Monogenic causes of pigmentary mosaicism

Clinical and Genetic Characterization of 8 Patients with Syndromic Patterned Cutaneous Hypopigmentation: A Descriptive Study

BACKGROUND

Patterned cutaneous hypopigmentation (PCH) can be associated with an array of extracutaneous findings.

OBJECTIVE

Determine clinical and genetic characteristics of patients with PCH with extracutaneous involvement.

METHOD

Thirty patients were identified, and eight patients with neurological involvement agreed to participate in this study. They were reassessed to collect clinical anomalies associated with their PCH. Exome sequencing was performed on patient blood and lesional skin biopsies as well as blood from both parents when available. Array comparative genomic hybridization was performed on patients' skin and blood samples.

RESULTS

Narrow and broad bands along the lines of Blaschko were observed in 8 and 5 patients, respectively. Musculoskeletal, acral; ophthalmologic; and dental anomalies were observed in 7 patients. A chromosomal or monogenic cause of syndromic PCH was identified in all patients: 3 mosaic chromosomal abnormalities found (trisomy 7, trisomy 14, and 13q13-ter deletion) and pathological de novo germline and somatic mutations in 3 (NBEA, USP9X, and DDX3X) and 2 patients (NIPBL and RHOA) respectively.

LIMITATIONS

Single-centre and retrospective study.

CONCLUSION

Through better clinical characterization of syndromic PCH, we will improve our understanding of these disorders. We believe that all patients with PCH with systemic findings should undergo comprehensive genomic evaluations of lesional skin and peripheral blood.

Biallelic variants in GTF3C3 encoding a subunit of the TFIIIC2 complex are associated with neurodevelopmental phenotypes in humans and zebrafish

RNA polymerase III transcribes essential non-coding RNAs, a process regulated by transcription factors TFIIIB and TFIIIC. Although germline variants in TFIIIC subunit genes have been described in a few patients with neurodevelopmental disorders, the associated pathogenesis and clinical spectrum are not yet well defined. Herein, we describe the identification of biallelic variants in GTF3C3, which encodes a key component of the TFIIIC subunit, in four patients from three unrelated families of different ethnicities collected through GeneMatcher. The patients exhibited microcephaly, developmental delay, intellectual disability and distinctive dysmorphic facies that appear recognizable in very young children. Their brain imaging showed brain atrophy with predominant cerebellar involvement, as well as hypoplasia of the frontal lobes and one patient had moderate to severe simplified gyral pattern. Seizures were observed in half of the patients. Exome/genome sequencing revealed four different GTF3C3 variants including three missense (p.Cys172Gly, p.Val427Phe and p.Ala509Thr) and one nonsense variant (p.Arg717Ter). Missense variants were not present in known genetic databases and occurred in highly conserved residues. Knockout of the GTF3C3 ortholog in zebrafish recapitulated the key clinical symptoms including microcephaly, brain anomalies and seizure susceptibility. We also observed reduced RNA polymerase III target gene expression in the zebrafish knockout model. This study describes a new neurodevelopmental syndrome in humans and zebrafish associated with biallelic GTF3C3 variants and underscores the need for further research into the biological impacts of variants in TFIIIC-linked genes and their contribution to RNA polymerase III-related pathologies.

Biallelic variants in GTF3C5, a regulator of RNA polymerase III-mediated transcription, cause a multisystem developmental disorder

General transcription factor IIIC subunit 5 (GTF3C5) encodes transcription factor IIIC63 (TFIIIC63). It binds to DNA to recruit another transcription factor, TFIIIB, and RNA polymerase III (Pol III) to mediate the transcription of small noncoding RNAs, such as tRNAs. Here, we report four individuals from three families presenting with a multisystem developmental disorder phenotype with biallelic variants in GTF3C5. The overlapping features include growth retardation, developmental delay, intellectual disability, dental anomalies, cerebellar malformations, delayed bone age, skeletal anomalies, and facial dysmorphism. Using lymphoblastoid cell lines (LCLs) from two affected individuals, we observed a reduction in TFIIIC63 protein levels compared to control LCLs. Genome binding of TFIIIC63 protein is also reduced in LCL from one of the affected individuals. Additionally, approximately 40% of Pol III binding regions exhibited reduction in the level of Pol III occupancy in the mutant genome relative to the control, while approximately 54% of target regions showed comparable levels of Pol III occupancy between the two, indicating partial impairment of Pol III occupancy in the mutant genome. Yeasts with subject-specific variants showed temperature sensitivity and impaired growth, supporting the notion that the identified variants have deleterious effects. gtf3c5 mutant zebrafish showed developmental defects, including a smaller body, head, and eyes. Taken together, our data show that GTF3C5 plays an important role in embryonic development, and that biallelic variants in this gene cause a multisystem developmental disorder. Our study adds GTF3C5-related disorder to the growing list of genetic disorders associated with Pol III transcription machinery.