Mutational analysis of exons 6 and 7 of the MYO7A gene in consanguineous Usher syndrome affected families from Sindh.

Abstract

Usher syndrome causes simultaneous loss of hearing and vision as well as vestibular dysfunction in some, but not all cases. It is the most common cause of deaf-blindness globally, with a prevalence of 4 to 17 per 100,000 individuals. It is inherited in an autosomal recessive manner. USH is classified into three clinical types: USH1, USH2, and USH3. Each of these types is further divided into 14 subtypes in total, brought on by alterations in distinct genes. Nine genes have been identified with accuracy causing the onset of USH: including MYO7A, USH1C, CDH23, PCDH15, USH1G (SANS for USH2A), ADGRV1, WHRN, and CLRN1. Many of the genes involved in USH are also found in non-syndromic RP and non-syndromic hearing variants, demonstrating the high genetic diversity of this condition. MYO7A gene is one of the most involved genes in the case of Usher syndrome and is located on chromosome 11q. It belongs to the superfamily of motor genes, reporting 70% of USH1B cases when mutated, causing severe to profound hearing loss, RP, and vestibular areflexia. Due to insufficient sources of diagnosis and rehabilitation, the prevalence rate of USH isn’t documented comprehensively on a global level. According to numerous studies, consanguineous marriages frequently result in the transmission of autosomal recessive traits, and Pakistan is one of the countries where such marriages are relatively common. In this study, we selected 4 USH affected families from Sindh, Pakistan, having a positive family history with consanguineous marriages practiced commonly. Blood samples were collected from the hospital. DNA from the blood sample was extracted in the lab of Molecular Biology, Quaid I Azam University, Islamabad. Followed by gel electrophoresis, PCR was run to amplify exons 6 and 7 of the MYO7A gene of probands from each of the 4 selected families. Sanger’s sequencing of purified PCR products showed a disease-causing variant c.640G>A in exon 7 of MYO7A in a clinically diagnosed USH patient in one of our 4 families, having congenital hearing loss, progressive RP, and vestibular areflexia, proving exon 7 as one of the hotspots in Pakistani population causing USH1B. In-silico analysis of c.640G>A highlights that this disease-causing variant changes the splice site, interfering with proper protein features and halting it from performing its function correctly, which could lead to the development of USH. However, the other 3 families do not exhibit any sequence variation in the targeted MYO7A exons, emphasizing the need for further sequencing. For future perspective, genetic counseling must be given to avoid consanguineous marriages and as the disease is highly heterogeneous, better innovations like whole genome sequencing should be done.