Analysis of chromosomal rearrangements that may predispose cells to oncogenesis
Description
Cancer-associated chromosomal rearrangements, which include translocations, large deletions, and inversions, are often important in carcinogenesis or tumor progression. Ultrasensitive polymerase chain reaction (PCR) assays to detect very low numbers of target molecules in the vast excess of normal human DNA, might allow testing the ability of genotoxins to induce chromosomal rearrangements. In one such assay that we developed, a PCR byproduct formed during amplification of unique sequences bordering a tandemly repeat in the low density lipoprotein receptor (LDLR) gene. In another PCR assay, we were able to detect one molecule containing the follicular lymphoma-associated bcl-2/$J\sb{H}$ translocation in a background of DNA from $5\times10\sp6$ normal human cells. Using this assay we determined the frequency of such translocations in blood samples from a population of normal people. Among 132 tested individuals, $\sim$47% had detectable levels of the bcl-2/$J\sb{H}$ translocation in their peripheral blood. The likelihood of being translocation-positive was age-related. The age correlation was also seen for the percentage of individuals with rather high translocation frequencies, although a few individuals of $\le$40 years of age had higher levels than their age mates. The individuals who have higher than average numbers of the translocation in their peripheral blood may be at risk for follicular lymphoma In the last part, I examined the relationship between DNA demethylation and chromosomal aberrations in patients with ICF (immunodeficiency, centromeric instability, and facial anomalies) and in breast cancer. Treatment of a human pro-B cell line with a DNA methyltransferase inhibitor (5-azacytidine) gave rise to a high frequency of unusual chromosomal abnormalities involving heterochromatin of the pericentromeric region of chromosome 1. These rearranged chromosomes are identical to diagnostic anomalies in ICF patients. In addition, these kinds of rearrangements are the same as those frequently observed in breast cancer. Furthermore, much hypomethylation of juxtacentromeric DNA was seen in an ICF lymphoblast cell line as well as in seven out of sixteen examined primary breast adenocarcinomas using Southern blot analysis. The data suggest that hypomethylation of these repetitive DNA sequences provides a hotspot for chromosomal rearrangements, although demethylation of other DNA sequences controlling expression of recombination-promoting genes may also be required