Novel Mutations in GPR68 and SLC24A4 Cause Hypomaturation Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a rare genetic condition affecting the quantity and/or quality of tooth enamel. Hypomaturation AI is characterized by brownish-yellow discoloration with increased opacity and poorly mineralized enamel prone to fracture and attrition. We recruited three families affected by hypomaturation AI and performed whole exome sequencing with selected individuals in each family. Bioinformatic analysis and Sanger sequencing identified and confirmed mutations and segregation in the families. Family 1 had a novel homozygous frameshift mutation in GPR68 gene (NM_003485.3:c.78_83delinsC, p.(Val27Cysfs*146)). Family 2 had a novel homozygous nonsense mutation in SLC24A4 gene (NM_153646.4:c.613C>T, NP_705932.2:p.(Arg205*)). Family 3 also had a homozygous missense mutation in SLC24A4 gene which was reported previously (c.437C>T, p.(Ala146Val)). This report not only expands the mutational spectrum of the AI-causing genes but also improves our understanding of normal and pathologic amelogenesis.

Novel Homozygous Nonsense Mutation Associated with Bardet-Biedl Syndrome in Fetus with Congenital Renal Malformation

Background: Bardet-Biedl syndrome (BBS) is a rare autosomal recessive genetic disorder with clinical and genetic heterogeneity. BBS is more commonly reported in adults and children than in fetuses. Method: Here, a retrospective study of 210 fetuses with congenital renal malformation was performed. These fetuses were performed invasive prenatal diagnosis. Chromosome karyotype analysis, whole exome sequencing (WES), and a single nucleotide polymorphism array (SNP-array) were used.Results: We found the intrauterine phenotype of a fetus with enlarged kidneys, enhanced echo, and oligohydramnios, and the molecular characterizations of the fetus with BBS. The results of chromosome karyotype analysis and SNP-array on the fetus were normal. WES, however, revealed homozygous mutation of c.1177C>T (p.Arg393*) on exon 12 of the BBS1 gene, and heterozygous variation of c.2704G>A (p.Asp902Asn) on exon 22 of the CC2D2A gene. According to ACMG guidelines, c.1177C> T was identified as a pathogenic mutation and c.2704G>A was identified as an uncertain significance. Sanger sequencing showed that there was heterozygous mutation of c.1177C>T and heterozygous variation of c.2704G>A in the parents of the fetus. Conclusion: WES identified a novel homozygous nonsense mutation c.1177C>T in the BBS1 gene of a Chinese fetus with congenital renal malformation. The finding provides more insight into BBS1 mutations in Asian populations in general, and provides a basis for genetic counseling.

Identification of a Novel Homozygous Nonsense Mutation in a Fetus with Bardet-Biedl Syndrome

Background: Bardet-Biedl syndrome (BBS) is a rare autosomal recessive genetic disorder with clinical and genetic heterogeneity. BBS is more commonly reported in adults and children than in fetuses. Here, we reported the intrauterine phenotype and molecular characterizations of a fetus with BBS. Methods: Chromosome karyotype analysis, whole exome sequencing (WES), and a single nucleotide polymorphism array (SNP-array) were used to analyze the genetic etiology of a fetus with enlarged kidneys, enhanced echo, and oligohydramnios. Results: The results of chromosome karyotype analysis and SNP-array on the fetus were normal. WES, however, revealed homozygous mutation of c.1177C>T (p.Arg393*) on exon 12 of the BBS1 gene, and heterozygous variation of c.2704G>A (p.Asp902Asn) on exon 22 of the CC2D2A gene. According to ACMG guidelines, c.1177C> T was identified as a pathogenic mutation and c.2704G>A was identified as an uncertain significance. Sanger sequencing showed that there was heterozygous mutation of c.1177C>T and heterozygous variation of c.2704G>A in the parents of the fetus. Conclusions: WES identified a novel homozygous nonsense mutation c.1177C>T in the BBS1 gene of a Chinese fetus. The finding provides more insight into BBS1 mutations in Asian populations in general, and provides a basis for genetic counseling.

