Diagnostic utility of whole exome sequencing inundiagnosed inborn errors of metabolism

Inborn errors of metabolism (IEMs) are diseases that involve disorders in biochemical pathways and have diagnostic difficulties due to clinical and genetic heterogeneity. Mitochondrial diseases are the most common form of IEMs associated with nuclear and mitochondrial pathogenic variants. Advances in DNA sequencing technologies have given an important support to elucidating the genetic etiology of such diseases, which are difficult to diagnose. The aim of this thesis is to determine the molecular etiologies for undiagnosed IEMs using whole-exome sequencing (WES). Twenty-six affected patients from 20 undiagnosed families, 14 of which were mitochondrial disease were included. Thermo Scientific™ IonGeneStudioS5 System was used for WES and mtDNA sequencing. Transcript studies were performed from cultured muscle/blood cells. Eighteen different pathogenic/likely pathogenic variants were shown in TK2, POLG, EARS2, MICOS13, ECHS1, NDUFAF6, FBXL4, CAPN3, DYSF, ALG8, OXCT1, DHTK2, SMARCAL1 genes in nineteen affected cases from thirteen families. All of the diagnosed variants confirmed by Sanger sequencing and segregation in families with available individuals. Altogether, three novel and five ultra-rare variants first time associated with disease in this study; EARS2 c.1283delC ((p.Pro428Leufs*)) and c.319C>T (p.Arg107Cys), ALG8 c.479A>T (p.His160Leu), ECHS1 c.202G>A (p.Glu68Lys), NDUFAF6 c.479delA (p.(N162Ifs*27)), OXCT1 c.1370C>T(p.Thr457Ile), c.1173-139G>T SMARCAL1 c.1027_1034delTTCGAGGC, (p.(F343Rfs*13)). The clinical, radiological, biochemical and histopathological results of the cases supported the gene and variant associations. Molecular genetic diagnosis was achieved at a rate of 65% for all of the families and 50% of the mitochondrial disease group. Genetic result allowed two patients to receive early access to the medicine for TK2 deficiency. Genetic counseling presented for all of the families. Six of the families remain undiagnosed, and planned for whole genome sequencing in future projects.