Abstract

The finding of a large number of recurrent fragile sites on human chromosomes provides suggestive evidence for the origin of recurrent chromosomal rearrangements that are a common feature in most neoplasias. These fragile sites can be induced by a wide variety of mutagens and carcinogens that are known to act through different molecular mechanisms. They correlate with two-thirds of the recurrent cancer chromosomal breakpoints known thus far and with the location of most mapped oncogenes and growth factors. Because we have now found that a large number of fragile sites can also be induced by agents such as DNase I and benzo[a]pyrene diol-epoxide (BPDE), which are known to induce hypersensitive chromatin sites, it is possible that a class of fragile sites represents regulatory regions of active genes. We have also introduced restriction enzymes into cultured lymphocytes and found that fragile sites are expressed in a pattern similar to that produced by DNase I and BPDE. Because fragile sites have a higher expression with advancing age, cigarette smoking, and deficiency of folic acid and B12 vitamins, they may represent active genomic sites that are vulnerable to physiological and environmental disturbance. [emphasis added]

Figure 2: High-resolution human chromosomes. Arrows indicate nonrandom oncogenic viral integration sites. Capital letters (eg. DBSH) indicate sites sensitive to nuclear enzymes or chemotherapeutic agents or both. Asterisks indicate chromosome fragile sites. Dots indicate locations of select oncogenes.

Discussion [abbreviated]

Recently, it has been found that 16 mutagens and carcinogens from eight different classes expressed a total of 110 fragile sites in cultured T lymphocytes from normal individuals (table, reference). The majority of the fragile sites were induced by at least half of all agents tested, even though the agents are known to act through different molecular mechanisms. Fifty of the 110 fragile sites coincided with the location of 50 of 75 recurrent chromosome breakpoints observed in more than 80 different types of cancer. In addition, 26 of 36 oncogenes that had been mapped to a chromosome band or subband had a corresponding fragile site (reference).

In the present study, chromosomes were found to break preferentially at 75 sites when cells were exposed to DNase I for six hours prior to mitosis. Sixty-three percent of DNase I-positive sites were found to correlate with known fragile sites. DNase I is known to induce hypersensitive chromatin sites, and these sites most often correlate with regulatory regions of active genes or genes that were previously active in a given cell type (2 references). In an earlier study we found that two mutagens known to have a DNase I-like activity (bromoacetaldehyde (reference) and dimethyl sulfate (reference)) attack fragile sites, and we have now found that DNase I acts in a basically similar manner at the chromosomal level (figure). Some of the DNase I sites may represent GC-rich regions (reference) because DNase I breaks many of the same sites that are vulnerable to attack by mutagens such as dimethyl sulfate, chlorambucil, actinomycin D, and bleomycin, all of which are believed to act on GC-rich DNA regions. This is particularly the case with BPDE, which is known to target poly G sequences (reference). It is known that the 5' promoter region of very active or potentially active genes such as oncogenes, growth factors, and housekeeping genes are 65 to 90% GC rich (reference)....

The possibility that chromosomal fragile sites may be related to oncogenes, hypersensitive chromatin sites, and cancer is also suggested by the finding that oncogenic viruses are preferentially integrated into chromosomes at promoter regions of oncogenes such as c-myc, c-erb B, c-myb, and pim-1 in mice and chickens (4 references) and at hypersensitive chromatin sites in virus-induced leukemia and lymphoma in mice and chickens (3 references). Further, the sites of integration of human adenovirus 12, simian virus 40, papillomavirus 18, hepatitis B virus, and Epstein-Barr virus have been recently localized to specific chromosome bands in human cells (8 references). It is intriguing that many reported sites of [viral] integration correlate with fragile site bands (figure), which may contain genes that are targeted for virally induced transcription. [emphasis added]

3 figures, 2 tables, 61 references