Homozygosity Mapping and Targeted Sanger Sequencing Identifies Three Novel CRB1 (Crumbs homologue 1) Mutations in Iranian Retinal Degeneration Families

Whole exome sequencing in 17 consanguineous Iranian pedigrees expands the mutational spectrum of inherited retinal dystrophies

Inherited retinal dystrophies (IRDs) constitute one of the most heterogeneous groups of Mendelian human disorders. Using autozygome-guided next-generation sequencing methods in 17 consanguineous pedigrees of Iranian descent with isolated or syndromic IRD, we identified 17 distinct genomic variants in 11 previously-reported disease genes. Consistent with a recessive inheritance pattern, as suggested by pedigrees, variants discovered in our study were exclusively bi-allelic and mostly in a homozygous state (in 15 families out of 17, or 88%). Out of the 17 variants identified, 5 (29%) were never reported before. Interestingly, two mutations (GUCY2D:c.564dup, p.Ala189ArgfsTer130 and TULP1:c.1199G > A, p.Arg400Gln) were also identified in four separate pedigrees (two pedigrees each). In addition to expanding the mutational spectrum of IRDs, our findings confirm that the traditional practice of endogamy in the Iranian population is a prime cause for the appearance of IRDs.

Whole exome sequencing and homozygosity mapping reveals genetic defects in consanguineous Iranian families with inherited retinal dystrophies

Inherited retinal dystrophies (IRDs), displaying pronounced genetic and clinical heterogeneity, comprise of a broad range of diseases characterized by progressive retinal cell death and gradual loss of vision. By the combined use of whole exome sequencing (WES), SNP-array and WES-based homozygosity mapping, as well as directed DNA sequencing (Sanger), we have identified nine pathogenic variants in six genes (ABCA4, RPE65, MERTK, USH2A, SPATA7, TULP1) in 10 consanguineous Iranian families. Six of the nine identified variants were novel, including a putative founder mutation in ABCA4 (c.3260A>G, p.Glu1087Gly), detected in two families from Northeastern Iran. Our findings provide additional information to the molecular pathology of IRDs in Iran, hopefully contributing to better genetic counselling and patient management in the respective families from this country.

Retinal Dystrophies and the Road to Treatment: Clinical Requirements and Considerations

: Retinal dystrophies (RDs) comprise relatively rare but devastating causes of progressive vision loss. They represent a spectrum of diseases with marked genetic and clinical heterogeneity. Mutations in the same gene may lead to different diagnoses, for example, retinitis pigmentosa or cone dystrophy. Conversely, mutations in different genes may lead to the same phenotype. The age at symptom onset, and the rate and characteristics of peripheral and central vision decline, may vary widely per disease group and even within families. For most RD cases, no effective treatment is currently available. However, preclinical studies and phase I/II/III gene therapy trials are ongoing for several RD subtypes, and recently the first retinal gene therapy has been approved by the US Food and Drug Administration for RPE65-associated RDs: voretigene neparvovec-rzyl (Luxturna). With the rapid advances in gene therapy studies, insight into the phenotypic spectrum and long-term disease course is crucial information for several RD types. The vast clinical heterogeneity presents another important challenge in the evaluation of potential efficacy in future treatment trials, and in establishing treatment candidacy criteria. This perspective describes these challenges, providing detailed clinical descriptions of several forms of RD that are caused by genes of interest for ongoing and future gene or cell-based therapy trials. Several ongoing and future treatment options will be described.

CRB1-Related Cystic Maculopathy in Twins Conceived Through Heterologous Fertilization With Variant-Carrying Oocytes

