Point Mutations Point mutations of a gene may remain without any consequence for the expression and function of the protein (gene polymorphism). However, point mutations can alter a codon sequence and subsequently disrupt the normal function of a gene product. The ras gene family is an example of oncogene-encoded proteins that disrupt the intracellular signal transduction system following point mutations. Transforming Ras proteins contain point mutations in critical codons (i.e., codons 11, 12, 59, 61) with decrease of GTPase activity and subsequent expression of constitutively active Ras. Point mutations of the p53 gene are the most common genetic mutations described in solid tumors. These mutations occur at preferential “hot spots” that coincide with the most highly conserved regions of the gene. The p53 tumor suppressor gene encodes for a phosphoprotein that is detectable in the nucleus of normal cells. When DNA damage occurs, p53 can arrest cell cycle progression to allow the DNA to be repaired or undergo apoptosis. The lack of function of normal p53 within a cancer cell results in a loss of control of cell proliferation with inefficient DNA repair and genetic instability. Mutations of the p53 gene occur in approximately 50% of advanced ovarian cancers and 30% to 40% of endometrial cancers but are uncommon in cervical cancer.
Point mutations in the BRCA1 and BRCA2 genes can alter the activity of these genes and predispose to the development of breast and ovarian cancer (67). The frequency of BRCA1 and BRCA2 mutations in the general population in the United States is estimated at 1:250. Specific
Figure 6.9 Genes can be amplified or undergo mutation, deletion, or rearrangement.
founder mutations were reported for various ethnic groups. For example, two BRCA1 mutations (185delAG and 5382insC) and one BRCA2 mutation (6174delT) are found in 2.5% of Ashkenazi Jews of Central and Eastern European descent. Additional founder mutations were described in other ethnic groups, including from the Netherlands (BRCA1, 2804delAA and several large deletion mutations), Iceland (BRCA2, 995del5), and Sweden (BRCA1, 3171ins5).
The BRCA proteins are involved in DNA repair. If DNA is damaged, for example, by ionizing radiation or chemotherapy, the BRCA2 protein binds to the RAD51 protein, which is central for the repair of double-stranded breaks via homologous recombination. BRCA2 regulates the availability and activity of RAD51 in this key reaction. Phosphorylation of the BRCA2/RAD51 complex allows RAD51 to bind to the site of DNA damage and, in conjunction with several other proteins, mediates repair of DNA by homologous recombination. BRCA1 functions within a complex network of protein–protein interactions, mediating DNA repair by homologous recombination and regulating transcription via the BRCA1-associated surveillance complex (BASC).
Deletions and rearrangements reﬂect gross changes in the DNA template that may result in the synthesis of a markedly altered protein product. Somatic mutations may involve chromosomal translocations that result in chimeric transcripts with juxtaposition of one gene to the regulatory region of another gene. This mutation type is most commonly reported in leukemias, lymphomas, and mesenchymal tumors. The Philadelphia chromosome in chronic myeloid leukemia, for example, is the result of a reciprocal translocation between one chromosome 9 and one chromosome 22. The DNA sequence removed from chromosome 9 contains the proto-oncogene c-ABL and inserts into the BCR gene sequence on chromosome 22 (Philadelphia chromosome). The resulting chimeric BCR-ABL gene product functions as a constitutively active tyrosine kinase and stimulates cellular proliferation by such mechanisms as an increase of growth factors.