IMAGe syndrome: Case report with a previously unreported feature and review of published literature

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

A novel mutation in the NNT gene causing familial glucocorticoid deficiency, with a literature review

Familial glucocorticoid deficiency (FGD) is an autosomal recessive disorder characterized by low cortisol levels despite elevated adrenocorticotropin (ACTH). Mineralocorticoid secretion is classically normal. Clinical manifestations are secondary to low cortisol levels (recurrent hypoglycemia, chronic asthenia, failure to thrive, seizures) and high levels of ACTH (cutaneous-mucosal hyperpigmentation). FGD is often caused by mutations in the ACTH melanocortin 2 receptor gene (MC2R, 18p11.21, FGD type 1) or melanocortin receptor 2 accessory protein gene (MRAP, 21q22.11, FGD type 2). But mutations have also been described in other genes: the steroidogenic acute regulatory protein (STAR, 8q11.2q13.2, FGD type 3), nicotinamide nucleotide transhydrogenase (NNT, 5p12, FGD type 4) and thioredoxin reductase 2 genes (TXNRD2, 22q11.21, FGD type 5). We report the case of a 3-year-old boy recently diagnosed with FGD type 4 due to a novel mutation in NNT gene. A homozygous variant in exon 18 of the NNT gene, NM_012343.3:c.2764C>T, p.(Arg922*), determines a stop codon and, consequently, a non-functional truncated protein or absence of protein due to the nonsense-mediated decay (NMD) mechanism. We review the recent literature on NNT mutations and clinical presentations, which are broader than suspected. This disorder can result in significant morbidity and is potentially fatal if untreated. Precise diagnosis allows correct treatment and follow-up.

An Update on Genetics of Adrenal Gland and Associated Disorders

The intricacies of human adrenal development have been under scrutiny for decades. Each year marks the identification of new genes and new interactions between gene products that ultimately will act to produce the fully functioning adult gland. Due to the complexity of this process, genetic missteps may lead to a constellation of pathologies. Recent years have identified several novel genetic causes of adrenal dysgenesis and provided new insights into previously delineated processes. SF1, DAX1 (NR0B1), CDKN1C, SAMD9, GLI3, TPIT, MC2R, MRAP, NNT, TXNRD2, AAAS, and MCM4 are among the genes which have had significant contributions to our understanding of the development and function of both adrenals and gonads. Collection and elucidation of these genetic and clinical insights are valuable tools for clinicians who diagnose and manage cases of adrenal dysfunction.

Common genetic variants with fetal effects on birth weight are enriched for proximity to genes implicated in rare developmental disorders

Birth weight is an important factor in newborn survival; both low and high birth weights are associated with adverse later-life health outcomes. Genome-wide association studies (GWAS) have identified 190 loci associated with maternal or fetal effects on birth weight. Knowledge of the underlying causal genes is crucial to understand how these loci influence birth weight and the links between infant and adult morbidity. Numerous monogenic developmental syndromes are associated with birth weights at the extreme ends of the distribution. Genes implicated in those syndromes may provide valuable information to prioritize candidate genes at the GWAS loci. We examined the proximity of genes implicated in developmental disorders (DDs) to birth weight GWAS loci using simulations to test whether they fall disproportionately close to the GWAS loci. We found birth weight GWAS single nucleotide polymorphisms (SNPs) fall closer to such genes than expected both when the DD gene is the nearest gene to the birth weight SNP and also when examining all genes within 258 kb of the SNP. This enrichment was driven by genes causing monogenic DDs with dominant modes of inheritance. We found examples of SNPs in the intron of one gene marking plausible effects via different nearby genes, highlighting the closest gene to the SNP not necessarily being the functionally relevant gene. This is the first application of this approach to birth weight, which has helped identify GWAS loci likely to have direct fetal effects on birth weight, which could not previously be classified as fetal or maternal owing to insufficient statistical power.

Pediatric Primary Adrenal Insufficiency: A 21-year Single Center Experience

OBJECTIVE

Primary adrenal insufficiency (PAI) is a rare but potentially life-threatening condition. In childhood, PAI is usually caused by monogenic diseases. Although congenital adrenal hyperplasia (CAH) is the most common cause of childhood PAI, numerous non-CAH genetic causes have also been identified.

METHODS

Patients aged 0-18 years and diagnosed with PAI between 1998 and 2019 in a tertiary care hospital were retrospectively evaluated. After the etiologic distribution was determined, non-CAH PAI patients were evaluated in detail.

