of Grb10 on glucose-stimulated insulin secretion (GSIS) and the insulin/IGF-1 signalling pathway were investigated in rat islets and/or dispersed islet cells with Grb10 overexpresion by adenovirus transfection. Protein expression was detected by western blot analysis. We found that Grb10 was expressed in both human and rat pancreas. Expression of Grb10 was increased in islets isolated from rats fed a high-fat plus high-sugar diet compared with islets isolated from rats fed normal chow diet, as well as in INS 832/13 cells exposed to high levels of glucose (20 mmol/L), palmitate (1 mmol/L) and interleukin-1β (50 U/mL). Overexpression of Grb10 in INS 832/13 cells or rat islets impaired GSIS compared with the respective control (all P < 0.05). Moreover, inhibition of GSIS by Grb10 overexpression was associated with a decrease in insulin- and IGF-1-induced Akt and extracellular signal-regulated kinase 1/2 phosphorylation. The results of the present study demonstrate that Grb10 is an important negative regulator of insulin/IGF-1 signalling in pancreatic β-cells and a potential target to improve β-cell function.

Growth factor receptor-bound protein 14: a potential new gene associated with oocyte competence

The Grb14 protein is a member of the Grb7 protein family. This protein family acts by binding to tyrosine kinase receptors, promoting cell proliferation and differentiation. There is evidence of the involvement of tyrosine kinase factors in the bovine oocyte maturation process. However, Grb14 has not been studied for bovine cumulus–oocyte complexes (COCs). The aim of the present study was to characterize Grb14 mRNA expression in bovine COCs during follicular development. Furthermore, we demonstrated that the expression of Grb14 mRNA is not regulated by estradiol. mRNA expression of Grb14 was assessed in 480 COCs from follicles of different sizes (1–3, 4–6, 6–8 or >8 mm) by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Grb14 mRNA expression decreased in COCs throughout follicular growth (P < 0.05). The role of estradiol in the expression of Grb14 mRNA in COCs was studied. Grb14 mRNA abundance did not differ in COCs cultured in the presence or absence of 17β-estradiol or fulvestrant. In conclusion, we showed that Grb14 mRNA is downregulated in COCs during antral follicle development, a finding that suggests a role for Grb14 in oocyte competence.

Lentivirus shRNA Grb10 targeting the pancreas induces apoptosis and improved glucose tolerance due to decreased plasma glucagon levels

AIMS/HYPOTHESIS

The physiological significance of growth factor receptor-bound protein-10 (GRB10) in the pancreas is unclear. We hypothesised that GRB10 is involved in pancreatic apoptosis, as GRB10 binds with a family of cell-survival-related proteins implicated in apoptosis.

METHODS

Lentiviral vector small hairpin RNA (shRNA) targeting Grb10 was injected in vivo via an intraductal pancreatic route to target pancreatic tissues in adult mice, which were studied 2 weeks post-injection.

RESULTS

Using the TUNEL assay, we demonstrated for the first time that in vivo injection of lentivirus shRNA Grb10 directly into the adult mouse pancreas induced apoptosis in both exocrine and endocrine (alpha and beta) cells. This effect was more pronounced in alpha cells. Levels of the pro-apoptotic protein BCL2-interacting mediator of cell death (BIM) in islets was higher in lentivirus shRNA Grb10 than in lentivirus shRNA scramble mice. In the apoptotic pathway, BIM initiates apoptosis signalling, leading to activation of the caspase cascade. We propose that, when complexed with GRB10, BIM is inactive. On activation by stress signalling or, in the present study, following injection of lentivirus shRNA Grb10 into pancreas, BIM becomes unbound from GRB10 and activates the caspase cascade. Indeed, caspase-3 activity in islets was higher in the experimental than in the control group. Apoptosis induced by shRNA Grb10 resulted in a 34% decrease in fasting plasma glucagon. Mice injected with shRNA Grb10 had improved glucose tolerance despite reduced insulin secretion compared with shRNA scramble control mice.

CONCLUSIONS/INTERPRETATION

GRB10 is critically involved in alpha cell survival and, as a result, plays an important role in regulating basal glucagon secretion and glucose tolerance in adult mice.

