Current mutation discovery approaches in Retinitis Pigmentosa

Abstract

With a worldwide prevalence of about 1 in 3500-5000 individuals, Retinitis Pigmentosa (RP) is the most common form of hereditary retinal degeneration. It is an extremely heterogeneous group of genetically determined retinal diseases leading to progressive loss of vision due to impairment of rod and cone photoreceptors. RP can be inherited as an autosomal-recessive, autosomal-dominant, or X-linked trait. Non-Mendelian inheritance patterns such as digenic, maternal (mitochondrial) or compound heterozygosity have also been reported. To date, more than 65 genes have been implicated in syndromic and non-syndromic forms of RP, which account for only about 60% of all RP cases. Due to this high heterogeneity and diversity of inheritance patterns, the molecular diagnosis of syndromic and non-syndromic RP is very challenging, and the heritability of 40% of total RP cases worldwide remains unknown. However new sequencing methodologies, boosted by the human genome project, have contributed to exponential plummeting in sequencing costs, thereby making it feasible to include molecular testing for RP patients in routine clinical practice within the coming years. Here, we summarize the most widely used state-of-the-art technologies currently applied for the molecular diagnosis of RP, and address their strengths and weaknesses for the molecular diagnosis of such a complex genetic disease.

Navigating the current landscape of clinical genetic testing for inherited retinal dystrophies

“…12,[17][18][19] As testing improved, small gene panels using Sanger sequencing became available in cases where both the specific clinical diagnosis (such as CRD) and the inheritance pattern was known. Given the complexities of inherited retinal disorders, however, these was not always known.…”

“…Chromosomal microarray to evaluate for copy-number variants or a contiguous gene deletion would be most appropriate for patients presenting with a retinal dystrophy in addition to other congenital anomalies, such as facial dysmorphism, developmental delay/intellectual disabilities, and/or other systemic diseases/malformations that do not seem to fit a known syndrome. 12 A simultaneous karyotype analysis could also be considered; however, its diagnostic yield would be lower than that of a microarray analysis. 22 Disease-specific Sanger or next-generation sequencing (NGS) gene panels are appropriate options for patients with a relatively certain clinical diagnosis that has genetic heterogeneity.…”

“…25 Because Sanger sequencing is unable and NGS platforms are not currently optimized to detect copy-number changes, commercial laboratories also offer disease-specific deletion and duplication panels to complement the sequencing panels. 12 Several commercial laboratories offer a gene-targeted comparative genomic hybridization array to identify larger deletions and duplications in all of the genes included in their sequencing panels. Most commercial laboratories also still offer single-gene dosage assays to detect possible large/whole-gene deletions.…”

“…Few Mendelian disorders exhibit the degree of genetic heterogeneity demonstrated by RP, one of the most common retinal dystrophies, with a worldwide prevalence of 1 in 4,000. [11][12][13] Over 100 genes have been associated with this condition. 14 Moreover, other retinal dystrophies, including cone-rod dystrophy (CRD), cone dystrophy, and Stargardt disease, also exhibit genetic heterogeneity.…”

“…For example, mutations in the ABCA4 gene (OMIM 601691) have been associated with several distinct hereditary retinal dystrophies and can have highly variable retinal appearance on clinical examination, ranging from normal to widespread pigmentary changes (Stargardt disease, CRD, cone dystrophy, and RP). 12 Genetic testing options have greatly expanded rapidly over the past few years with the development of new technologies. Now multigene panels and whole-exome sequencing (exome sequencing) are available in addition to traditional Sanger sequencing and polymerase chain reaction-based testing.…”