RESULTS

Seventy-three PAI patients were identified. The most common etiology was CAH (69.9%, n=51). Non-CAH etiologies accounted for 30.1% (n=22) and included adrenoleukodystrophy (ALD; n=8), familial glucocorticoid deficiency (n=3), Triple A syndrome (n=5), autoimmune adrenalitis (n=1), adrenal hypoplasia congenital (n=1), IMAGe syndrome (n=1), and other unknown etiologies (n=3). The median age at the time of AI diagnosis for non-CAH etiologies was 3.52 (0.03-15.17) years. The most frequent symptoms/clinical findings at onset were hyperpigmentation of skin (81.8%), symptoms of hypoglycemia (40.9%), and weakness/fatigue (31.8%). Hypoglycemia (50.0%), hyponatremia (36.4%) and hyperkalemia (22.7%) were prominent biochemical findings. Diagnosis of specific etiologies were proven genetically in 13 of 22 patients. A novel p.Q301* hemizygous frameshift mutation of the DAX1 gene was identified in one patient.

CONCLUSION

Etiology was determined in 86.3% of children with non-CAH PAI through specific clinical and laboratory findings with/ without molecular analysis of candidate genes. ALD was the most common etiology. Currently, advanced molecular analysis can be utilized to establish a specific genetic diagnosis for PAI in patients who have no specific diagnostic features.

Deep exploration of a CDKN1C mutation causing a mixture of Beckwith-Wiedemann and IMAGe syndromes revealed a novel transcript associated with developmental delay

Background

Loss-of-function mutations in CDKN1C cause overgrowth, that is, Beckwith-Wiedemann syndrome (BWS), while gain-of-function variants in the gene’s PCNA binding motif cause a growth-restricted condition called IMAGe syndrome. We report on a boy with a remarkable mixture of both syndromes, with developmental delay and microcephaly as additional features.

Methods

Whole-exome DNA sequencing and ultra-deep RNA sequencing of leucocyte-derived and fibroblast-derived mRNA were performed in the family.

Results

We found a maternally inherited variant in the IMAGe hotspot region: NM_000076.2(CDKN1C) c.822_826delinsGAGCTG. The asymptomatic mother had inherited this variant from her mosaic father with mild BWS features. This delins caused tissue-specific frameshifting resulting in at least three novel mRNA transcripts in the boy. First, a splice product causing CDKN1C truncation was the likely cause of BWS. Second, an alternative splice product in fibroblasts encoded IMAGe-associated amino acid substitutions. Third, we speculate that developmental delay is caused by a change in the alternative CDKN1C-201 (ENST00000380725.1) transcript, encoding a novel isoform we call D (UniProtKB: A6NK88). Isoform D is distinguished from isoforms A and B by alternative splicing within exon 1 that changes the reading frame of the last coding exon. Remarkably, this delins changed the reading frame back to the isoform A/B type, resulting in a hybrid D–A/B isoform.

Conclusion

Three different cell-type-dependent RNA products can explain the co-occurrence of both BWS and IMAGe features in the boy. Possibly, brain expression of hybrid isoform D–A/B is the cause of developmental delay and microcephaly, a phenotypic feature not previously reported in CDKN1C patients.

Genetic causes of proportionate short stature

Human growth is a very complex phenomenon influenced by genetic, hormonal, nutritional and environmental factors, from fetal life to puberty. Although the GH-IGF axis has a central role with specific actions on growth, numerous genes are involved in the control of stature. Genome-wide association studies have identified >600 variants associated with human height, still explaining only a small fraction of phenotypic variation. Since short stature in childhood is a common reason for referral, pediatric endocrinologists must be aware of the multifactorial and polygenic contributions to height. Multiple disorders characterized by growth failure of prenatal and/or postnatal onset due to single gene defects have been described. Their early diagnosis, facilitated by advances in genomic technologies, is of upmost importance for their clinical management and to provide genetic counseling. Here we review the current clinical and genetic information regarding different syndromes and hormone abnormalities with proportionate short stature as the main feature, and provide an update of the approach for diagnosis and management.

Rare monogenic causes of primary adrenal insufficiency

PURPOSE OF REVIEW

Monogenic disorders play significant roles in the pathogenesis of childhood-onset primary adrenal insufficiency (PAI). The most common form of PAI is congenital adrenal hyperplasia (CAH), which includes the enzymatic defects of the steroidogenic pathway. This review focuses on less common forms of monogenic PAI (i.e. non-CAH monogenic PAI) with particular attention on their cause, clinical phenotypes and genetic epidemiology.

