Fig 2. Retroviruses hijack innate chromatin loops to drive oncogene expression.

(a) A model for the existing chromatin state around Ccnd1. (b) When a retrovirus integrates proximal to the Ccnd1 promoter, it drives overexpression through its retroviral enhancer elements and this leads to tumorigenesis. (c) When a retrovirus integrates upstream of Ccnd1 within the chromatin loop, the retroviral enhancers have no physical contact with the Ccnd1 promoter. This integration has no effect on gene expression and no tumor develops. (d) When a retrovirus integrates distal to Ccnd1 but at the base of a chromatin loop, it is in physical contact with the gene promoter and the enhancer elements can act on this promoter. This scenario leads to tumorigenesis.