Genomic organization and control of the grb7 gene family

Grb7 and their related family members Grb10 and Grb14 are adaptor proteins, which participate in the functionality of multiple signal transduction pathways under the control of a variety of activated tyrosine kinase receptors and other tyrosine-phosphorylated proteins. They are involved in the modulation of important cellular and organismal functions such as cell migration, cell proliferation, apoptosis, gene expression, protein degradation, protein phosphorylation, angiogenesis, embryonic development and metabolic control. In this short review we shall describe the organization of the genes encoding the Grb7 protein family, their transcriptional products and the regulatory mechanisms implicated in the control of their expression. Finally, the alterations found in these genes and the mechanisms affecting their expression under pathological conditions such as cancer, diabetes and some congenital disorders will be highlighted.

Genome-wide assessment of imprinted expression in human cells

Background

Parent-of-origin-dependent expression of alleles, imprinting, has been suggested to impact a substantial proportion of mammalian genes. Its discovery requires allele-specific detection of expressed transcripts, but in some cases detected allelic expression bias has been interpreted as imprinting without demonstrating compatible transmission patterns and excluding heritable variation. Therefore, we utilized a genome-wide tool exploiting high density genotyping arrays in parallel measurements of genotypes in RNA and DNA to determine allelic expression across the transcriptome in lymphoblastoid cell lines (LCLs) and skin fibroblasts derived from families.

Results

We were able to validate 43% of imprinted genes with previous demonstration of compatible transmission patterns in LCLs and fibroblasts. In contrast, we only validated 8% of genes suggested to be imprinted in the literature, but without clear evidence of parent-of-origin-determined expression. We also detected five novel imprinted genes and delineated regions of imprinted expression surrounding annotated imprinted genes. More subtle parent-of-origin-dependent expression, or partial imprinting, could be verified in four genes. Despite higher prevalence of monoallelic expression, immortalized LCLs showed consistent imprinting in fewer loci than primary cells. Random monoallelic expression has previously been observed in LCLs and we show that random monoallelic expression in LCLs can be partly explained by aberrant methylation in the genome.

Conclusions

Our results indicate that widespread parent-of-origin-dependent expression observed recently in rodents is unlikely to be captured by assessment of human cells derived from adult tissues where genome-wide assessment of both primary and immortalized cells yields few new imprinted loci.

Large de novo deletion of 7p15.1 to 7p12.1 involving the imprinted gene GRB10 associated with a complex phenotype including features of Beckwith Wiedemann syndrome

We present an infant with a de novo cytogenetically visible interstitial deletion of approximately 21.9Mb involving chromosome bands 7p15.1-7p12.1, with the loss of 119 genes confirmed by array CGH. The infant had a ventricular septal defect, hand and skull anomalies, and hyperglycaemia compatible with haploinsufficiency of TBX20, GLI3, and GCK genes, respectively. In addition, the infant had some features reminiscent of Beckwith Wiedemann syndrome including macroglossia, umbilical hernia, and a relatively large birth weight and we speculate that this is due to the deletion of GRB10, an imprinted gene on chromosome 7. This report illustrates how knowledge of genes within a deleted interval facilitates optimal medical management, can explain observed phenotypes, and stimulates research questions.

Study of transcriptional effects in Cis at the IFIH1 locus

BACKGROUND

The Thr allele at the non-synonymous single-nucleotide polymorphism (nsSNP) Thr946Ala in the IFIH1 gene confers risk for Type 1 diabetes (T1D). The SNP is embedded in a 236 kb linkage disequilibrium (LD) block that includes four genes: IFIH1, GCA, FAP and KCNH7. The absence of common nsSNPs in the other genes makes the IFIH1 SNP the strongest functional candidate, but it could be merely a marker of association, due to LD with a variant regulating expression levels of IFIH1 or neighboring genes.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the effect of the T1D-associated variation on mRNA transcript expression of these genes. Heterozygous mRNA from lymphoblastoid cell lines (LCLs), pancreas and thymus was examined by allelic expression imbalance, to detect effects in cis on mRNA expression. Using single-nucleotide primer extension, we found no difference between mRNA transcripts in 9 LCLs, 6 pancreas and 13 thymus samples, suggesting that GCA and FAP are not involved. On the other hand, KCNH7 was not expressed at a detectable level in all tissues examined. Moreover, the association of the Thr946Ala SNP with T1D is not due to modulation of IFIH1 expression in organs involved in the disease, pointing to the IFIH1 nsSNP as the causal variant.

