Tunisian Maturity-Onset Diabetes of the Young: A Short Review and a New Molecular and Clinical Investigation

Abstract

Introduction/aims: Maturity-Onset Diabetes of the Young (MODY) is a monogenic non-autoimmune diabetes with 14 different genetic forms. MODY-related mutations are rarely found in the Tunisian population. Here, we explored MODY related genes sequences among seventeen unrelated Tunisian probands qualifying the MODY clinical criteria.

Materials and methods: The GCK and HNF1A genes were systematically analyzed by direct sequencing in all probands. Then, clinical exome sequencing of 4,813 genes was performed on three unrelated patients. Among them, 130 genes have been reported to be involved in the regulation of glucose metabolism, β-cell development, differentiation and function. All identified variants were analyzed according to their frequencies in the GnomAD database and validated by direct sequencing.

Results: We identified the previously reported GCK mutation (rs1085307455) in one patient. The clinical features of the MODY2 proband were similar to previous reports. In this study, we revealed rare and novel alterations in GCK (rs780806456) and ABCC8 (rs201499958) genes with uncertain significance. We also found two likely benign alterations in HNF1A (rs1800574) and KLF11 (rs35927125) genes with minor allele frequencies similar to those depicted in public databases. No pathogenic variants have been identified through clinical exome analysis.

Conclusions: The most appropriate patients were selected, following a strict clinical screening approach, for genetic testing. However, the known MODY1-13 genes could not explain most of the Tunisian MODY cases, suggesting the involvement of unidentified genes in the majority of Tunisian affected families.

Keywords: GCK; HNF1A; MODY; Sanger sequencing; clinical exome sequencing; genetic testing.

Figure 1

Clinical and genetic characterization of Tunisian MODY patients reported in previous studies. The clinical and familial inclusion criteria were specific for each study. Pathogenic variants and polymorphisms found in Tunisian patients were mainly reported in the most prevalent MODY genes. Functional studies have not been performed. OHA, oral hypoglycemic agents.

Figure 2

Mutation analysis of the GCK gene. Panel (A) Pedigree of the family F1 identified with heterozygous GCK variant (NM_000162.5: c.571C>T, p.Arg191Trp). The generations within the family are indicated by roman numerals. Squares and circles represent male and female family members, respectively. Normal individuals are shown as a clear symbol. Black-filled symbols denote patients with diabetes. A line through a symbol denotes the deceased. Green arrows indicate available DNA members. OHA, oral hypoglycemic agents; FPG, fasting plasma glucose; NT, not tested. The genotype is shown underneath each symbol. M and N denote mutant and wild-type alleles, respectively. The age of diabetes onset, glycemic control (the latest FPG or HbA1c measurements), and treatment follow-up are indicated directly below the genotype. Panel (B) Electropherogram analysis of GCK gene in the family F1. Mutated nucleotide on the chromatographs is depicted with an arrow. Amino acid substitution is indicated in red.

Figure 3

Mutation analysis of the GCK and KLF11 genes. Panel (A) Pedigree of the family F2 identified with GCK [NM_000162.5: c.774C>T, p.(Gly258Gly)] and KLF11 variants (NM_003597.5: c.185A>G, p.Gln62Arg). OHA, oral hypoglycemic agents; NT, not tested; n.a, Not available. M and N denote mutant and wild-type alleles, respectively. Clinical information such as the age of diabetes onset, treatment follow-up, and last HbA1c measurement is indicated directly below the genotype for p.(Gly258Gly) and p.Gln62Arg variants. Panel (B) Electropherogram analysis of GCK gene in the family F2. Panel (C) Validation of the KLF11 c.185A>G variant segregation by Sanger sequencing. Mutated nucleotides on the chromatographs are depicted with an arrow. Amino acid substitution is indicated in red.

Figure 4

Mutation analysis of the HNF1A gene. Panel (A) Pedigree of the family F3 identified with HNF1A variant (NM_000545.8: c.293C>T, p.Ala98Val). OHA, oral hypoglycemic agents; NT, not tested; n.a, Not available. M and N denote mutant and wild-type alleles, respectively. The age of diabetes onset, treatment follow-up, and last HbA1c measurement are indicated directly below the genotype (were available). Panel (B) Electropherograms showing co-segregation of the heterozygous variant c.293C>T in HNF1A gene to the phenotype observed in the proband and his father. Proband III.1 and his father II.10 carry heterozygous alleles (C/T) linked to the phenotype whereas non-affected subjects (mother II.9 and siblings III.2, III.3) carry homozygous alleles (C/C). Mutated nucleotide on the chromatographs is depicted with an arrow. Amino acid substitution is indicated in red.

Figure 5

Mutation analysis of the ABCC8 gene. Panel (A) Pedigree of the family F4 identified with ABCC8 variant [NM_001287174.2: c.2978G>A, p.(Arg993His)]. FPG, fasting plasma glucose; NT, not tested; n.a, Not available. M and N denote mutant and wild-type alleles, respectively. The age of diabetes onset, glycemic control (the latest FPG or HbA1c measurements), complications of diabetes, and treatment follow-up are indicated directly below the genotype. Panel (B) Validation of the ABCC8 c.2978G>A variant segregation by Sanger sequencing. Mutated nucleotide on the chromatographs is depicted with an arrow. Amino acid substitution is indicated in red.