Figure 1. Schematic diagram showing RLF expression in self-renewing tissues. The model includes stem cell, dividing-transit and functional compartments [1]. The flux of cells through these compartments is continuous; new cells are supplied from the stem cell compartment (S) and their number is amplified in the dividing-transit compartment (T). Cells become fully differentiated and functionally competent as they enter the mature compartment (M). The stem cell compartment shows low MCM expression [25]. Mcm2-7 levels rapidly increase as cells enter the dividing-transit compartment. There is a gradual downregulation of Mcm2-7 as cells differentiate and adopt a fully differentiated functional phenotype [44]. However, proliferative capacity is lost at an earlier point during execution of the differentiation programme as cells exit the division-transit compartment and is coupled to downregulation of the MCM loading factor Cdc6 [24]. Notably, the arrested differentiation that characterizes cancer, particularly in high-grade tumours, is associated with a failure to downregulate Mcm2-7.

Figure 2. MCM cancer detection test. Mcm2-7 protein expression in normal epithelium is restricted to the basal stem/transit compartments and is absent from surface layers as cells adopt a fully differentiated phenotype. In premalignant/dysplastic epithelial lesions there is an expansion of the proliferative compartment coupled to arrested differentiation, resulting in the appearance of cycling, MCM positive cells in superficial layers. Superficial cells obtained either through exfoliation or by surface sampling should therefore be negative for Mcm2-7 proteins. Detection of Mcm2-7 in exfoliated or surface sampled cells is thus indicative of an underlying premalignant/dysplastic lesion or malignancy [^{[22•], [49] and [50]}].

Figure 3. Cell cycle phase progression in breast cancer. Two breast cancer biopsy specimens immunostained for Mcm2, Ki67, geminin, Aurora A and H3S10ph are shown in (a) and (b). Both cases are characterized by high Mcm2 protein expression, indicating that the majority of tumour cells are engaged in the cell division cycle. Although both tumour specimens show a high growth fraction as defined by Mcm2 expression, there are striking differences in expression of the S-G2-M markers geminin and Aurora A and the mitotic marker H3S10ph. (a) This tumour shows very low expression levels of geminin and Aurora A, and a small number of cells show phosphorylation of Histone H3 at serine 10 (H3S10ph), indicating an arrested or prolonged G1 phase. (b) By contrast, this tumour shows high expression levels of geminin and Aurora A and increased H3S10 phosphorylation, indicating rapid cell cycle phase progression. Thus, it can be postulated that the tumour shown in (b) is more responsive to S or G2/M cell cycle phase specific drugs.

Presence of Ki67, Mcm2-7, geminin, Aurora A and H3S10ph during the mitotic cell division cycle