Head-to-Head (H2H) gene pairs are regulated by bidirectional promoters and divergently transcribed from opposite DNA strands with transcription start sites (TSSs) separated within 1 kb. H2H organization is ancient and conserved, and H2H pairs tend to exhibit similar expression patterns. Although some H2H genes have been reported to be associated with disease and cancer, there is a lack of systematic studies on H2H organization in the scenario of cancer development.
Human H2H gene pairs were identified based on GENCODE hg19 and the functional relevance of H2H pairs was explored through function enrichment and semantic similarity analysis. To investigate the association between H2H organization and carcinogenesis, pan-cancer differential analysis of H2H genes about transcriptional activity, co-expression and transcriptional regulation by transcription factors and enhancers were performed based on data from The Cancer Genome Atlas. Cox proportional hazards regression model and log-rank test were used to determine the prognostic powers of H2H pairs.
In the present study, we first updated H2H genes from 1,447 to 3,150 pairs, from which the peak group with TSS distance of 1–100 was observed as expected in our previous work. It was found that housekeeping genes, mitochondrial-functional associated genes and cancer genes tend to be organized in H2H arrangement. Pan-cancer analysis indicates that H2H genes are transcriptionally active than random genes in both normal and cancer tissues, but H2H pairs display higher correlation in cancer than in normal. Particularly, housekeeping H2H pairs are differentially correlated much more significantly than non-housekeeping H2H pairs are. Some of differentially correlated H2H pairs were found to be associated with prognosis. The alteration of TF similarity seems to contribute to differential co-expression of H2H pairs during carcinogenesis; meanwhile remote enhancers also at least partly explain the differential co-expression and co-regulation of H2H pairs.
H2H pairs tend to show much stronger positive expression correlation in cancer than in normal due to differential regulation of bidirectional promoters. The study provides insights into the significance of H2H organization in carcinogenesis and the underlying dysfunctional regulation mechanisms. Those differentially correlated H2H pairs associated with survival have the potential to be prognostic biomarkers and therapeutic targets for cancer.