CONCLUSIONS/SIGNIFICANCE

The mechanism of the association of the nsSNP with T1D remains to be determined, but does not involve mRNA modulation. It becomes necessary to study differential function of the IFIH1 protein alleles at Thr946Ala to confirm that it is responsible for the disease association.

Review and hypothesis: syndromes with severe intrauterine growth restriction and very short stature--are they related to the epigenetic mechanism(s) of fetal survival involved in the developmental origins of adult health and disease?

Diagnosing the specific type of severe intrauterine growth restriction (IUGR) that also has post-birth growth restriction is often difficult. Eight relatively common syndromes are discussed identifying their unique distinguishing features, overlapping features, and those features common to all eight syndromes. Many of these signs take a few years to develop and the lifetime natural history of the disorders has not yet been completely clarified. The theory behind developmental origins of adult health and disease suggests that there are mammalian epigenetic fetal survival mechanisms that downregulate fetal growth, both in order for the fetus to survive until birth and to prepare it for a restricted extra-uterine environment, and that these mechanisms have long lasting effects on the adult health of the individual. Silver-Russell syndrome phenotype has recently been recognized to be related to imprinting/methylation defects. Perhaps all eight syndromes, including those with single gene mutation origin, involve the mammalian mechanism(s) of fetal survival downsizing. Insights into those mechanisms should provide avenues to understanding the natural history, the heterogeneity and possible therapy not only for these eight syndromes, but for the common adult diseases with which IUGR is associated.

Reciprocal imprinting of human GRB10 in placental trophoblast and brain: evolutionary conservation of reversed allelic expression

Genomic imprinting may have evolved not only to regulate fetal growth and development, but also behaviour. The mouse Grb10 gene provides a remarkable model to explore this idea because it shows paternal expression in brain, whereas in the placenta and most other embryonic tissues, expression is from the maternal allele. To assess the biological relevance of this reciprocal pattern of imprinting, we explored its conservation in humans. As in mice, we find the human GRB10 gene to be paternally expressed in brain. Maternal allele-specific expression is conserved only in the placental villous trophoblasts, an essential part of the placenta involved in nutrient transfer. All other fetal tissues tested showed equal expression from both alleles. These data suggest that the maternal GRB10 expression in placenta is evolutionarily important, presumably in the control of fetal growth. As in the mouse, the maternal transcripts originate from several kilobases upstream of the imprinting control region (ICR) of the domain, from a promoter region at which we find no allelic chromatin differences. The brain-specific paternal expression from the ICR shows mechanistic similarities with the mouse as well. This conserved CpG island is DNA-methylated on the maternal allele and is marked on the paternal allele by developmentally regulated bivalent chromatin, with the presence of both H3 lysine-4 and H3 lysine-27 methylation. The strong conservation of the opposite allelic expression in placenta versus brain supports the hypothesis that GRB10 imprinting evolved to mediate diverse roles in mammalian growth and behaviour.

Paternal deletion of Meg1/Grb10 DMR causes maternalization of the Meg1/Grb10 cluster in mouse proximal Chromosome 11 leading to severe pre- and postnatal growth retardation

Mice with maternal duplication of proximal Chromosome 11 (MatDp(prox11)), where Meg1/Grb10 is located, exhibit pre- and postnatal growth retardation. To elucidate the responsible imprinted gene for the growth abnormality, we examined the precise structure and regulatory mechanism of this imprinted region and generated novel model mice mimicking the pattern of imprinted gene expression observed in the MatDp(prox11) by deleting differentially methylated region of Meg1/Grb10 (Meg1-DMR). It was found that Cobl and Ddc, the neighboring genes of Meg1/Grb10, also comprise the imprinted region. We also found that the mouse-specific repeat sequence consisting of several CTCF-binding motifs in the Meg1-DMR functions as a silencer, suggesting that the Meg1/Grb10 imprinted region adopted a different regulatory mechanism from the H19/Igf2 region. Paternal deletion of the Meg1-DMR (+/DeltaDMR) caused both upregulation of the maternally expressed Meg1/Grb10 Type I in the whole body and Cobl in the yolk sac and loss of paternally expressed Meg1/Grb10 Type II and Ddc in the neonatal brain and heart, respectively, demonstrating maternalization of the entire Meg1/Grb10 imprinted region. We confirmed that the +/DeltaDMR mice exhibited the same growth abnormalities as the MatDp(prox11) mice. Fetal and neonatal growth was very sensitive to the expression level of Meg1/Grb10 Type I, indicating that the 2-fold increment of the Meg1/Grb10 Type I is one of the major causes of the growth retardation observed in the MatDp(prox11) and +/DeltaDMR mice. This suggests that the corresponding human GRB10 Type I plays an important role in the etiology of Silver-Russell syndrome caused by partial trisomy of 7p11-p13.