Cystic maculopathy has been associated with genetic disorders such as retinitis pigmentosa, X-linked retinoschisis, cone dystrophy, and foveal retinoschisis. Familial foveal retinoschisis was recently described as a rare disease caused by CRB1 variants. The authors report the phenotype-genotype pattern of a pair of dizygotic twins with early-onset cystic maculopathy due to CRB1 pathogenic variants. The twins were conceived by heterologous fertilization with variant-carrying oocytes. The probands were monitored for a period of 4 years. Next generation sequencing of a panel of genes responsible for retinal dystrophies was performed. Both children carried three pathogenic variants in CRB1: a novel heterozygous truncating variant p.(Val855*) inherited from the father and two known heterozygous missense variants, p.[(Phe144Val; Thr745Met)], inherited from the oocyte donor. The findings confirm that CRB1 variants can be responsible for foveal retinoschisis with variable clinical expressivity ranging from schitic macular alteration to early-onset forms of cystic maculopathy. The authors highlight the importance of exome analysis of gamete donors to assess the likelihood of recessively inherited disorders by means of a prediction algorithm able to combine parent and donor exome data. [J Pediatr Ophthalmol Strabismus. 2020;57:e19-e24.].

A novel deletion mutation in GUCY2D gene may be responsible for Leber congenital amaurosis-1 disease: A case report

Purpose

To investigate genetic mutation(s) underlying retinal degeneration in a male patient.

Methods

A seven-year-old male patient was referred to receive genetic counseling and molecular testing. Clinical examination was performed by slit-lamp examination and electroretinography (ERG). Molecular testing was undertaken through arrayed-primer extension (APEX) and Sanger sequencing.

Results

Slit-lamp examination and flat ERG were in favor of Leber congenital amaurosis (LCA) disease as well as fundus findings. The genetic screening revealed two novel homozygote deletion and duplication variants in intron 15 and exon 16 of the GUCY2D gene. Segregation analysis in the family supports the probable contribution of these two novel mutations in clinical representations of the patient.

Conclusions

This report provides more information about LCA disease and its relevant mutations in Iran. Considering the overlapping phenotypes observed in retinal degenerative disorders, comprehensive molecular testing is needed for precise diagnosis.

Possible dual contribution of a novel GUCY2D mutation in the development of retinal degeneration in a consanguineous population

Molecular characterization of novel mutations in Leber Congenital Amaurosis (LCA) disease improves the disease diagnosis and contributes to the development of preventive and therapeutic approaches. We studied an isolated inbred population in Iran with a high prevalence of retinal degeneration with clinical variability. The clinical examinations were performed on eight patients belonging to three consanguineous families. The identical-by-descent (IBD) mapping technique was employed to identify the shared loci in patients. Subsequently, Sanger sequencing of the GUCY2D gene, in silico analysis, as well as segregation study were conducted. The whole-exome sequencing method was applied for negative cases of GUCY2D mutation, followed by segregation study in suspected variants among families. A novel deletion mutation in the GUCY2D gene can explain the emergence of LCA-1 in most patients but not all. Besides, a heterozygous variant of uncertain significance (VUS) was observed in the BEST1 gene in some healthy and participant patients. These results further support inter/intra-familial clinical heterogeneity in retinal dystrophy and suggest that screening the GUCY2D gene would be needed for the diagnosis of LCA in Iranian people living in the central regions. The variant in the BEST1 gene might be considered a benign heterozygous variant; however, we hypothesized a possible double heterozygosity in both GUCY2D and BEST1 genes that may cause the pathogenesis of cone-rod dystrophy-6 (CRD-6) disease. This would propose a new scenario for the pathogenesis of a monogenic disorder such as CRD-6 disease in which other genetic elements may be involved in the development of the disease.

Targeted next generation sequencing reveals genetic defects underlying inherited retinal disease in Iranian families

Purpose

Inherited retinal diseases (IRDs) are clinically and genetically heterogeneous showing progressive retinal cell death which results in vision loss. IRDs include a wide spectrum of disorders, such as retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), cone-rod dystrophy (CRD), and Stargardt disease (STGD1).

Methods

In this study, we performed targeted next-generation sequencing based on molecular inversion probes (MIPs) that allowed the sequence analysis of 108 IRD-associated genes in 50 Iranian IRD probands.

Results

The sequencing and variant filtering led to the identification of putative pathogenic variants in 36 out of 50 (72%) probands. Among 36 unique variants, we identified 20 novel variants in 15 genes. Four out of 36 probands carry compound heterozygous variants, and 32 probands carry homozygous variants.

Conclusions

Employing a cost-effective targeted next-generation sequencing procedure, we identified the genetic causes of different retinal disorders in the majority of Iranian families in this study.