Molecular genetic study of Usher Syndrome in a cohort of consanguineous families from the local population

Abstract

Retinitis pigmentosa is most prevalent form of inherited retinal degenerations (IRDs) that affect 1/4000 individuals worldwide. Most of the RP cases are non-syndromic while 20 to 30% of cases are associated with syndromic RP. Usher syndrome is most common form of RP that affects 1 to 4 in 25000 individuals. Congenital severe-to profound sensorineural hearing loss with RP and, in rare cases, vestibular dysfunction defines USH. USH is divided into three clinical types; USH1, USH2 and USH3 depending upon the severity, age of onset and progression of disease. Each type is genetically heterogenous with many genes that are cause of each specific type.75% cases in USH1 have USH1B subtype that occur by mutation in MYO7A gene that code for myosin VIIA. This USH1 subtype USH1B is most common in Pakistani population with 42.9% affected people followed by USH1D with 28.6% and USH1C and USH1F that affect 12.2%. Mutation in many other genes like ADGRV1, CDH23 and PCDH15 cause Usher syndrome. In Pakistani population, autosomal recessive diseases are common due to consanguineous marriages specially in backward areas. In this study, we selected 6 families with Usher symptoms with recessive pattern of inheritance to screen genetic mutation and polymorphisms. Blood samples were collected from hospitals and DNA extracted from white blood cells. Electrophoresis was done to check integrity of DNA followed by PCR to amplify exon 6,7 and 8 of MYO7A gene. Sanger’s sequencing of purified PCR products was done to analyze genetic variant. Novel polymorphism were detected at position g.29179T>G, g.29186C>G and g.29181G>A in MYO7A gene (table 3.2). In silico analysis show that these polymorphisms affect the splice site that further change protein features including folding, integrity and structure of protein. Genetic counselling of affected families was done to avoid cousin marriages. However no homozygous disease-causing variant was found in this study