👤 Miriam Bauwens

A homozygous loss-of-function (LoF) variant in POC5 was previously described in an individual with retinitis pigmentosa. We identified POC5 variants in 12 probands with a syndromic phenotype. We aim to define the phenotype spectrum and molecular mechanism associated with biallelic POC5 LoF variants. We studied a cohort of 12 families with bi-allelic LoF POC5 variants and performed detailed phenotype analysis. POC5 localization studies were performed in 3 proband-derived fibroblast cell lines. Detailed phenotyping of probands with POC5 variants expands the phenotype spectrum beyond ocular manifestations. This syndrome causes not only rod-cone dystrophy but also diabetes mellitus with severe insulin resistance and partial lipodystrophy, kidney disease, and muscle cramps. The POC5 protein plays an essential role during cell cycle and cilium formation. Interestingly, POC5 localization studies in 3 proband-derived fibroblast cell lines show aberrant localization suggesting a ciliary defect. The phenotypes of the 12 families in this study fit well within the ciliopathy phenotype spectrum, except for lipodystrophy, which is not common in ciliopathies. We describe a multiorgan syndrome caused by bi-allelic LoF variants in POC5. This underscores the pleiotropic effects of POC5 variants and highlights the significance of adipose tissue and metabolic dysfunction in ciliopathies. Show less

Autosomal recessive retinal dystrophies are clinically and genetically heterogeneous, which hampers molecular diagnosis. We evaluated identity-by-descent-guided Sanger sequencing or whole-exome sequencing in 26 families with nonsyndromic (19) or syndromic (7) autosomal recessive retinal dystrophies to identify disease-causing mutations. Patients underwent genome-wide identity-by-descent mapping followed by Sanger sequencing (16) or whole-exome sequencing (10). Whole-exome sequencing data were filtered against identity-by-descent regions and known retinal dystrophy genes. The medical history was reviewed in mutation-positive families. We identified mutations in 14 known retinal dystrophy genes in 20/26 (77%) families: ABCA4, CERKL, CLN3, CNNM4, C2orf71, IQCB1, LRAT, MERTK, NMNAT1, PCDH15, PDE6B, RDH12, RPGRIP1, and USH2A. Whole-exome sequencing in single individuals revealed mutations in either the largest or smaller identity-by-descent regions, and a compound heterozygous genotype in NMNAT1. Moreover, a novel deletion was found in PCDH15. In addition, we identified mutations in CLN3, CNNM4, and IQCB1 in patients initially diagnosed with nonsyndromic retinal dystrophies. Our study emphasized that identity-by-descent-guided mutation analysis and/or whole-exome sequencing are powerful tools for the molecular diagnosis of retinal dystrophy. Our approach uncovered unusual molecular findings and unmasked syndromic retinal dystrophies, guiding future medical management. Finally, elucidating ABCA4, LRAT, and MERTK mutations offers potential gene-specific therapeutic perspectives. Show less

We investigated the influence of telomere proximity and composition on the expression of an EGFP reporter gene in human cells. In transient transfection assays, telomeric DNA does not repress EGFP but rather slightly increases its expression. In contrast, in stable cell lines, the same reporter construct is repressed when inserted at a subtelomeric location. The telomeric repression is transiently alleviated by increasing the dosage of the TTAGGG repeat factor 1 (TRF1). Upon a prolongated treatment with trichostatin A, the derepression of the subtelomeric reporter gene correlates with the delocalization of HP1alpha and HP1beta. In contrast, treating the cells with 5 azacytidin, a demethylating agent, or with sirtinol, an inhibitor of the Sir2 family of deacetylase, has no apparent effect on telomeric repression. Overall, position effects at human chromosome ends are dependent on a specific higher-order organization of the telomeric chromatin. The possible involvement of HP1 isoforms is discussed. Show less