RECENT FINDINGS

Non-CAH monogenic PAI can be classified into three major categories: first, adrenocorticotropic hormone resistance, second, impaired adrenal redox homeostasis and third, defective organogenesis of the adrenal glands. The clinical phenotypes of the mutation-carrying patients vary depending on the responsible gene, and they are partially explained by the tissue RNA expression patterns. Genetic epidemiology studies conducted in Turkey and Japan showed that about 80% of PAI of unknown cause was monogenic.

SUMMARY

Genetic basis of non-CAH monogenic PAI had been less clearly understood than CAH; however, significant advances have been made with use of new research techniques such as next-generation sequencing. Understanding of these rare forms of PAI may contribute to clarifying the physiology and pathology of the adrenal glands.

Hypercalcemic Disorders in Children

Hypercalcemia is defined as a serum calcium concentration that is greater than two standard deviations above the normal mean, which in children may vary with age and sex, reflecting changes in the normal physiology at each developmental stage. Hypercalcemic disorders in children may present with hypotonia, poor feeding, vomiting, constipation, abdominal pain, lethargy, polyuria, dehydration, failure to thrive, and seizures. In severe cases renal failure, pancreatitis and reduced consciousness may also occur and older children and adolescents may present with psychiatric symptoms. The causes of hypercalcemia in children can be classified as parathyroid hormone (PTH)-dependent or PTH-independent, and may be congenital or acquired. PTH-independent hypercalcemia, ie, hypercalcemia associated with a suppressed PTH, is commoner in children than PTH-dependent hypercalcemia. Acquired causes of PTH-independent hypercalcemia in children include hypervitaminosis; granulomatous disorders, and endocrinopathies. Congenital syndromes associated with PTH-independent hypercalcemia include idiopathic infantile hypercalcemia (IIH), William's syndrome, and inborn errors of metabolism. PTH-dependent hypercalcemia is usually caused by parathyroid tumors, which may give rise to primary hyperparathyroidism (PHPT) or tertiary hyperparathyroidism, which usually arises in association with chronic renal failure and in the treatment of hypophosphatemic rickets. Acquired causes of PTH-dependent hypercalcemia in neonates include maternal hypocalcemia and extracorporeal membrane oxygenation. PHPT usually occurs as an isolated nonsyndromic and nonhereditary endocrinopathy, but may also occur as a hereditary hypercalcemic disorder such as familial hypocalciuric hypercalcemia, neonatal severe primary hyperparathyroidism, and familial isolated primary hyperparathyroidism, and less commonly, as part of inherited complex syndromic disorders such as multiple endocrine neoplasia (MEN). Advances in identifying the genetic causes have resulted in increased understanding of the underlying biological pathways and improvements in diagnosis. The management of symptomatic hypercalcemia includes interventions such as fluids, antiresorptive medications, and parathyroid surgery. This article presents a clinical, biochemical, and genetic approach to investigating the causes of pediatric hypercalcemia. © 2017 American Society for Bone and Mineral Research.

MECHANISMS IN ENDOCRINOLOGY: Update on pathogenesis of primary adrenal insufficiency: beyond steroid enzyme deficiency and autoimmune adrenal destruction

Primary adrenal insufficiency (PAI) is potentially life threatening, but rare. In children, genetic defects prevail whereas adults suffer more often from acquired forms of PAI. The spectrum of genetic defects has increased in recent years with the use of next-generation sequencing methods and now has reached far beyond genetic defects in all known enzymes of adrenal steroidogenesis. Cofactor disorders such as P450 oxidoreductase (POR) deficiency manifesting as a complex form of congenital adrenal hyperplasia with a broad clinical phenotype have come to the fore. In patients with isolated familial glucocorticoid deficiency (FGD), in which no mutations in the genes for the ACTH receptor (MC2R) or its accessory protein MRAP have been found, non-classic steroidogenic acute regulatory protein (StAR) and CYP11A1 mutations have been described; and more recently novel mutations in genes such as nicotinamide nucleotide transhydrogenase (NNT) and thioredoxin reductase 2 (TRXR2) involved in the maintenance of the mitochondrial redox potential and generation of NADPH important for steroidogenesis and ROS detoxication have been discovered. In addition, whole exome sequencing approach also solved the genetics of some syndromic forms of PAI including IMAGe syndrome (CDKN1C), Irish traveler syndrome (MCM4), MIRAGE syndrome (SAMD9); and most recently a syndrome combining FGD with steroid-resistant nephrotic syndrome and ichthyosis caused by mutations in the gene for sphingosine-1-phosphate lyase 1 (SGPL1). This review intends do give an update on novel genetic forms of PAI and their suggested mechanism of disease. It also advocates for advanced genetic work-up of PAI (especially in children) to reach a specific diagnosis for better counseling and treatment.

