Genetic variation and complex disease: The examination of an X-linked disorder and a multifactorial disease

Two genetic studies are presented in this dissertation. The studies share a common theme of the influence of genetic variability in complex disease. The first study is entitled, "The Association of Antioxidant-Related Gene Polymorphisms and Second Primary Malignant Neoplasms in Pediatric Hodgkin Lymphoma.";Improved treatment strategies in pediatric Hodgkin Lymphoma (HL) have resulted in a cure rate approaching 95%, yet the development of a second primary malignant neoplasm (SMN) is a risk for long-term survivors. In a pediatric HL population, between 6-26% of patients will develop a SMN within 30 years following treatment. The etiology of HL is still largely unknown, as is the cause of certain types of SMNs.;Oxidative stress has been linked to the development of cancer due to the damaging effects of reactive oxygen species (ROS) on DNA, lipids, and proteins. During periods of extended oxidative stress, the damaging effects of ROS are likely to increase which in turn raises the risk of genetic change. In a multi-stage model of tumorigenesis, genetic change is considered to be the initiating event of cancer development. Under normal circumstances, antioxidant enzymes within the cell mitigate the effects of ROS by converting reactive species into non-toxic molecules. Mechanisms within the cell maintain a steady state balance between ROS and antioxidant enzyme levels. We hypothesize that individuals predisposed to lower levels of antioxidant enzyme activity due to polymorphic variants within those genes may be at risk for increased damage caused by ROS. Although decreased amounts of antioxidant enzymes have been found in a variety of cancers, a correlation between polymorphisms in antioxidant genes and predisposition to secondary cancer has not been studied.;The purpose this study was to assess the association of antioxidant gene alleles with the risk of developing a SMN in HL survivors. DNA samples were obtained from 768 HL patients enrolled in the Childhood Cancer Survivor Study (CCSS). The samples were genotyped for 90 polymorphisms in antioxidant related genes including SOD, GPX, NOS, CAT, and CYP2C9. Statistical analysis methods to determine risk of developing a SMN included association, haplotype, and multiple regression models.;In our HL cohort, 131 patients developed a SMN. An additional 117 patients developed nonmelanoma skin cancer, and 34 patients developed 2 SMNs. Out of the 36 SNPs that were included in the final analysis, 4 SNPs in the GPX1, GPX3, GPX4, and SOD2 genes, were potentially suggestive (p<0.05) of an association between genotype and the development of a SMN. A general trend observed in the genotyping data suggested that an increased risk of SMN was conferred by the presence of the minor allele, while a decreased risk of SMN was conferred by the presence of the major allele in the population. Replication studies are necessary, though it is notable that polymorphisms within the GPX family may be associated with the development SMN in our cohort. Additionally, hazard ratios were only moderately increased in this population suggesting that the development of a SMN in a pediatric HL is likely multi-factorial in nature. The elucidation of critical genes involved in SMN development could influence patient follow-up and intervention strategies, and potentially aid in the prevention of SMN.;The second study in this thesis is entitled, "Atypical X-Inactivation in an X;1 Translocation Patient Diagnosed with Otopalatodigital Syndrome." X-chromosome inactivation (XCI) is an epigenetic process used to regulate gene dosage in mammalian females by silencing one X-chromosome. While the pattern of XCI is typically random in normal females, abnormalities of the X-chromosome may result in skewing due to disadvantaged cell growth. We describe a female patient with an X;1 translocation [46,X, t(X;1)(q28;q21)dn] and unusual pattern of XCI who was clinically diagnosed with Otopalatodigital syndrome (OPD) type 1. There was complete skewing of XCI in the patient, along with the atypical findings of an active normal X-chromosome and an inactive derivative X. An X-linked disorder, OPD1 is characterized by multiple congenital anomalies including skeletal abnormalities, craniofacial defects, and hearing loss. Mutations within the FLNA gene (Xq28) are known to cause OPD, though none were detected in our patient. Additionally, no abnormalities in FLNA mRNA or protein were detected in our patient. Characterization of the translocation revealed that the patient's Xq28 breakpoint interrupts the DKC1 gene, located 400kB distal to FLNA. Molecular analysis of the breakpoint region revealed functional disomy of Xq28 genes distal to DKC1. No monosomy of 1q genes was detected. Possible explanations for the patient's phenotype include a position effect due to the translocation breakpoint, an undetected FLNA-related mutation, or altered gene dosage due to consequences of atypical XCI.