Ellis-van Creveld syndrome resulting from segmental uniparental disomy of chromosome 4

Genetics of ncHH: from a peculiar inheritance of a novel GNRHR mutation to a comprehensive review of the literature

BACKGROUND

Normosmic congenital hypogonadotropic hypogonadism (ncHH) is caused by the deficient production, secretion, or action of gonadotropin-releasing hormone (GnRH). Its typical clinical manifestation is delayed puberty and azoospermia. Homozygous and compound heterozygous mutations in the GNRHR gene (4q13.2) are the most frequent genetic causes of ncHH.

OBJECTIVES

(i) Characterization at the molecular level (genetic origin and functional effect) of a unique homozygous mutation (p.Gly99Glu) in a ncHH man; (ii) to provide a comprehensive catalog of GNRHR mutations with genotype-phenotype correlation and comparison of in vitro studies vs. in silico prediction tools.

MATERIAL AND METHODS

A ncHH man and his parents, in whom we performed the following: (i) Sanger sequencing, qPCR of the GNRHR gene; (ii) chromosome 4 SNP array; and (iii) competition binding assay and inositol phosphate signaling assay. PubMed and Human Genome Mutation Database (HGMD) search for GNRHR mutations. Bioinformatic analysis of 55 reported variants.

RESULTS

qPCR showed two GNRHR copies in the index case. SNP array revealed the inheritance of two homologous chromosomes 4 from the mother (maternal heterodisomy; hUPD) with two loss of heterozygosity regions, one of them containing the mutated gene (maternal isodisomy; iUPD). Functional studies for the p.Gly99Glu mutation demonstrated a right-shifted GnRH-stimulated signaling response. Bioinformatic tools show that commonly used in silico tools are poor predictors of the function of ncHH-associated GNRHR variants.

DISCUSSION

Functional analysis of the p.Gly99Glu mutation is consistent with severely decreased GnRH binding affinity (a severe partial loss-of-function mutation). Complete LOF variants are associated with severe and severe/moderate phenotype, whereas partial LOF variants show wide range of clinical manifestations.

CONCLUSION

This is the first ncHH patient carrying a novel causative missense mutation of GNRHR with proven 'severe pLOF' due to maternal hUPD/iUPD of chromosome 4. Our literature review shows that functional studies remain essential both for diagnostic and potential therapeutic purposes.

Maternal chromosome 4 heterodisomy/isodisomy and Bβ chain Trp323X mutation resulting in severe hypodysfibrinogenaemia

We report a rare case of congenital hypodysfibrinogenaemia due to maternal uniparental disomy of chromosome 4 (mat UPD 4) and a maternally inherited novel nonsense mutation Trp323X in the fibrinogen Bβ chain (FGB) gene. Western blot analysis of patient's plasma revealed an abnormal fibrinogen which consisted of truncated Bβ chain and normal Aα and γ chains. Patient's clinical history and laboratory evidence are presented. Microsatellite genotyping analysis revealed a mixed nature of heterodisomy and isodisomy along chromosome 4. High density SNP genotyping array analysis further confirmed the mat UPD 4 and defined two segments of chromosome 4 (4pter-p15.33 and 4q31.21–4q32.3) as maternal isodisomy (iUPD4) and the remaining regions as maternal heterodisomy (hUPD4), with the FGB gene carrying the mutation resided in the iUPD4 region on the long (q) arm. It was predicted that the segmental nature of iUPD and hUPD was caused by three recombination events at positions around 167.96 cM, 145.51 cM and 14.40 cM on chromosome 4 followed by a meiosis I non-disjunction. This case is clinically and molecularly unique and offers an opportunity for understanding novel mechanisms of congenital hypodysfibrinogenaemia associated with complex UPD and fibrinogen secretion.

Prenatal diagnosis of complete maternal uniparental isodisomy of chromosome 4 in a fetus without congenital abnormality or inherited disease-associated variations

Background

The prenatal diagnosis of subjects with complete uniparental isodisomy of chromosome 4 (iUPD4) has rarely been reported and poses a great challenge for genetic counseling. In this study, a prenatal case with a high (1 in 58) risk of Down syndrome was diagnosed with iUPD4 by combined chromosomal microarray analysis (CMA), whole exome sequencing (WES) and ultrasound morphology scan.

