Role of the dependence receptor DCC in colorectal cancer pathogenesis

More than a decade ago, the DCC (deleted in colorectal cancer) gene was proposed as a putative tumor suppressor gene. Data supporting this proposal included observations that one DCC allele was deleted in roughly 70% of colorectal cancers, some cancers had somatic mutations of the DCC gene, and DCC expression was often reduced or absent in colorectal cancer tissues and cell lines. Despite subsequent studies which have supported DCC's potential role as a tumor suppressor gene, the rarity of point mutations identified in DCC coding sequences, the lack of a tumor predisposition phenotype in mice heterozygous for DCC inactivating mutations, and the presence of other known and candidate tumor suppressor genes on chromosome 18q have raised questions about DCC's candidacy. Following its initial characterization, the DCC protein was identified as a transmembrane receptor for netrins, key factors in axon guidance in the developing nervous system. At first glance, the established role of DCC and netrin-1 during organization of the spinal cord could be viewed as a further challenge to the position that DCC inactivation might play a significant role in tumorigenesis. However, recent observations on DCC's functions in intracellular signaling have renewed interest in the potential contribution of DCC inactivation to cancer. In particular, data indicate that, when engaged by netrin ligands, DCC may activate downstream signaling pathways. Moreover, in settings where netrin is absent or at low levels, DCC can promote apoptosis. Here, we review DCC's candidacy as a tumor suppressor gene, with an emphasis on how recent molecular analyses of DCC have offered support for the notion that DCC may function as a tumor suppressor gene.