Imprinted genes and neuroendocrine function

Imprinted genes are monoallelically expressed in a parent-of-origin dependent manner. Whilst the full functional repertoire of these genes remains obscure, they are generally highly expressed in the brain and are often involved in fundamental neural processes. Besides influencing brain neurochemistry, imprinted genes are important in the development and function of the hypothalamus and pituitary gland, key sites of neuroendocrine regulation. Moreover, imprinted genes may directly modulate hormone-dependent signalling cascades, both in the brain and elsewhere. Much of our knowledge about imprinted gene function has come from studying knockout mice and human disorders of imprinting. One such disorder is Prader-Willi syndrome, a neuroendocrine disorder characterised by hypothalamic abnormalities and aberrant feeding behaviour. Through examining the role of imprinted genes in neuroendocrine function, it may be possible to shed light on the neurobiological basis of feeding and aspects of social behaviour and underlying cognition, and to provide insights into disorders where these functions go awry.

3-M syndrome: a report of three Egyptian cases with review of the literature

The 3-M syndrome is a rare autosomal recessive disorder. It is characterized by prenatal and postnatal growth retardation associated with characteristic features. In this study, we report on three patients from two unrelated families, including two male sibs, with the characteristic features and radiological findings of the 3-M syndrome. The main features in our cases were low birth weight, short stature, malar hypoplasia, anteverted nostrils with a fleshy nasal tip, long philtrum, pointed full chin, short broad neck, broad chest with transverse grooves of anterior thorax and hyperlordosis. An orodental examination revealed characteristic findings, some of which were not reported before. Prominent premaxilla, hypoplastic maxilla, thick patulous lips, high-arched palate, median fissured tongue, delayed eruption of teeth with enamel hypocalcification and malocclusion were present in our three studied cases. Radiographic studies showed slender long bones and ribs, a narrow pelvis and foreshortened vertebral bodies. Our reported cases are the offspring of healthy consanguineous parents, confirming the autosomal recessive pattern of inheritance in the syndrome. Cases were reported from different countries all over the world. To our knowledge, these are the first reported Egyptian patients with this rare disorder. This syndrome may be underreported because of the phenotypic overlap with other low birth dwarfism syndromes. Recent identification of a gene mutated in some cases of 3-M syndrome will aid diagnosis.

Regulation of growth and metabolism by imprinted genes

A small sub-set of mammalian genes are subject to regulation by genomic imprinting such that only one parental allele is active in at least some sites of expression. Imprinted genes have diverse functions, notably including the regulation of growth. Much attention has been devoted to the insulin-like growth factor signalling pathway that has a major influence on fetal size and contains two components encoded by the oppositely imprinted genes, Igf2 (a growth promoting factor expressed from the paternal allele) and Igf2r (a growth inhibitory factor expressed from the maternal allele). These genes fit the parent-offspring conflict hypothesis for the evolution of genomic imprinting. Accumulated evidence indicates that at least one other fetal growth pathway exists that has also fallen under the influence of imprinting. It is clear that not all components of growth regulatory pathways are encoded by imprinted genes and instead it may be that within a pathway the influence of a single gene by each of the parental genomes may be sufficient for parent-offspring conflict to be enacted. A number of imprinted genes have been found to influence energy homeostasis and some, including Igf2 and Grb10, may coordinate growth with glucose-regulated metabolism. Since perturbation of fetal growth can be correlated with metabolic disorders in adulthood these imprinted genes are considered as candidates for involvement in this phenomenon of fetal programming.

