A Review of Functional Characterization of Single Amino Acid Change Mutations in HNF Transcription Factors in MODY Pathogenesis.

Mutations in HNF transcription factor genes cause the most common subtypes of maturity-onset of diabetes of youth (MODY), a monogenic form of diabetes mellitus. Mutations in the HNF1-α, HNF4-α, and HNF1-ß genes are primarily considered as the cause of MODY3, MODY1, and MODY5 subtypes, respectively. Although patients with different subtypes display similar symptoms, they may develop distinct diabetes-related complications and require different treatments depending on the type of the mutation. Genetic analysis of MODY patients revealed more than 400 missense/nonsense mutations in HNF1-α, HNF4-α, and HNF1-ß genes, however only a small portion of them are functionally characterized. Evaluation of nonsense mutations are more direct as they lead to premature stop codons and mostly in mRNA decay or nonfunctional truncated proteins. However, interpretation of the single amino acid change (missense) mutation is not such definite, as effect of the variant may vary depending on the location and also the substituted amino acid. Mutations with benign effect on the protein function may not be the pathologic variant and further genetic testing may be required. Here, we discuss the functional characterization analysis of single amino acid change mutations identified in HNF1-α, HNF4-α, and HNF1-ß genes and evaluate their roles in MODY pathogenesis. This review will contribute to comprehend HNF nuclear family-related molecular mechanisms and to develop more accurate diagnosis and treatment based on correct evaluation of pathologic effects of the variants.

Whole exome sequencing reveals novel candidate gene variants for MODY.

Maturity-onset diabetes of the young (MODY) is a monogenic subtype of diabetes mellitus. Although 14 genes were associated to different subtypes of MODY, 30-40% of MODY patients have unidentified genetic mutations. In this study, we conducted whole exome sequencing (WES) in four Turkish MODY suspected patients in two families who do not carry any pathological variants in four frequent MODY genes (HNF1A, GCK, HNF1B and HNF4A). Initially, the variants were scanned for the known 14 MODY genes and a gene set related to glucose metabolism. Secondly, the destructive (frameshift, inframe insertion/deletion, initiator codon variant, splice acceptor/ donor variant, stop gained/lost) and missense variants in novel candidate genes shared by two patients of the same family were assessed by different bioinformatic tools. As a result, a total of three rare and novel probably pathogenic heterozygous missense variants (p.His307Gln in c-Myc, p.Asp129Asn in CDK4 and p.Gly107Ser in ARHGDIA) in candidate genes were identified in two families. This study revealed the presence of nonsynonymous alterations in novel candidate genes which were implicated in the insulin secretion. This is the first WES study which reveals novel candidate genes in Turkish MODY patients although functional analyses are required to confirm the pathogenicity of these variants.

SCN1A gene sequencing in 46 Turkish epilepsy patients disclosed 12 novel mutations.

PURPOSE: The SCN1A gene is one of the most commonly mutated human epilepsy genes associated with a spectrum of phenotypes with variable degrees of severity. Despite over 1200 distinct mutations reported, it is still hard to draw clear genotype-phenotype relationships, since genetic and environmental modifiers contribute to the development of a particular disease caused by an SCN1A mutation. We aimed to initiate mutational screening of the SCN1A gene in Turkey and advance further our understanding of the relationship between the SCN1A sequence alterations and disease phenotypes such as GEFS+, DS and related epileptic encephalopathies. METHODS: Mutational analysis of the SCN1A gene was carried out in 46 patients with DS, late-onset DS, GEFS+ and unspecified EE using either direct Sanger sequencing of the coding regions and exon/intron boundaries or massively parallel sequencing. RESULTS: Nineteen point mutations, 12 of which were novel were identified, confirming the clinical diagnosis of the patients. Patients with a mutation (either truncating or missense) on linker regions had significantly later disease onset than patients with mutations in homology regions. The presence of SCN1A mutations in two clinically unclassified patients supported the association of SCN1A mutations with a wide range of phenotypes. CONCLUSION: The conventional Sanger sequencing method was successfully initiated for the detection of SCN1A point mutations in Turkey in epilepsy patients. Furthermore, a modified strategy of massively parallel pyro-sequencing was also established as a rapid and effective mutation detection method for large genes as SCN1A.