Second Malignant Neoplasms
Second Malignant Neoplasms and Genetic Susceptibility
Literature clearly supports the role of chemotherapy and radiation in the development of SMNs. However, interindividual variability exists, suggesting that genetic variation has a role in susceptibility to genotoxic exposures, or that genetic susceptibility syndrome confers an increased risk of cancer, such as Li-Fraumeni syndrome. Previous studies have demonstrated that childhood cancer survivors with either a family history of cancer, but more so, presence of Li-Fraumeni syndrome, carry an increased risk of developing an SMN.[27,28] The risk of SMNs could potentially be modified by mutations in high-penetrance genes that lead to these serious genetic diseases (e.g., Li-Fraumeni syndrome). However, the attributable risk is expected to be very small because of the extremely low prevalence of mutations in high-penetrance genes. The interindividual variability in risk of SMNs is more likely related to common polymorphisms in low-penetrance genes that regulate the availability of active drug metabolites or are responsible for DNA repair. Gene-environment interactions may magnify subtle functional differences resulting from genetic variations.
Metabolism of genotoxic agents occurs in two phases. Phase I involves activation of substrates into highly reactive electrophilic intermediates that can damage DNA, a reaction principally performed by the cytochrome p450 (CYP) family of enzymes. Phase II enzymes (conjugation) function to inactivate genotoxic substrates. The phase II proteins comprise the glutathione S-transferase (GST), NAD(P)H:quinone oxidoreductase-1 (NQO1), and others. The balance between the two sets of enzymes is critical to the cellular response to xenobiotics; for example, high activity of a phase I enzyme and low activity of a phase II enzyme can result in DNA damage.
DNA repair mechanisms protect somatic cells from mutations in tumor suppressor genes and oncogenes that can lead to cancer initiation and progression. An individual's DNA repair capacity appears to be genetically determined. A number of DNA repair genes contain polymorphic variants, resulting in large interindividual variations in DNA repair capacity. Evaluation of the contribution of polymorphisms influencing DNA repair to the risk of SMN represents an active area of research.
Screening and Follow-up for Second Malignant Neoplasms
Vigilant screening is important for those at risk. Because of the relatively small size of the pediatric cancer survivor population and the prevalence and time to onset of therapy-related complications, undertaking clinical studies to assess the impact of screening recommendations on the morbidity and mortality associated with the late effect is not feasible. However, well-conducted studies on large populations of childhood cancer survivors have provided compelling evidence linking specific therapeutic exposures and late effects. This evidence has been used by several national and international cooperative groups (Scottish Collegiate Guidelines Network, Children's Cancer and Leukaemia Group, Children's Oncology Group [COG]) to develop consensus-based clinical practice guidelines to increase awareness and standardize the immediate care needs of medically vulnerable childhood cancer survivors. The COG Guidelines employ a hybrid approach that is both evidence-based (utilizing established associations between therapeutic exposures and late effects to identify high-risk categories) and grounded in the collective clinical experience of experts (matching the magnitude of the risk with the intensity of the screening recommendations). The screening recommendations in these guidelines represent a statement of consensus from a panel of experts in the late effects of pediatric cancer treatment.