Epigenetic deregulation of imprinting in congenital diseases of aberrant growth

Human chromosome 11p15 comprises two imprinted domains important in the control of fetal and postnatal growth. Novel studies establish that imprinting at one of these, the IGF2-H19 domain, is epigenetically deregulated (with loss of DNA methylation) in Silver-Russell Syndrome (SRS), a congenital disease of growth retardation and asymmetry. Previously, the exact opposite epigenetic alteration (gain of DNA methylation) had been detected at the domain's 'imprinting control region' (ICR) in patients with Beckwith-Wiedemann Syndrome (BWS), a complex disorder of fetal overgrowth. However, more frequently, BWS is caused by loss of DNA methylation at the ICR that regulates the second imprinted domain at 11p15. Interestingly, a similar epigenetic alteration (with loss of methylation) at a putative ICR on human chromosome 6q24, is involved in transient neonatal diabetes mellitus (TNDM), a congenital disease with intrauterine growth retardation and a transient lack of insulin. Thus, fetal and postnatal growth is epigenetically controlled by different ICRs, at 11p15 and other chromosomal regions.

Grb10 and Grb14: enigmatic regulators of insulin action--and more?

The Grb proteins (growth factor receptor-bound proteins) Grb7, Grb10 and Grb14 constitute a family of structurally related multidomain adapters with diverse cellular functions. Grb10 and Grb14, in particular, have been implicated in the regulation of insulin receptor signalling, whereas Grb7 appears predominantly to be involved in focal adhesion kinase-mediated cell migration. However, at least in vitro, these adapters can bind to a variety of growth factor receptors. The highest identity within the Grb7/10/14 family occurs in the C-terminal SH2 (Src homology 2) domain, which mediates binding to activated receptors. A second well-conserved binding domain, BPS [between the PH (pleckstrin homology) and SH2 domains], can act to enhance binding to the IR (insulin receptor). Consistent with a putative adapter function, some non-receptor-binding partners, including protein kinases, have also been identified. Grb10 and Grb14 are widely, but not uniformly, expressed in mammalian tissues, and there are various isoforms of Grb10. Binding of Grb10 or Grb14 to autophosphorylated IR in vitro inhibits tyrosine kinase activity towards other substrates, but studies on cultured cell lines have been conflicting as to whether Grb10 plays a positive or negative role in insulin signalling. Recent gene knockouts in mice have established that Grb10 and Grb14 act as inhibitors of intracellular signalling pathways regulating growth and metabolism, although the phenotypes of the two knockouts are distinct. Ablation of Grb14 enhances insulin action in liver and skeletal muscle and improves whole-body tolerance, with little effect on embryonic growth. Ablation of Grb10 results in disproportionate overgrowth of the embryo and placenta involving unidentified pathways, and also impacts on hepatic glycogen synthesis, and probably on glucose homoeostasis. This review discusses the extent to which previous studies in vitro can account for the observed phenotype of knockout animals, and considers evidence that aberrant function of Grb10 or Grb14 may contribute to disorders of growth and metabolism in humans.

Allelic effects on gene regulation at the autoimmunity-predisposing CTLA4 locus: a re-evaluation of the 3' +6230G>A polymorphism

Genetic variation at a linkage disequilibrium block encompassing the cytotoxic T-lymphocyte antigen-4 (CTLA4) gene influences susceptibility to autoimmunity, but identifying the polymorphism(s) responsible for this effect has been challenging. Recently, a single-nucleotide polymorphism (SNP) located 3' to the known polyadenylation site of CTLA4 (+6230G>A) and strongly associated with autoimmune disease was reported to regulate levels of soluble CTLA4 isoform (sCTLA4) but not the full-length isoform. The purpose of the present study is to define the mechanistic effect of the 3'SNP on the isoforms of CTLA4 (alternative splicing vs polyadenylation vs effects on RNA stability). However, using allele-specific single-nucleotide primer extension, we found no difference between mRNA transcripts derived from either +6230G>A allele in 11 heterozygous individuals, in either of the two known CTLA4 isoforms. We also found no effect of this polymorphism on ICOS (inducible costimulator), a putative downstream target. In addition, repeated attempts at 3' RACE (3'rapid amplification of cDNA ends) were unsuccessful in amplifying any contiguous sequence past the known CTLA4 polyadenylation site and no such sequence was found in the EST databases. We conclude that the mechanism of the observed association of the +6230 SNP with autoimmune disease remains to be determined, but does not involve modulation of steady-state mRNA of any known CTLA4 isoform.

