Whole exome sequencing to identify mutations in genes in Nepalese patients with rare bleeding disorders

Abstract

Rare bleeding disorders (RBDs) are among the oldest described genetic diseases, generally leading to lifelong hemorrhagic complications. These are monogenic in nature and are inherited in Mendelian patterns. The genetic cause of RBDs is the defect(s) in gene(s) coding or regulating various clotting factor(s). RBDs manifest themselves in the form of either severe or moderately severe or mild and have affected approximately 400,000 individuals worldwide. Von Willebrand disease and hemophilia A are the most common type of RBDs. Since the clinical presentations of various types of RBDs intersect with each other, only the laboratory studies may not be sufficient for the accurate diagnosis of the RBDs. Genetic studies are required in such cases. Moreover, genetic studies allow better understanding of the biology of rare bleeding disorders and the genetic information can be used for the translational application, prenatal diagnosis and the detection of carrier status, prediction of development of inhibitors and can also assist in genetic counseling. However, traditional molecular techniques have shown limitations in efficient characterization of mutations causing RBDs. In present era of high through-put sequencing, Next Generation Sequencing (whole genome sequencing and whole exome sequencing) which has emerged as a gold-standard for the identification of diseasecausing mutations in various other rare Mendelian diseases has also shown a convincing

potential

to explore the underlying genetic lesions in the patients with rare bleeding disorders. In our current study whole exome sequencing has been used for the screening of mutations in patients suffering from two rare bleeding disorders viz. Type 2 Normandy von Willebrand disease (2N VWD) and Factor X Deficiency (FXD). Sequencing was performed in Illumina platform (HiSeq 2500). We developed our own bioinformatics analysis pipeline for WES data and ended up with only one causative mutation in both the RBDs following rigorous prioritization of the variants. The causative mutation identified in FXD, c.T212C:p.F71S, which is reported as a founder effect in Algerian population has not yet been reported from the other parts of the world. In case of 2N VWD, the causative mutation identified, c.C2446T:p.R816W is one of a very common variant reported all over the world. Both the causative mutations were validated by capillary sequencing and also the carrier status among the family members was checked. We found two daughters of male patient of 2N VWD are carrier for the disorder. Key words: rare bleeding disorders, 2N VWD, FXD, whole exome sequencing, bioinformatics analysis of WES data, validation of WES results, detection of carrier status