Lhx6 and Lhx8 promote palate development through negative regulation of a cell cycle inhibitor gene, p57Kip2

Cleft palate is a common birth defect in humans. Therefore, understanding the molecular genetics of palate development is important from both scientific and medical perspectives. Lhx6 and Lhx8 encode LIM homeodomain transcription factors, and inactivation of both genes in mice resulted in profound craniofacial defects including cleft secondary palate. The initial outgrowth of the palate was severely impaired in the mutant embryos, due to decreased cell proliferation. Through genome-wide transcriptional profiling, we discovered that p57(Kip2) (Cdkn1c), encoding a cell cycle inhibitor, was up-regulated in the prospective palate of Lhx6(-/-);Lhx8(-/-) mutants. p57(Kip2) has been linked to Beckwith-Wiedemann syndrome and IMAGe syndrome in humans, which are developmental disorders with increased incidents of palate defects among the patients. To determine the molecular mechanism underlying the regulation of p57(Kip2) by the Lhx genes, we combined chromatin immunoprecipitation, in silico search for transcription factor-binding motifs, and in vitro reporter assays with putative cis-regulatory elements. The results of these experiments indicated that LHX6 and LHX8 regulated p57(Kip2) via both direct and indirect mechanisms, with the latter mediated by Forkhead box (FOX) family transcription factors. Together, our findings uncovered a novel connection between the initiation of palate development and a cell cycle inhibitor via LHX. We propose a model in which Lhx6 and Lhx8 negatively regulate p57(Kip2) expression in the prospective palate area to allow adequate levels of cell proliferation and thereby promote normal palate development. This is the first report elucidating a molecular genetic pathway downstream of Lhx in palate development.

Anesthetic and dental management of a child with IMAGe syndrome

IMAGe syndrome (OMIM 300290) is a rare multisystem disorder that has a broad phenotypic presentation. Though variable, this disorder mainly consists of Intrauterine growth retardation, Metaphyseal dysplasia, Adrenal hypoplasia congenita, and Genital abnormalities. Patients with IMAGe syndrome present as an uncommon yet important challenge for dentists and anesthesiologists due to their wide range of dysmorphic facial features, adrenal insufficiency, electrolyte imbalances, and need for steroid replacement. The purpose of this case report is to describe the successful anesthetic management of a pediatric patient diagnosed with IMAGe syndrome who presented for full mouth dental rehabilitation.

Is Hyperpigmentation on the First Day of Life Always Associated with IMAGe Syndrome?

IMAGe syndrome is an exceedingly rare condition first described in 1999. Components of the syndrome are intrauterine growth retardation (IUGR), metaphyseal dysplasia, congenital adrenal hypoplasia and genital anomalies. Cases generally present with life-threatening adrenal insufficiency in the neonatal period. Herein, we describe a patient with pronounced IUGR diagnosed with severe hyperpigmentation and adrenal insufficiency in the neonatal term in order to attract the attention to this rare entity.

Beckwith-Wiedemann and IMAGe syndromes: two very different diseases caused by mutations on the same gene

Genomic imprinting is an epigenetically regulated mechanism leading to parental-origin allele-specific expression. Beckwith-Wiedemann syndrome (BWS) is an imprinting disease related to 11p15.5 genetic and epigenetic alterations, among them loss-of-function CDKN1C mutations. Intriguing is that CDKN1C gain-of-function variations were recently found in patients with IMAGe syndrome (intrauterine growth restriction, metaphyseal dysplasia, congenital adrenal hypoplasia, and genital anomalies). BWS and IMAGe share an imprinted mode of inheritance; familial analysis demonstrated the presence of the phenotype exclusively when the mutant CDKN1C allele is inherited from the mother. Interestingly, both IMAGe and BWS are characterized by growth disturbances, although with opposite clinical phenotypes; IMAGe patients display growth restriction whereas BWS patients display overgrowth. CDKN1C codifies for CDKN1C/KIP2, a nuclear protein and potent tight-binding inhibitor of several cyclin/Cdk complexes, playing a role in maintenance of the nonproliferative state of cells. The mirror phenotype of BWS and IMAGe can be, at least in part, explained by the effect of mutations on protein functions. All the IMAGe-associated mutations are clustered in the proliferating cell nuclear antigen-binding domain of CDKN1C and cause a dramatic increase in the stability of the protein, which probably results in a functional gain of growth inhibition properties. In contrast, BWS mutations are not clustered within a single domain, are loss-of-function, and promote cell proliferation. CDKN1C is an example of allelic heterogeneity associated with opposite syndromes.