Trisomy 7 mosaicism, maternal uniparental heterodisomy 7 and Hirschsprung's disease in a child with Silver–Russell syndrome

Prenatal trisomy 7 is usually a cell culture artifact in amniocytes with normal diploid karyotype at birth and normal fetal outcome. In the same way, true prenatal trisomy 7 mosaicism usually results in a normal child except when trisomic cells persist after birth or when trisomy rescue leads to maternal uniparental disomy, which is responsible for 5.5-7% of patients with Silver-Russell syndrome (SRS). We report here on the unusual association of SRS and Hirschsprung's disease (HSCR) in a patient with maternal uniparental heterodisomy 7 and trisomy 7 mosaicism in intestine and skin fibroblasts. HSCR may be fortuitous given its frequency, multifactorial inheritance and genetic heterogeneity. However, the presence of the trisomy 7 mosaicism in intestine as well as in skin fibroblasts suggests that SRS and HSCR might possibly be related. Such an association might result from either an increased dosage of a nonimprinted gene due to trisomy 7 mosaicism in skin fibroblasts (leading to SRS) and in intestine (leading to HSCR), or from an overexpression, through genomic imprinting, of maternally expressed imprinted allele(s) in skin fibroblasts and intestine or from a combination of trisomy 7 mosaicism and genomic imprinting. This report suggests that the SRS phenotype observed in maternal uniparental disomy 7 (mUPD(7)) patients might also result from an undetected low level of trisomy 7 mosaicism. In order to validate this hypothesis, we propose to perform a conventional and molecular cytogenetic analysis in different tissues every time mUPD7 is displayed.

An interstitial deletion of chromosome 7 at band q21: A case report and review

We report on a girl with moderate developmental delay and mild dysmorphic features. Cytogenetic investigations revealed a de novo interstitial deletion at the proximal dark band on the long arm of chromosome 7 (7q21.1-q21.3) in all analyzed G-banded metaphases of lymphocytes and fibroblasts. Fluorescence in situ hybridization (FISH) and molecular studies defined the breakpoints at 7q21.11 and 7q21.3 on the paternal chromosome 7, with the proximal deletion breakpoint between the elastin gene (localized at 7q11.23) and D7S2517, and the distal breakpoint between D7S652 and the COL1A2 gene (localized at 7q21.3-q22.1). Deletions of interstitial segments at the proximal long arm of chromosome 7 at q21 are relatively rare. The karyotype-phenotype correlation of these patients is reviewed and discussed. The clinical findings of patients with a deletion at 7q21 significantly overlap with those of patients with maternal uniparental disomy of chromosome 7 (matUPD(7)) and Silver-Russell syndrome (SRS, OMIM 180860). Therefore, 7q21 might be considered a candidate chromosomal region for matUPD(7) and SRS.

Lack of association of birth size with polymorphisms of two imprinted genes, IGF2R and GRB10

Little is known about the determinants of birth size variability among individuals. Maternal and nutritional factors have been studied, but familial clustering suggests genetic factors as well. As a first step in testing this hypothesis, we examined common sequence variants in IGF2R and GRB10, two genes involved in the regulation of growth and subject to parental imprinting. The IGF2R gene was scanned with five polymorphisms spanning the coding and 3'-UTR for possible association with birth size in a set of 97 normal newborns in Greece. In addition, a silent SNP in GRB10 exon 2 was similarly tested as an exploratory first step. Birth weight and length were compared between groups of newborns divided according to which allele they had received from heterozygous parents. No significant differences were found between alleles in either gene, examined either by parental origin or in aggregate. Thus, we found no evidence that IGF2R variants modulate intrauterine growth within the normal range. If such variants exist in GRB10, they are not in linkage disequilibrium with the marker studied.

