PROJECT SUMMARY: Children diagnosed with acute myeloid leukemia (AML) continue to have an overall poor outcome with rates of relapse that approach 40%. Relapsed disease is particularly resistant to conventional therapy. Unfortunately, the molecular alterations that are common in relapsed pediatric AML have been poorly defined. Recently our group reported on the spectrum of genetic changes in 136 children with relapsed AML and identified tandem duplications of exon 13 of UBTF (upstream binding transcription factor) in nearly 10% of children with relapsed AML. We further demonstrated that UBTF-tandem duplication (UBTF-TD) AMLs are also present in 4% of children at diagnosis, yet are rare in adults, and commonly occur with FLT3-ITD and WT1 mutations along with either normal karyotype cytogenetics or trisomy 8. Importantly, we demonstrated that children with UBTF-TD AML have an inferior overall survival and high rates of minimal residual disease after induction chemotherapy. Our preliminary functional studies have confirmed that UBTF-TD expression is sufficient to drive proliferation and self-renewal of primary human hematopoietic cells and that UBTF-TD proteins maintain canonical interactions of wild-type UBTF, but also interact with new proteins/networks important in leukemia development, such as KMT2A and XPO1. Collectively these genomic, functional and clinical findings suggest that UBTF-TD AMLs represents a new molecular category of pediatric AML and establishes a strong scientific premise to investigate the molecular impact of UBTF tandem duplications in primary hematopoietic cells. We hypothesize that UBTF-TD represents in new initiating lesion that drives the expression of specific transcriptional networks, in particular the HOXB program, through new interactions with the genome and from collaboration with unique cooperating mutations and interacting proteins. We will test our hypothesis with the following specific aims using a combination of genetic tools in human and mouse hematopoietic cells. Specific Aim 1: Decipher the molecular mechanisms of UBTF-TD in leukemogenesis; Specific Aim 2: Dissect the contribution of UBTF-TD and co-occurring mutations to leukemogenesis using in vivo models; Specific Aim 3. Establish the contribution of UBTF domains and interacting proteins to UBTF- TD mediated transformation. Not only will the proposed studies elucidate the transcriptional and epigenetic impact of UBTF-TD expression in primary hematopoietic cells, including patient samples, but they will also establish multiple mouse and human model systems for this new subtype of high-risk pediatric AML and evaluate potential vulnerabilities. The successful completion of these proposed studies, and the resulting model systems, will ultimately be used to develop therapeutic approaches to target UBTF-TD AMLs and most importantly to improve the long-term outcome of children with these leukemias.