Heterozygous Germline ATM Mutations Do Not Contribute to Radiation-Associated Malignancies After Hodgkin's Disease
Reviewers: John Han-Chih Chang, MD
Source: Journal of Clinical Oncology 1999, Volume 17, Number 4: pages 1259-1266.
Hodgkin's disease (HD) in children and young adults has been very successfully treated. Nearly 80% or more are "cured" or have become long-term survivors of those diagnosed with early-stage HD. Unfortunately, these survivors are at significant risk for secondary malignancies, the most common of which are acute myeloid leukemia (AML), non-Hodgkin's lymphoma (NHL), breast cancer in women and a host of other solid tumors (soft tissue sarcomas). By 20 years out from therapy for HD, the late mortality from secondary cancers becomes significantly higher than the mortality from the initial HD itself.
Ataxia-telangiectasia (AT) is an autosomal recessive disease resulting from the inactivation of the ATM gene by mutations. This leads to a defect in the p53 dependent DNA-damage response pathway. Essentially, patients are at a higher than normal risk for other genetic mutations. Thus, they are more susceptible to developing lymphocytic leukemias, HD, NHL, breast and gastric carcinomas. Studies have demonstrated increased susceptibility of cultured cells with the mutated ATM to the ionizing effects of radiation therapy. Approximately 1% of the general population are AT carriers or heterozygotes. In vitro studies of AT heterozygote cell lines have shown an impaired response to the DNA damage induced by ionizing radiation, intermediate in effect between that of AT homozygotes and normal controls. AT families have demonstrated in epidemiological studies that there is an increased incidence of breast cancer among relatives considered to be carriers.
The focus of this article was to elucidate whether the high rate of second malignancies in young patients treated with HD could be correlated to an increased incidence of ATM mutations.
Patients and Methods
The patients in this study were derived from the Harvard Joint Center for Radiation Therapy and the Massachusetts General Hospital in Boston. They identified 123 patients with HD diagnosed between 1969 and 1991, who developed secondary malignancies. This was 10% of the patients diagnosed and treated for HD. Only 52 agreed to participate in the study. Forty-four of the 123 were unavailable secondary to having died of their secondary malignant neoplasms. The remainder either refused to be entered or were lost to follow-up. The median age at diagnosis of their HD was 22 years. Median follow-up was 22 years. Another cohort of 68 HD patients who had not developed secondary malignancies were utilized for comparative measures. Median follow-up was 16 years on this cohort. They were all female and reportedly treated in a similar fashion as the ones who had developed secondary cancers. No data were given as to patient characteristics.
Reverse transcriptase polymerase chain reaction (RT-PCR) was utilized for gene amplification from the immortalized cells developed from each patient. The subsequent mutational analysis method is described in detail in the paper. Briefly, the method required that the amplified genes were to be inserted into a certain type of yeast. The yeast was then plated onto agar plates. The wild-type ATM gene (non-mutated) would yield a high number of colonies (> 75%), whereas heterozygotes (35 - 55%) and homozygotes ( < 5%) had a much lower yield. Intermediate results lead to a repeat of the test to confirm the findings.
Seventy-six neoplasms were observed in the cohort of 52. Thus, 18 patients had more than one secondary malignant neoplasms. No secondary leukemia patients were part of the 52. This is likely due to the high mortality rate in the leukemics. The most common types of secondary cancers were breast, skin, lymphoma, head & neck, sarcoma and female genitourinary cancer (table 2 in the article). Forty-eight (92%) had at least one lesion within the RT field. The median time to diagnosis of the secondary neoplasm was 14 years, which is consistent with other studies if you exclude leukemias. The median time to developing subsequent secondary malignant neoplasms was 21, 27 and 35 years for third, fourth and more secondary cancers, respectively. Most were treated with RT alone (n = 32). Others got combined chemoradiation (n = 18) or chemotherapy alone (n = 2).
The incidence of positive family histories for malignancy in association with secondary neoplasms was evaluated. Eleven of those that had developed secondary malignant neoplasms had a positive family history of cancer. In the HD cohort that did not develop a secondary cancer, only 3 of the 68 had a positive family history. Therefore, those that had developed a secondary malignancy had a higher rate of cancer in their family history (21% versus 4%). No significant differences were seen between those that had multiple secondary malignant neoplasms versus those that had only one, concerning family history of cancer.
The ATM mutational analysis using a yeast-based protein truncation assay found no patient in the cohort of 52 who had developed secondary cancers to be carriers or heterozygotes of the AT mutation. No mention was made in reference to the control cohort as to the frequency of AT heterozygotes or homozygotes.
Discussion and Critique
The authors' general conclusion was that there must be a genetic link in those that developed a secondary malignancy based on the family history data, but AT heterozygocity had no role, since NONE of the patients in the cohort of 52 that participated were AT heterozygotes. The main objection I have to the conclusion stated above is that a total of 123 patients developed a secondary malignant neoplasm. Only 52 were available for analysis because most had died before being able to be evaluated. Could it be that the AT heterozygotes predominated that population of those who had been deceased. In order for the conclusion to be valid, prospective registration and molecular/genetic tests should be performed before treatments begin and the patients should be followed to analyze the rate of secondary cancers. The utility of the comparative cohort of 68 women with HD who had NOT developed a secondary malignancy is also called into question. No details were given as to the patient characteristics besides a blanket statement that they were treated similarly. Was it out of convenience that they were used, since they had been used for another study. Besides a low rate of cancer in their family history, little else was gleaned from those patients. I agree that this study has demonstrated that there appears to be genetic factors contributing to the development of secondary cancers in these HD patients. However, analysis of these selective 52 patients cannot eliminate the possibility that AT mutations do not play significant role.