IMAGe association: report of two cases in siblings with adrenal hypoplasia and review of the literature

We report the postmortem findings of two siblings with gross and microscopic features consistent with IMAGe association (Intrauterine growth retardation, Metaphyseal dysplasia, Adrenal hypoplasia congenita, and Genital anomalies) with an emphasis on the histopathology of the adrenal gland in this rare syndrome. The first sibling was an 8-week old male diagnosed postnatally with primary adrenal insufficiency. There was no deletion of the DAX1 gene by FISH. Examination at autopsy revealed dysmorphic features including frontal bossing, epicanthal folds, flat philtrum, cryptorchidism, penile chordee, overriding fourth toe, and height and weight below 3rd percentile. Grossly, the adrenal glands were not identified; however, microscopic examination of the suprarenal soft tissue revealed a 3 mm focus of disorganized fetal adrenal cortex with distended "cytomegalic" cells with abundant pink eosinophilic cytoplasm, vesicular nuclei, and cytoplasmic vacuolization. A minute focus of permanent adult cortex was also seen, but no adrenal medulla was identified. An autopsy of the sibling, who died 12 years previously at day 9 of life, revealed dysmorphic facial features with cryptorchidism and a large phallus. The adrenal glands were grossly hypoplastic (11 mm). Histologically, the adrenal glands showed disorganized fetal cortex with cytomegalic cells, a larger amount of permanent adult cortex, and bizarre nuclei with numerous pseudoinclusions. While there is currently limited information regarding the histopathologic adrenal findings in IMAGe association, our small case series suggests overlapping features between X-linked recessive congenital adrenal hypoplasia (cytomegalic cells with lack of permanent adult cortex) and autosomal recessive congenital adrenal hypoplasia (diminished permanent adult cortex without cytomegalic cells).

IMAGe syndrome: clinical and genetic implications based on investigations in three Japanese patients

OBJECTIVE

Arboleda et al. have recently shown that IMAGe (intra-uterine growth restriction, metaphyseal dysplasia, adrenal hypoplasia congenita and genital abnormalities) syndrome is caused by gain-of-function mutations of maternally expressed gene CDKN1C on chromosome 11p15.5. However, there is no other report describing clinical findings in patients with molecularly studied IMAGe syndrome. Here, we report clinical and molecular findings in Japanese patients.

PATIENTS

We studied a 46,XX patient aged 8·5 years (case 1) and two 46,XY patients aged 16·5 and 15·0 years (cases 2 and 3).

RESULTS

Clinical studies revealed not only IMAGe syndrome-compatible phenotypes in cases 1-3, but also hitherto undescribed findings including relative macrocephaly and apparently normal pituitary-gonadal endocrine function in cases 1-3, familial glucocorticoid deficiency (FGD)-like adrenal phenotype and the history of oligohydramnios in case 2, and arachnodactyly in case 3. Sequence analysis of CDKN1C, pyrosequencing-based methylation analysis of KvDMR1 and high-density oligonucleotide array comparative genome hybridization analysis for chromosome 11p15.5 were performed, showing an identical de novo and maternally inherited CDKN1C gain-of-function mutation (p.Asp274Asn) in cases 1 and 2, respectively, and no demonstrable abnormality in case 3.

CONCLUSIONS

The results of cases 1 and 2 with CDKN1C mutation would argue the following: [1] relative macrocephaly is consistent with maternal expression of CDKN1C in most tissues and biparental expression of CDKN1C in the foetal brain; [2] FGD-like phenotype can result from CDKN1C mutation; and [3] genital abnormalities may primarily be ascribed to placental dysfunction. Furthermore, lack of CDKN1C mutation in case 3 implies genetic heterogeneity in IMAGe syndrome.

