Moody Only Monogenic? A Narrative Review of the Rare and Low-Penetrant Genetic Variants

Abstract

Introduction: Maturity-onset diabetes of the young (MODY) represents the most frequent form of monogenic diabetes mellitus (DM), currently classified in 14 distinct subtypes according to single gene mutations involved in the differentiation and function of pancreatic β-cells.
 
 Case report: The female patient at the age of 16 years old was diagnosed with non-autoimmune DM with glycated haemoglobin (HbA1c) value of 6.5%. After 6 years of dietary and lifestyle interventions due to the worsening of the glycometabolic profile, oral hypoglicemic therapy was initiated including at first only metformin, and subsequently also sulfonylureas and DPP-IV inhibitor. 11 years after diagnosis glargine insulin was started, associated 3 years later with Lispro insulin because of the worsening of the post-prandial glycaemia. According to the genetic testing a compound heterozygosis of two novel variants of uncertain/unknown clinical significance of BLK and RFX6 genes, respectively c.1013T>C (Ile338Thr) and c.1072G>A (Val358Ile), was identified. The RFX6 variant was also detected in the patient’s mother genome, while the BLK variant in her father’s.
 
 Discussion: Up to 11% of MODY has unknown etiology, suggesting that the genetic landscape is still to be explored. Recently, novel causal genes, involved in the differentiation and function of β-cells, have been identified such as RFX6, NKX2.2, NKX6.1, WFS1 and PCBD1. Genetic and clinical features of MODY variants remain highly heterogeneous, with no direct genotype-phenotype correlation especially in the low-penetrant subtypes. Thus, the aim of our paper is to present additionally to such rare case of a compound heterozygosity BLK/RFX6 underlying MODY, also a brief review of literature focused on the rare and reduced-penetrant variants, as well as on some controversies regarding this subtype of monogenic diabetes. 
 
 Conclusions: Due to the limited data available, the assessment of MODY related-genes pathogenicity remains challenging especially in the setting of rare and low-penetrant subtypes. In consideration of the crucial importance of an accurate diagnosis, prognosis and management of MODY, more studies are warranted to further investigate its genetic landscape, the genotype-phenotype correlation, as well as the pathogenetic contribution of the non-genetic modifiers in this cohort of patients.