Molecular biology of colorectal cancer

Accumulated clonal genetic alterations in familial and sporadic colorectal carcinomas with widespread instability in microsatellite sequences

A subset of hereditary and sporadic colorectal carcinomas is defined by microsatellite instability (MSI), but the spectra of gene mutations have not been characterized extensively. Thirty-nine hereditary nonpolyposis colorectal cancer syndrome carcinomas (HNPCCa) and 57 sporadic right-sided colonic carcinomas (SRSCCa) were evaluated. Of HNPCCa, 95% (37/39) were MSI-positive as contrasted with 31% (18/57) of SRSCCa (P < 0.000001), but instability tended to be more widespread in SRSCCa (P = 0.08). Absence of nuclear hMSH2 mismatch repair gene product by immunohistochemistry was associated with germline hMSH2 mutation (P = 0.0007). The prevalence of K-ras proto-oncogene mutations was similar in HNPCCa and SRSCCa (30% (11/37) and 30% (16/54)), but no HNPCCa from patients with germline hMSH2 mutation had codon 13 mutation (P = 0.02), and two other HNPCCa had multiple K-ras mutations attributable to subclones. 18q allelic deletion and p53 gene product overexpression were inversely related to MSI (P = 0.0004 and P = 0.0001, respectively). Frameshift mutation of the transforming growth factor beta type II receptor gene was frequent in all MSI-positive cancers (85%, 46/54), but mutation of the E2F-4 transcription factor gene was more common in HNPCCa of patients with germline hMSH2 mutation than in those with germline bMLH1 mutation (100% (8/8) versus 40% (2/5), P = 0.04), and mutation of the Bax proapoptotic gene was more frequent in HNPCCa than in MSI-positive SRSCCa (55% (17/31) versus 13% (2/15), P = 0.01). The most common combination of mutations occurred in only 23% (8/35) of evaluable MSI-positive cancers. Our findings suggest that the accumulation of specific genetic alterations in MSI-positive colorectal cancers is markedly heterogeneous, because the occurrence of some mutations (eg, ras, E2F-4, and Bax genes), but not others (eg, transforming growth factor beta type II receptor gene), depends on the underlying basis of the mismatch repair deficiency. This genetic heterogeneity may contribute to the heterogeneous clinical and pathological features of MSI-positive cancers.

A retinoblastoma-binding protein that affects cell-cycle control and confers transforming ability

The retinoblastoma (RB) gene is one of the most extensively studied tumour-suppressor genes. Deletion or inactivation of both RB alleles is an essential, rate-limiting step in the formation of retinoblastoma and osteosarcoma that arise in families that carry mutant RB (ref. 2). RB inactivation is also found in other human tumours. Whereas loss of RB function is associated with the loss of cellular proliferative control, introduction of a wild-type RB can suppress cell growth and tumorigenicity. Thus, identification of factors that interfere with and/or control the function of the RB protein is critical for understanding both cell-cycle control and oncogenesis. Here we describe a new gene, Bog (for B5T over-expressed gene), which was identified and shown to be overexpressed in several transformed rat liver epithelial (RLE) cell lines resistant to the growth-inhibitory effect of TGF-beta1, as well as in primary human liver tumours. The Bog protein shares homology with other retinoblastoma-binding proteins and contains the Rb-binding motif LXCXE. Using the yeast two-hybrid system and co-immunoprecipitation, we demonstrated that Bog binds to Rb. In vivo, Bog/Rb complexes do not contain E2F-1, and Bog can displace E2F-1 from E2F-1/Rb complexes in vitro. Overexpression of Bog in normal RLE cells conferred resistance to the growth-inhibitory effect of TGF-beta1. Furthermore, normal RLE cells are rapidly transformed when Bog is continuously overexpressed and form hepatoblastoma-like tumours when transplanted into nude mice. These data suggest that Bog may be important in the transformation process, in part due to its capacity to confer resistance to the growth-inhibitory effects of TGF-beta1 through interaction with Rb and the subsequent displacement of E2F-1.