DNA sequence analysis of the complete p53 tumor-suppressor gene provides predictive information about breast cancer patients' response to therapy, according to a study published in the October 1995 issue of Nature Medicine. The study analyzes the associations between tumor mutations and patient outcome--especially in relation to therapy--using Sequence-Based Diagnosis, (SBD), a new concept for complete DNA sequencing. Although previous studies have examined p53 using various molecular biologic methods, this study represents the first complete sequencing of the p53 gene in a large retrospective study of a population-based cohort. The study also confirms that since mutations are found over the entire coding sequence, some could be missed using traditional DNA analysis protocols.
Since cancer develops in stages through a step-by-step breakdown of the mechanisms that control normal cellular growth, an accumulation of genetic changes has been identified as a key event in progression of the disease. p53 is a tumor-suppressor gene located on the short arm of chromosome 17. In breast cancer, mutations of the p53 gene have been considered to be a critical step in the development of certain tumors. These alterations can be determined using immunohistochemistry or DNA analysis, which reveals prognostic information.
"We hope that the results from this study will, in the future, lead to the development of more tailored treatments for breast cancer patients," said Jonas Bergh, MD, associate professor of oncology and director of the study. "If we can more accurately determine the effects of therapy, we may be able to reduce the costs of treatment in patients with less aggressive tumors, and at the same time, develop more aggressive regimens for those with poor prognosis."
Commenting on the findings, Barbara Lynn Weber, MD, associate professor of medicine and genetics and director of the Breast Cancer Program, University of Pennsylvania School of Medicine, said, "Increasing numbers of breast cancers are being diagnosed at the early stages, when the tumors are small and have not spread to distant parts of the body. Traditional prognostic factors, including tumor size and lymph node involvement, are limited and, as tumors are detected earlier, new, more precise methods are needed to select individualized treatment regimens."
Study Results Differ for Lymph Node-Positive vs Node-Negative Patients
Conducted at the Uppsala Akademiska University Hospital in Sweden in cooperation with Pharmacia Biotech and the Swedish Cancer Society, the study analyzed frozen tumor material from a total of 316 consecutive Scandinavian women who underwent surgery for breast cancer from January 1987 through December 1989. The complete coding region of the p53 gene was sequenced and 69 mutations were discovered using the automated laser fluorescence ALF DNA sequencer, developed by Pharmacia Biotech.
A total of 97 patients had primary lymph node metastases and 206 were node-negative. In 13 cases, the node status of the patient was unknown, since axillary exploration was not performed. The median follow-up time was 57 months.
Lymph node-positive patients with p53 mutations had a significantly shorter survival rate, compared with those without these mutations. Therapy, especially adjuvant tamoxifen(Drug information on tamoxifen), seemed to be of less value in lymph node-positive patients with p53 mutations than in patients without these mutation. For node-negative patients with p53 mutations, tamoxifen and/or locoregional radiotherapy resulted in improved relapse-free survival; however, adjuvant tamoxifen and/or locoregional therapy seemed to be of less value in node-negative patients with no mutations.
In addition, the location of the mutations in the p53 gene differed between the lymph-node-positive and lymph-node-negative groups, which may explain the discriminative power of p53 alterations to predict response to adjuvant therapy.
"For more than a decade it has been apparent that molecular techniques can provide the answers for improved understanding of tumor development," said Margaret Bywater, an initial pioneer of the SBD technique and director, molecular medicine, market development, Molecular Systems Division, at Pharmacia Biotech. "With the automation of these techniques, the true relevance of each mutation can be evaluated and genetic information obtained from tumor biopsy will, in the future, be used for prognosis and guiding the choice of therapies."