The majority of autosomal recessive nanophthalmos and posterior microphthalmia can be attributed to biallelic sequence and structural variants in MFRP and PRSS56

This study aimed to genetically and clinically characterize a unique cohort of 25 individuals from 21 unrelated families with autosomal recessive nanophthalmos (NNO) and posterior microphthalmia (MCOP) from diferent ethnicities. An ophthalmological assessment in all families was followed by targeted MFRP and PRSS56 testing in 20 families and whole-genome sequencing in one family. Three families underwent homozygosity mapping using SNP arrays. Eight distinct MFRP mutations were found in 10/21 families (47.6%), fve of which are novel including a deletion spanning the 5′ untranslated region and the frst coding part of exon 1. Most cases harbored homozygous mutations (8/10), while a compound heterozygous and a monoallelic genotype were identifed in the remaining ones (2/10). Six distinct PRSS56 mutations were found in 9/21 (42.9%) families, three of which are novel. Similarly, homozygous mutations were found in all but one, leaving 2/21 families (9.5%) without a molecular diagnosis. Clinically, all patients had reduced visual acuity, hyperopia, short axial length and crowded optic discs. Retinitis pigmentosa was observed in 5/10 (50%) of the MFRP group, papillomacular folds in 12/19 (63.2%) of MCOP and in 3/6 (50%) of NNO cases. A considerable phenotypic variability was observed, with no clear genotype-phenotype correlations. Overall, our study represents the largest NNO and MCOP cohort reported to date and provides a genetic diagnosis in 19/21 families (90.5%), including the frst MFRP genomic rearrangement, ofering opportunities for gene-based therapies in MFRP-associated disease. Finally, our study underscores the importance of sequence and copy number analysis of the MFRP and PRSS56 genes in MCOP and NNO

Loss of function of RIMS2 causes a syndromic congenital cone-rod synaptic disease with neurodevelopmental and pancreatic involvement

Congenital cone-rod synaptic disorder (CRSD), also known as incomplete congenital stationary night blindness (iCSNB), is a non-progressive inherited retinal disease (IRD) characterized by night blindness, photophobia, and nystagmus, and distinctive electroretinographic features. Here, we report bi-allelic RIMS2 variants in seven CRSD-affected individuals from four unrelated families. Apart from CRSD, neurodevelopmental disease was observed in all affected individuals, and abnormal glucose homeostasis was observed in the eldest affected individual. RIMS2 regulates synaptic membrane exocytosis. Data mining of human adult bulk and single-cell retinal transcriptional datasets revealed predominant expression in rod photoreceptors, and immunostaining demonstrated RIMS2 localization in the human retinal outer plexiform layer, Purkinje cells, and pancreatic islets. Additionally, nonsense variants were shown to result in truncated RIMS2 and decreased insulin secretion in mammalian cells. The identification of a syndromic stationary congenital IRD has a major impact on the differential diagnosis of syndromic congenital IRD, which has previously been exclusively linked with degenerative IRD

Unraveling the molecular basis genetically heterogeneous developmental eye disorders

Vision is one of the most important senses of a human being. It is estimated that 285 million people worldwide suffer from visual impairment. Although the etiology of visual impairment is rather complex, genetic factors play an important role. In this respect, developmental eye disorders have a huge impact on childhood visual impairment and blindness. Here, we focus on three types of developmental eye disorders including: idiopathic infantile nystagmus (IIN), Leber congenital amaurosis (LCA), nanophthalmos and posterior microphthalmia (NNO and MOCP) caused by mutations encoding many different signaling and structural proteins in the developing eye. The pursuit of the molecular diagnosis in these conditions is challenging as they have been hampered by a tremendous clinical and genetic heterogeneity where different modes of Mendelian inheritance can be found. In the first study of this thesis the role of mutations and copy number variations (CNV) of FRMD7 and GPR143 was investigated in 49 unrelated Belgian probands with a possible diagnosis of X-linked idiopathic infantile nystagmus (XLIIN). Our comprehensive molecular workflow consisted of targeted gene screening using Sanger sequencing and next generation sequencing (NGS), followed by multiplex ligation–dependent probe amplification. This revealed a genetic defect in FRMD7 in 11 of 49 (22.4%) of the studied probands, including a deletion of 1.29 Mb containing FRMD7 in a syndromic patient with nystagmus and autism spectrum disorder. This study expanded the mutational spectrum of FRMD7 in XLIIN and generated a discovery cohort of IIN patients potentially harboring either hidden genetic variation such as deep intronic or cis-regulatory mutations within FRMD7 or mutations in other genes at known or novel loci. The second study was targeting 15 consanguineous Saudi families including 20 patients with an initial diagnosis of LCA. A combined approach of homozygosity mapping, targeted gene screening and whole exome sequencing (WES) was applied to unveil the underlying genetic cause in these families. Overall, this strategy identified the causative mutations in all but one (14/15, 93.3%) of the studied families, guiding refinement of the clinical diagnosis. Specifically, twelve unique homozygous mutations were identified in eight known LCA genes: CABP4, CEP290, CRB1, GUCY2D, MERTK, RDH12, RPGRIP1 and SPATA7. In addition, one novel mutation was found in the recently identified achromatopsia gene ATF6, leading to a clinical diagnosis of achromatopsia in retrospect. Apart from mutations in known disease genes, a homozygous pathogenic variant was found in RIMS2 (Regulating Synaptic Membrane Exocytosis 2) in two affected sibs with LCA and autism spectrum disorder. The expression of RIMS2 in the retina and several brain structures of human, mouse and zebrafish is consistent with the observed phenotype in these sibs. Moreover, RIMS2 is known to play a role in maintaining the photoreceptor ribbon synapse and has been shown previously to be associated with Asperger syndrome, an autism subtype. Interestingly, the paralog RIMS1 was previously shown to be mutated in autosomal dominant cone-rod dystrophy (CORD7) and to be implicated in autism. The identification of RIMS2 as novel candidate gene for LCA and autism link a synaptic protein with autosomal recessive IRD and autism. Finally, the third study of this doctoral project investigated the role of MFRP and PRSS56 mutations in nanophthalmos (NNO) and posterior microphthalmia (MCOP) in a cohort of 21 unrelated patients from a different ethnic origin. The genetic approach consisted of targeted gene screening and homozygosity mapping in families with a consanguineous background. This revealed the underlying genetic cause in all but two (19/21, 90.5%) of the studied families, with either homozygous or compound heterozygous distinct mutations in MFRP (10/21, 47.6%) and PRSS56 (9/21, 42.9%). Genotype-phenotype correlations provided insight into the phenotypic variability among these genes. Particularly, the MFRP-positive group showed a higher tendency to develop peripheral retinal pigmentation while this was rare in the PRSS56- associated group. Moreover, the families with a diagnosis of MCOP had papillomacular folds in common, which was infrequent in families diagnosed with NNO. The identification of new families with MFRP mutations might offer opportunities for potential gene-based therapies