Variants At 6q21 Associated with Radiation-Induced Second Malignancies After Hodgkin Lymphoma Differentially Regulate PRDM1 Expression Following Radiation Exposure

Abstract 431

Childhood Hodgkin lymphoma (HL) survivors are particularly susceptible to second malignant neoplasms (SMNs) caused by radiation therapy (RT). To identify genetic risk factors for SMNs, we undertook a genome-wide association study of 96 SMN cases and 86 SMN-free controls. Both cases and control sets consisted of long-term survivors of childhood HL who received antecedent RT. Two variants at chromosome 6q21 intergenic between PRDM1 and ATG5 were associated with SMNs (rs4946728: P = 1.26×10⁻⁹, OR = 3.17 [2.16–4.65] and rs1040411: P = 4.68×10⁻⁸, OR= 2.41 [1.76–3.32]). These results were replicated in an independent set of 63 SMN cases and 75 SMN free controls (rs4946728: P =. 003, and rs1040411: P =. 04). The risk variants form a haplotype associated with decreased basal expression of PRDM1, as well as impaired induction of the PRDM1 protein by radiation exposure. An analysis of the genomic region spanned by the SMN-associated haplotype revealed four evolutionarily conserved putative enhancers, characterized by both H3K27 acetylation and DNase I hypersensitivity. To determine whether these putative enhancers modulate expression following radiation exposure, we cloned each into the pGL3 vector downstream of a luciferase reporter gene. We then transiently transfected each construct into lymphoblastoid cell lines. Data will be presented showing how carriage of either the risk or protective haplotype alters the activity of these enhancers in response to radiation. Furthermore, ChIP-Seq data indicate that NFkB variably binds each putative enhancer. As NFkB has previously been shown to induce PRDM1 expression and is itself activated by radiation, we treated LCLs with an NFkB inhibitor to determine whether NFkB is involved in induction of PRDM1 following radiation exposure. These data demonstrate that incorporating etiologic exposures, such as radiation, can enhance the discovery disease-associated genetic variation and guide functional follow-up experiments.