The next-generation sequencing era has repeatedly demonstrated that pediatric cancers harbor qualitatively fewer somatic mutations compared to adult cancers. Often, the somatic mutations that are present are not considered “drivers” or fail to recapitulate transformation in vitro. These observations suggest that additional factors are involved in pediatric tumorigenesis, and the Druley lab is interested in characterizing the role of germline variability.
Our model predicts that multiple germline variants, some inherited from each parent and potentially de novo mutation, combine to mitigate the need for extensive somatic mutation for transformation. To explore this hypothesis, we have elected to study infantile leukemia (IL), which demonstrates the highest mortality and morbidity rates of any pediatric leukemia. While more than two-thirds of these cases harbor rearrangements in the MLL1 gene, physiologic expression of these rearrangements fails to induce leukemic transformation in laboratory models, suggesting that early onset leukemogenesis requires additional factors. We have performed the largest series of matched maternal – infant germline exome sequencing. Consistent with our hypothesis, we find that these infants harbor a significant enrichment of rare, non-synonymous, deleterious germline variation compared to their mothers or healthy pediatric controls, and this variation is localized in genes associated with acute leukemia (Valentine MC, Leukemia 2014).
Furthermore, we find evidence of cooperativity between germline variation and somatic mutation in genes that encode critical protein complexes necessary for mesoderm and hematopoietic development. To explore the functional consequences of this variation, we have established IL patient-specific iPSC lines. By implementing CRISPR-engineered modifications in functional domains of candidate genes followed by differentiating these lines to definitive hematopoietic ontogeny, we will functionally characterize the genetic and epigenetic mechanisms of development that are compromised via germline variation and may lead to in utero leukemogenesis. Our hope is that a better mechanistic understanding of IL will lead to improved pre-natal screening and therapeutic options for these children as well as providing a model for the role of germline variation in pediatric tumorigenesis.