Results

By CMA, a pathogenic copy number variant was not detected; however, a complete maternal iUPD4 was identified in this fetus after analyzing the parental genotype results. To detect potentially autosomal recessive variants, WES was performed. Two missense and two frameshift variants were identified but were predicted with uncertain significance; none of the mutations were definitively associated with congenital abnormality or inherited disease. In addition, a detailed ultrasound morphology scan did not identify any structural abnormalities, facial dysmorphisms or intrauterine growth restriction. The family history was unremarkable. The couple was counseled with the prenatal diagnostic results, and they opted to give birth to the child. No phenotypic abnormalities were observed in this child after the first year of life.

Conclusion

This study provides further evidence that iUPD4 can result in a healthy live birth and demonstrates that the combined use of CMA, WES and ultrasound technology provides additional information for the prenatal diagnosis and clinical management of rare UPD events.

Electronic supplementary material

The online version of this article (doi:10.1186/s13039-015-0190-z) contains supplementary material, which is available to authorized users.

The consequences of uniparental disomy and copy number neutral loss-of-heterozygosity during human development and cancer

UPD (uniparental disomy) describes the inheritance of a pair of chromosomes from only one parent. Mechanisms that lead to UPD include trisomy rescue, gamete complementation, monosomy rescue and somatic recombination. Most of these mechanisms can involve aberrant chromosomes, particularly isochromosomes and Robertsonian translocations. In the last decade, the number of UPD cases reported in the literature has increased exponentially. This is partly due to the advances in genomic technologies that have allowed for high-resolution SNP (single nucleotide polymorphism) studies, which have complemented traditional methods relying on polymorphic microsatellite markers. In this review, we discuss aberrant cellular mechanisms leading to UPD and their impact on gene expression. Special emphasis is placed on the unmasking of mutant recessive alleles and the disruption of imprinted gene dosage, which give rise to specific and recurrent imprinting phenotypes. Finally, we discuss how copy number maps determined from SNP array datasets have helped identify not only deletions and duplications but also recurrent copy number neutral regions of loss-of-heterozygosity, which have been reported in many cancer types and that may constitute an important driving force in cancer. These tiny regions of UPD also alter imprinted gene dosage, which may have cumulative tumourgenic effects in addition to that of unmasking homozygous cancer-associated mutations.

Ellis-van Creveld syndrome and Weyers acrodental dysostosis are caused by cilia-mediated diminished response to hedgehog ligands

Ellis-van Creveld syndrome (EvC; OMIM 225500) is a recessive disorder comprising chondrodysplasia, polydactyly, nail dysplasia, orofacial abnormalities and, in a proportion of patients, cardiovascular malformations. Weyers acrodental dysostosis (Weyers; OMIM 193530) is an allelic dominant disorder comprising polydactyly, nail dysplasia, and orofacial abnormalities. EvC results from loss-of-function mutations in EVC or EVC2, the phenotype associated with the mutations in these two genes being indistinguishable. Three convincing causative mutations have been identified in patients with Weyers acrodental dysostosis, which are clustered in the last coding exon of EVC2 and lead to production of a truncated protein lacking the final 43 amino acids. Localization and function of EVC and EVC2 are inferred from studying the murine orthologs. Both Evc and Evc2 proteins localize to the basal bodies of primary cilia and analysis of an Ellis-van Creveld mouse model, which includes the limb shortening and tooth abnormalities of EvC patients, has demonstrated Hedgehog signaling defects in the absence of Evc. The loss of Evc2 has not been studied directly, but Hedgehog signaling is impaired when a mutant murine Evc2 Weyer variant is expressed in vitro. We conclude that the phenotypic abnormalities in EvC and Weyers syndrome result from tissue specific disruption of the response to Hh ligands.

Anesthetic implications of Ellis-van Creveld syndrome

Ellis-van Creveld syndrome (chondroectodermal or mesoectodermal dysplasia) is an autosomal recessive disorder. Initial reports of the syndrome identified a tetrad of clinical manifestations, including chondrodysplasia, postaxial polydactyly, ectodermal dysplasia, and congenital heart disease (CHD). Additional involvement may occur in organs of endodermal origin, such as the pulmonary, renal, gastrointestinal (hepatic and pancreatic), hematologic, and central nervous systems. The perioperative care of a 2-year, 8-month-old girl who underwent surgical correction of CHD is presented.

