DNA replication licensing in somatic and germ cells

Abstract

The DNA replication (or origin) licensing system ensures precise duplication of the genome in each cell cycle and is a powerful regulator of cell proliferation in metazoa. Studies in yeast, Drosophila melanogaster and Xenopus laevis have characterised the molecular machinery that constitutes the licensing system, but it remains to be determined how this important evolutionary conserved pathway is regulated in Homo sapiens. We have investigated regulation of the origin licensing factors Cdc6, Cdt1, Mcm2 and Geminin in human somatic and germ cells. Cdc6 and Cdt1 play an essential role in DNA replication initiation by loading the Mcm2-7 complex, which is required for unwinding the DNA helix, onto chromosomal origins. Geminin is a repressor of origin licensing that blocks Mcm2-7 loading onto origins. Our studies demonstrate that Cdc6, Cdt1 and Mcm2 play a central role in coordinating growth during the proliferation-differentiation switch in somatic self-renewing systems and that Cdc6 expression is rate-limiting for acquisition of replication competence in primary oocytes. In striking contrast, we show that proliferation control during male gametogenesis is not linked to Cdc6 or Mcm2, but appears to be coordinated by the negative regulator Geminin with Cdt1 becoming rate-limiting in late prophase. Our data demonstrate a striking sexual dimorphism in the mechanisms repressing origin licensing and preventing untimely DNA synthesis during meiosis I, implicating a pivotal role for Geminin in maintaining integrity of the male germline genome.

Minichromosome maintenance protein 6, a proliferation marker superior to Ki-67 and independent predictor of survival in patients with mantle cell lymphoma

“…This fact could also explain why the blastoid and classical subtype had no significant difference in the MCM6 expression as a marker for the G1 arrest and not as a proliferation marker. As this cell cycle arrest has been shown by others Eward et al, 2004;Obermann et al, 2005;Tachibana et al, 2005), we could demonstrate that this has a clinical relevance in patients with MCLs. Our data on expression of repp86 in MCL (Schrader et al, 2005) showed that real proliferation activity is an important prognostic factor in MCL.…”

The cell cycle and cancer

“…and are absent from the functional compartment [63] ( Figure 1A, B). The important role that repression of the licensing system plays in proliferation control is highlighted by the finding that Cdc6 over-expression sustains the proliferative capacity of differentiating cells [66].…”

“…Investigation of the DNA replication initiation machinery in different organisms, tissues and cell types has revealed that cell cycle withdrawal and loss of proliferative capacity are linked to a 'shut-down' of the licensing system [3,43,[62][63][64]. During the proliferation-differentiation switch, the MCM clamp loaders Cdc6 and Cdt1 are rapidly down-regulated as cells migrate from the transit amplifying compartment to the functionally differentiated compartment of self-renewing tissues.…”

“…There is a more gradual down-regulation of Mcm2-7 proteins as cells mature and adopt a fully differentiated phenotype. The conversion of replication origins into an unlicensed state also characterizes the quiescent (G 0 ) and senescent out-of-cycle states and therefore appears to be a common mechanism by which proliferation is restrained in multicellular organisms [3,63] ( Figure 1A, B). Interestingly, the regulation of Cdc6 protein levels appears to coordinate the proliferative capacity of cells during cell cycle withdrawal and re-entry.…”

“…Indeed, many components of the DNA replication initiation machinery are under E2F transcriptional control (eg Cdc6, There is a gradual down-regulation of Mcm2-7 as cells mature and adopt a fully differentiated functional phenotype. Proliferative capacity, however, is lost at an earlier point in the differentiation programme as cells exit the transit/amplifying compartment and is linked to down-regulation of the loading factors Cdc6 and Cdt1 [62,63,66]. Notably, the arrested differentiation that characterizes cancer, particularly in high-grade tumours, is associated with failure to down-regulate the replication initiation proteins.…”