G-quadruplex formation in GCK and TM6SF2 are targets for differential DNA methylation in metabolic disease and hepatocellular carcinoma patients

Abstract

The alarming increase in global rates of metabolic diseases (MetDs) and their association with cancer risk renders them a considerable burden on our society. The interplay of environmental and genetic factors in causing MetDs may be reflected in DNA methylation patterns, particularly at non-canonical (non-B) DNA structures, such as G-quadruplexes (G4s) or R-loops. To gain insight into the mechanisms of MetD progression, we focused on DNA methylation and functional analyses on intragenic regions of two MetD risk genes, the glucokinase (GCK) exon 7 and the transmembrane 6 superfamily 2 (TM6SF2) intron 2-exon 3 boundary, which harbor non-B DNA motifs for G4s and R-loops. Pyrosequencing of 148 blood samples from a nested cohort study revealed significant differential methylation in GCK and TM6SF2 in MetD patients versus healthy controls. Furthermore, these regions harbor hypervariable and differentially methylated CpGs also in hepatocellular carcinoma versus normal tissue samples from The Cancer Genome Atlas (TCGA). Permanganate/S1 nuclease footprinting with direct adapter ligation (PDAL-Seq), native polyacrylamide DNA gel electrophoresis and circular dichroism (CD) spectroscopy revealed the formation of G4 structures in these regions and demonstrated that their topology and stability is affected by DNA methylation. Detailed analyses including histone marks, chromatin conformation capture data, and luciferase reporter assays, highlighted the cell-type specific regulatory function of the target regions. Based on our analyses, we hypothesize that changes in DNA methylation lead to topological changes, especially in GCK exon 7, and cause the activation of alternative regulatory elements or potentially play a role in alternative splicing. Our analyses provide a new view on the mechanisms underlying the progression of MetDs and their link to hepatocellular carcinomas, unveiling non-B DNA structures as important key players already in early disease stages.