Overlaps Between CDS Regions of Protein-Coding Genes in the Human Genome: A Case Study on the NR1D1-THRA Gene Pair

For several decades, it has been known that a substantial number of genes within human DNA exhibit overlap; however, the biological and evolutionary significance of these overlaps remain poorly understood. This study focused on investigating specific instances of overlap where the overlapping DNA region encompasses the coding DNA sequences (CDSs) of protein-coding genes. The results revealed that proteins encoded by overlapping CDSs exhibit greater disorder than those from nonoverlapping CDSs. Additionally, these DNA regions were identified as GC-rich. This could be partially attributed to the absence of stop codons from two distinct reading frames rather than one. Furthermore, these regions were found to harbour fewer single-nucleotide polymorphism (SNP) sites, possibly due to constraints arising from the overlapping state where mutations could affect two genes simultaneously.While elucidating these properties, the NR1D1-THRA gene pair emerged as an exceptional case with highly structured proteins and a distinctly conserved sequence across eutherian mammals. Both NR1D1 and THRA are nuclear receptors lacking a ligand-binding domain at their C-terminus, which is the region where these gene pairs overlap. The NR1D1 gene is involved in the regulation of circadian rhythm, while the THRA gene encodes a thyroid hormone receptor, and both play crucial roles in various physiological processes. This study suggests that, in addition to their well-established functions, the specifically overlapping CDS regions of these genes may encode protein segments with additional, yet undiscovered, biological roles.