Although many cancers result from mutations in prototype oncogenes and tumour-suppressor genes that regulate cell proliferation and apoptosis (see Milestones 10, 11, 12, 20 and 21), cancer can also arise indirectly from defects in the protective cellular mechanisms that repair DNA damage. This idea originated in the study by Theodor Boveri of chromosomal imbalances in somatic cells (see Milestone 2). The type of DNA damage can range from the subtle, such as a single unrepaired base lesion, through small deletions or insertions, to macroscopic changes that manifest as non-reciprocal chromosome translocations (see Milestone 10). Genetic instability at any of these levels can predispose to cancer by increasing the rate at which potentially oncogenic mutations and chromosomal alterations occur.
When cells are exposed to ultraviolet (UV) light, base adducts are formed that must be excised for replication to occur. This process, nucleotide-excision repair (NER), involves recognition of distortion of the DNA helix, assembly of a complex on and around the lesion, and excision of a single-strand fragment containing the modified base. Several human syndromes show UV hypersensitivity, and one, xeroderma pigmentosum (XP), displays a strong predisposition to skin cancer.
XP patients were originally classified in eight complementation groups. In 1990, two human genes with roles in NER were cloned, and both were linked to XP. The sequence of excision-repair cross-complementing 3 (ERCC3; also known as XPB), cloned by Geert Weeda et al., implied that it encoded a DNA helicase. Complementation studies showed that in the unique XP group B individual, a splicing mutation in ERCC3 resulted in a frameshift. Kiyoji Tanaka et al. later cloned the XP group A-complementing protein (XPA; also known as XPAC) gene, the mRNA of which was reduced in cells from XP-A individuals. From its sequence, XPA was proposed to promote incision surrounding the lesion.
The association between XP and DNA repair deficiency arose from the extreme UV sensitivity of the patients, rather than specific observations of damage at the DNA level. In patients with hereditary non-polyposis colon cancer (HNPCC), however, the link to defective repair was obvious: microsatellite repeat sequences in their cells had changes similar to those found in bacterial mismatch repair (MMR)-deficient mutants.
This observation encouraged efforts to locate human genes with homology to the bacterial MMR proteins MutS, MutH and MutL. In 1993, two groups using complementary approaches identified MutS homologue 2 (MSH2) as an HNPCC-associated gene. Whereas Richard Fishel et al. went directly after homologues of MutS using a degenerate primer strategy, Frederick Leach et al. used markers linked to HNPCC to define the disease locus, and then isolated the candidate MMR gene. Additionally, Leach et al. reported that chromosome 2-linked HNPCC families had mutations in MSH2. A few months later, in 1994, the gene responsible for chromosome 3-linked HNPCC was cloned by Nickolas Papadopoulos et al. and Eric Bronner et al. Not surprisingly, this turned out to be the human MutL homologue, MLH1.
Gross chromosomal changes are consistently observed in human cancers, and their mechanistic basis is the subject of active investigation. One line of research indicates that the combination of telomerase dysfunction and p53 inactivation leads to chromosome instability. Late-passage telomerase-deficient mice were known to have shortened telomeres and chromosome instability, but cell viability was compromised. By introducing p53 deficiency into this background, Ronald DePinho and colleagues were able to show that cell survival could be promoted, allowing neoplastic transformation to occur. Furthermore, Steven Artandi et al. found that in telomerase- and p53-deficient epithelial cells, telomeres become progressively shortened, leading to a rise in chromosomal instability (non-reciprocal translocations and end-to-end fusions) and accelerated carcinogenesis. Another line of research implies that the maintenance of the mitotic-spindle checkpoint is essential for chromosome stability in cancer cells. Sandra Hanks et al. found that mutations of the spindle-checkpoint gene BUB1B caused a cancer-predisposition syndrome characterized by premature chromosome separation. Other cancer-predisposition syndromes caused by alterations in genes associated with chromosome-level repair are ataxia telangiectasia, Bloom syndrome and hereditary breast cancers (see Milestone 21).
These and other studies established that DNA repair defects of various forms and severity initiate genetic instability that affects cancer development. Whether genetic instability is actually mandatory for cancer development in non-familial cancers remains controversial, although these findings stress the importance of protecting the integrity of the genome as a tumour-suppression mechanism.
