Oncogenic activity of Cdc6 through repression of the INK4/ARF locus

Abstract

The INK4/ARF locus encodes three tumour suppressors (p15(INK4b), ARF and p16(INK4a)) and is among the most frequently inactivated loci in human cancer. However, little is known about the mechanisms that govern the expression of this locus. Here we have identified a putative DNA replication origin at the INK4/ARF locus that assembles a multiprotein complex containing Cdc6, Orc2 and MCMs, and that coincides with a conserved noncoding DNA element (regulatory domain RD(INK4/ARF)). Targeted and localized RNA-interference-induced heterochromatinization of RD(INK4/ARF) results in transcriptional repression of the locus, revealing that RD(INK4/ARF) is a relevant transcriptional regulatory element. Cdc6 is overexpressed in human cancers, where it might have roles in addition to DNA replication. We have found that high levels of Cdc6 result in RD(INK4/ARF)-dependent transcriptional repression, recruitment of histone deacetylases and heterochromatinization of the INK4/ARF locus, and a concomitant decrease in the expression of the three tumour suppressors encoded by this locus. This mechanism is reminiscent of the silencing of the mating-type HM loci in yeast by replication factors. Consistent with its ability to repress the INK4/ARF locus, Cdc6 has cellular immortalization activity and neoplastic transformation capacity in cooperation with oncogenic Ras. Furthermore, human lung carcinomas with high levels of Cdc6 are associated with low levels of p16(INK4a). We conclude that aberrant expression of Cdc6 is oncogenic by directly repressing the INK4/ARF locus through the RD(INK4/ARF) element.

The cell cycle and cancer

“…However, deregulation of the licensing system may also be a primary driver of oncogenesis, at least in some tumour types. For example, over-expression of Cdc6 or Cdt1 have been shown to be oncogenic, and deregulated Mcm7 expression has been linked to tumour formation, progression and malignant transformation in animal models [84][85][86][87][88][89][90]. Oncogenic mutations in genes upstream of the licensing machinery (eg RAS, CYCLINE and CYCLIND1 ) can also impact on tumourigenesis by causing deregulation of the licensing machinery.…”

CDC6 expression is regulated by lineage-specific transcription factor GATA1

“…5 It has also been related to leukemias that involve PML-RARA chromosomal translocations 6 and other types of cancer, probably through a direct role in overreplication 7 or silencing of the tumor suppressor Ink4a. 8 CDC6 expression is also differentially regulated during physiological endoreplication of plants, 9 and previous work in our laboratory showed that CDC6 expression is actively maintained upon cell cycle exit following terminal megakaryocytic polyploidization. 10 This endoreplication-related expression appears to be independent on E2F binding sites.…”

Vieillissement cellulaire, vieillissement et cancer

“…Gonzalez et al [19] ont décrit un méca-nisme similaire liant la répression du locus Ink4a/Arf/Ink4b et la réplication de l'ADN. Ces auteurs ont identifié une origine de répli-cation à proximité immédiate du locus INK4, qui permet la répression de p16, p19 et p15 et qui est dépendante du recrutement de cdc6 associée aux autres protéines de la réplication.…”