A fascination with chromosome rescue in uniparental disomy: Mendelian recessive outlaws and imprinting copyrights infringements

With uniparental disomy (UPD), the presence in a diploid genome of a chromosome pair derived from one genitor carries two main types of developmental risk: the inheritance of a recessive trait or the occurrence of an imprinting disorder. When the uniparentally derived pair carries two homozygous sequences (isodisomy) with a duplicated mutant, this 'reduction to homozygosity' determines a recessive phenotype solely inherited from one heterozygote. Thus far, some 40 examples of such recessive trait transmission have been reported in the medical literature and, among the current 32 known types of UPDs, UPD of chromosomes 1, 2, and 7 have contributed to the larger contingent of these conditions. Being at variance with the traditional mode of transmission, they constitute a group of 'Mendelian outlaws'. Several imprinted chromosome domains and loci have been, for a large part, identified through different UPDs. Thus, disomies for paternal 6, maternal 7, paternal 11, paternal and maternal 14 and 15, maternal 20 (and paternal 20q) and possibly maternal 16 cause as many syndromes, as at the biological level the loss or duplication of monoparentally expressed allele sequences constitutes 'imprinting rights infringements'. The above pitfalls represent the price to pay when, instead of a Mendelian even segregation and independent assortment of the chromosomes, the fertilized product with a nondisjunctional meiotic error undergoes correction (for unknown or fortuitous reasons) through a mitotic adjustment as a means to restore euploidy, thereby resulting in UPD. Happily enough, UPDs leading to the healthy rescue from some chromosomal mishaps also exist.

Congenital afibrinogenaemia caused by uniparental isodisomy of chromosome 4 containing a novel 15-kb deletion involving fibrinogen Aα-chain gene

Among rare inherited deficiencies of coagulation factors, congenital afibrinogenaemia is characterised by the lack of fibrinogen in plasma. In the last few years, several genetic defects underlying afibrinogenaemia (mostly point mutations) have been described in the fibrinogen gene cluster. In this study, the molecular basis responsible for afibrinogenaemia in a Thai proband was defined. Point mutation screening was accomplished by directly sequencing the three fibrinogen genes. The impossibility to amplify fibrinogen Aalpha-chain gene (FGA) exons 5 and 6 suggested the presence of a homozygous deletion. A specific long-range PCR assay enabled the identification of a novel 15-kb deletion, representing the largest afibrinogenaemia-causing deletion described so far. Direct sequencing of the deletion junction allowed mapping of the breakpoints in FGA intron 4 and in the intergenic region between Aalpha- and Bbeta-chain genes. Since the mutation was inherited only from the mother and nonpaternity was ruled out, a maternal uniparental disomy (UPD) was hypothesised. UPD test, carried out with markers covering the whole chromosome 4, revealed that maternal isodisomy was responsible for homozygosity of the 15-kb deletion in the proband. The apparently normal phenotype of the proband, except for afibrinogenaemia, suggests that UPD for chromosome 4 is clinically silent. This represents the first case of a documented complete isodisomy of chromosome 4 causing the phenotypic expression of a recessive disorder. In silico analyses of the regions surrounding the breakpoints suggested that the 15-kb deletion might have originated from an inappropriate repair of a double-strand break by the nonhomologous end joining mechanism.

Segmental maternal heterodisomy of the proximal part of chromosome 15 in an infant with Prader–Willi syndrome

Uniparental disomy (UPD) 15, detected in patients with Prader-Willi (PWS) and Angelman syndromes, has to date always involved the entire chromosome 15. We report the first case of segmental maternal uniparental heterodisomy confined to a proximal part of chromosome 15 in a child with clinical features of PWS. This unusual finding can be explained by the rare combination of three consecutive events: a trisomy 15 zygote caused by a maternal meiosis I error, early postzygotic mitotic recombination between maternal and paternal chromatids, and, finally, trisomy rescue by the loss of the rearranged chromosome 15 containing the paternal 15q11-q13 segment.