A Novel Mutation in the VPS13B Gene in a Cohen Syndrome Patient with Positive Antiphospholipid Antibodies

Cohen syndrome is an autosomal recessive disorder with the primary symptoms of mental deficiency, progressive retinopathy, hypotonia, microcephaly, obesity of midchildhood onset, intermittent neutropenia, and dysmorphic facial features. The syndrome has high phenotypic heterogeneity and is caused by loss-of-function mutations in the VPS13B gene. Here, we introduce a novel homozygous nonsense mutation (c.8698G > T, p.E2900X) in the VPS13B gene in an 11-year-old Iranian boy with major symptoms of Cohen syndrome. He also had mild anemia accompanied by positive antiphospholipid antibodies, the latter has never been previously reported in Cohen syndrome.

P–577 Impaired double strand DNA repair in isolated Primary Ovarian Insufficiency with homozygous nonsense mutation of SPIDR

Study question To identify the etiology of isolated Primary Ovarian Insufficiency (POI) in a patient from an Indian consanguineous family. Summary answer A homozygous nonsense mutation of SPIDR in the patient yielded chromosomal instability: first evidence of a role of this gene in DNA repair. What is known already POI, affecting 1% of women under 40, is a public health problem. To date ∼ 70% of cases remain idiopathic. The leap due to exome sequencing, led to the identification of ∼ 80 genes, often in single or few cases. SPIDR was recently identified as a scaffolding protein connecting RAD51, a central player in homologous recombination, to BLM, a helicase implicated in the integrity of the genome. But its precise role is still unknown. A SPIDR mutation was previously associated with POI. However, contradictory conclusions were reported on the mechanism of SPIDR action and on its pathogenic role in POI. Study design, size, duration Prospective genetic study of a cohort of 150 pateints with POI worldwide using a custom-made targeted next generation sequencing (NGS) panel comprising 60 known POI-causing genes. A single patient was found mutated in SPIDR. Cytogenetic studies were performed to analyse the consequences of the mutation on DNA repair and sister chromatide exchanges (SCE). Participants/materials, setting, methods The patient with SPIDR mutation had POI with primary amenorrhea, delayed puberty and streaks ovaries. She was born to consanguineous Indian parents. No other mutation was detected in our cohort of 150 patients with POI. Targeted NGS was performed in the proposita. Familial segregation was performed by Sanger sequencing. Mitomycin C (MMC)-induced chromosomal breakages were studied and a sister chromatid exchange (SCE) assay was performed in patient’s peripheral lymphocytes. Main results and the role of chance We identified a novel homozygous nonsense mutation in the exon 7 of SPIDR (KIAA0146) c.814C>T, R272*, predicted to yield either a truncated protein, or a non-sense-mediated mRNA decay. The patient’s cells display increased chromosomal fragility with high MMC-induced chromosomal breaks when compared to a control. Remarkably, there was no increased SCE. In the previous report of a SPIDR mutation in POI, no cytogenetic studies were performed, and contradictory results were obtained on a homologous recombination test between the two sisters, either enhanced or reduced. In conclusion, we show here that inactivation of SPIDR results in a defect of double strand DNA damage repair, similar to alteration of the RAD51 pathway. There was no increased SCE, the hallmark of the BLM pathway. This observation has major consequences for this patient’s care : indeed mutations of DNA-repair genes may also yield to tumors/cancers. A long follow-up of the patient is needed in a multidisciplinary team to detect possible comorbidities. Indeed, even in the absence of somatic symptomatology, the patient has enhanced chromosomal instability highlighted by cytogenetic studies, that may yield tumor-predisposition. Limitations, reasons for caution No other mutation of SPIDR in the replication cohort of 150 POI patients. SPIDR mutation are thus very rare world-wide. Wider implications of the findings: This is the first evidence of chromosomal instability associated with SPIDR defect, providing strong evidence for a role of SPIDR in double strand DNA damage repair in humans and for its causal role in POI. Our study improves the knowledge on SPIDR function and confirms its involvement in POI worldwide. Trial registration number Not applicable