Increased protein stability of CDKN1C causes a gain-of-function phenotype in patients with IMAGe syndrome

Mutations in the proliferating cell nuclear antigen (PCNA)-binding domain of the CDKN1C gene were recently identified in patients with IMAGe syndrome. However, loss of PCNA binding and suppression of CDKN1C monoubiquitination by IMAGe-associated mutations hardly explain the reduced-growth phenotype characteristic of IMAGe syndrome. We demonstrate here that IMAGe-associated mutations in the CDKN1C gene dramatically increased the protein stability. We identified a novel heterozygous mutation, c.815T>G (p.Ile272Ser), in the CDKN1C gene in three siblings manifesting clinical symptoms associated with IMAGe syndrome and their mother (unaffected carrier). PCNA binding to CDKN1C was disrupted in the case of p.Ile272Ser, and for two other IMAGe-associated mutations, p.Asp274Asn and p.Phe276Val. Intriguingly, the IMAGe-associated mutant CDKN1C proteins were fairly stable even in the presence of cycloheximide, whereas the wild-type protein was almost completely degraded via the proteasome pathway, as shown by the lack of degradation with addition of a proteasome inhibitor, MG132. These results thus suggested that the reduced-growth phenotype of IMAGe syndrome derives from CDKN1C gain-of-function due to IMAGe-associated mutations driving increased protein stability.

CDKN1C mutation affecting the PCNA-binding domain as a cause of familial Russell Silver syndrome

BACKGROUND

Russell Silver syndrome (RSS) leads to prenatal and postnatal growth retardation. About 55% of RSS patients present a loss-of-methylation of the paternal ICR1 domain on chromosome 11p15. CDKN1C is a cell proliferation inhibitor encoded by an imprinted gene in the 11p15 ICR2 domain. CDKN1C mutations lead to Beckwith Wiedemann syndrome (BWS, overgrowth syndrome) and in IMAGe syndrome which associates growth retardation and adrenal insufficiency. We searched for CDKN1C mutations in a cohort of clinically diagnosed RSS patients with no molecular anomaly.

METHOD

The coding sequence and intron-exon boundaries of CDKN1C were analysed in 97 RSS patients. The impact of CDKN1C variants on the cell cycle in vitro were determined by flow cytometry. Stability of CDKN1C was studied by western immunoblotting after inhibition of translation with cycloheximide.

RESULTS

We identified the novel c.836G>[G;T] (p.Arg279Leu) mutation in a familial case of intrauterine growth retardation (IUGR) with RSS phenotype and no evidence of IMAGe. All the RSS patients inherited this mutation from their mothers (consistent with monoallelic expression from the maternal allele of the gene). A mutation of this amino acid (p.Arg279Pro) has been reported in cases of IMAGe. Functional analysis showed that Arg279Leu (RSS) did not affect the cell cycle, whereas the Arg279Pro mutation (IMAGe) led to a gain of function. Arg279Leu (RSS) led to an increased stability which could explain an increased activity of CDKN1C.

CONCLUSIONS

CDKN1C mutations cause dominant maternally transmitted RSS, completing the molecular mirror with BWS. CDKN1C should be investigated in cases with family history of RSS.

Mutations in the PCNA-binding domain of CDKN1C cause IMAGe syndrome

IMAGe syndrome (intrauterine growth restriction, metaphyseal dysplasia, adrenal hypoplasia congenita and genital anomalies) is an undergrowth developmental disorder with life-threatening consequences. An identity-by-descent analysis in a family with IMAGe syndrome identified a 17.2-Mb locus on chromosome 11p15 that segregated in the affected family members. Targeted exon array capture of the disease locus, followed by high-throughput genomic sequencing and validation by dideoxy sequencing, identified missense mutations in the imprinted gene CDKN1C (also known as P57KIP2) in two familial and four unrelated patients. A familial analysis showed an imprinted mode of inheritance in which only maternal transmission of the mutation resulted in IMAGe syndrome. CDKN1C inhibits cell-cycle progression, and we found that targeted expression of IMAGe-associated CDKN1C mutations in Drosophila caused severe eye growth defects compared to wild-type CDKN1C, suggesting a gain-of-function mechanism. All IMAGe-associated mutations clustered in the PCNA-binding domain of CDKN1C and resulted in loss of PCNA binding, distinguishing them from the mutations of CDKN1C that cause Beckwith-Wiedemann syndrome, an overgrowth syndrome.