Imprinting analysis of 10 genes and/or transcripts in a 1.5-Mb MEST-flanking region at human chromosome 7q32

MEST is one of the imprinted genes in human. With the assistance of our integration map and the complete sequence in the registry, we mapped a total of 16 genes/transcripts at the 1.5-Mb MEST-flanking region at 7q32. This region has been suggested to form an imprinted gene cluster, because MEST and its three flanking genes/transcripts (MESTIT1, CPA4, and COPG2IT1) were reported to be imprinted, although two (TSGA14 and COPG2) were shown to escape imprinting. In this study, 10 other genes/transcripts were examined for their imprinting status in human fetal tissues. The results indicated that 8 genes/transcripts (NRF1, UBE2H, HSPC216, KIAA0265, FLJ14803, CPA2, CPA1, and DKFZp667F0312) were expressed biallelically. The imprinting status of two (TSGA13 and CPA5) was not conclusive, because of their weak and/or tissue-specific expression and inconstant results. These findings provided evidence that only 4 of the 16 genes/transcripts located to the region show monoallelic expression, while others are not involved in imprinting. Therefore, it is less likely that the MEST-flanking 7q32 region forms a large imprinted domain.

Growth factor receptor-binding protein 10 (Grb10) as a partner of phosphatidylinositol 3-kinase in metabolic insulin action

The regulation of the metabolic insulin response by mouse growth factor receptor-binding protein 10 (Grb10) has been addressed in this report. We find mouse Grb10 to be a critical component of the insulin receptor (IR) signaling complex that provides a functional link between IR and p85 phosphatidylinositol (PI) 3-kinase and regulates PI 3-kinase activity. This regulatory mechanism parallels the established link between IR and p85 via insulin receptor substrate (IRS) proteins. A direct association was demonstrated between Grb10 and p85 but was not observed between Grb10 and IRS proteins. In addition, no effect of mouse Grb10 was observed on the association between IRS-1 and p85, on IRS-1-associated PI 3-kinase activity, or on insulin-mediated activation of IR or IRS proteins. A critical role of mouse Grb10 was observed in the regulation of PI 3-kinase activity and the resulting metabolic insulin response. Dominant-negative Grb10 domains, in particular the SH2 domain, eliminated the metabolic response to insulin in differentiated 3T3-L1 adipocytes. This was consistently observed for glycogen synthesis, glucose and amino acid transport, and lipogenesis. In parallel, the same metabolic responses were substantially elevated by increased levels of Grb10. A similar role of Grb10 was confirmed in mouse L6 cells. In addition to the SH2 domain, the Pro-rich amino-terminal region of Grb10 was implicated in the regulation of PI 3-kinase catalytic activity. These regulatory roles of Grb10 were extended to specific insulin mediators downstream of PI 3-kinase including PKB/Akt, glycogen synthase kinase, and glycogen synthase. In contrast, a regulatory role of Grb10 in parallel insulin response pathways including p70 S6 kinase, ubiquitin ligase Cbl, or mitogen-activated protein kinase p38 was not observed. The dissection of the interaction of mouse Grb10 with p85 and the resulting regulation of PI 3-kinase activity should help elucidate the complexity of the IR signaling mechanism.

Disruption of the imprinted Grb10 gene leads to disproportionate overgrowth by an Igf2-independent mechanism

To investigate the function of the Grb10 adapter protein, we have generated mice in which the Grb10 gene was disrupted by a gene-trap insertion. Our experiments confirm that Grb10 is subject to genomic imprinting with the majority of Grb10 expression arising from the maternally inherited allele. Consistent with this, disruption of the maternal allele results in overgrowth of both the embryo and placenta such that mutant mice are at birth approximately 30% larger than normal. This observation establishes that Grb10 is a potent growth inhibitor. In humans, GRB10 is located at chromosome 7p11.2-p12 and has been associated with Silver-Russell syndrome, in which approximately 10% of those affected inherit both copies of chromosome 7 from their mother. Our results indicate that changes in GRB10 dosage could, in at least some cases, account for the severe growth retardation that is characteristic of Silver-Russell syndrome. Because Grb10 is a signaling protein capable of interacting with tyrosine kinase receptors, we tested genetically whether Grb10 might act downstream of insulin-like growth factor 2, a paternally expressed growth-promoting gene. The result indicates that Grb10 action is essentially independent of insulin-like growth factor 2, providing evidence that imprinting acts on at least two major fetal growth axes in a manner consistent with parent-offspring conflict theory.