Exploring the Role of a Novel Tumor Suppressor – OSR2 in T-ALL

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer, representing >25% of all cancers in children 0-14 years. Despite major advancements in pediatric ALL treatment, it remains the second most lethal childhood cancer, accounting for ~25% of deaths. The two types of ALL are precursor-B, or B-ALL, and precursor-T, or T-ALL, which have distinct molecular landscapes. Of these types, T-ALL comprises about 15% and 25% of pediatric and adult cases, respectively, and is historically considered more aggressive and treatment-resistant, with an inferior prognosis. In the precision medicine era, it is imperative to identify genetic alterations and aberrant gene expression patterns, to better understand tumor biology and improve treatment outcomes by identifying new therapeutic targets.

Our study investigates a novel transcription factor, odd-skipped related transcription factor 2 (OSR2), which we hypothesize is a putative T-ALL tumor suppressor. We are using a zebrafish T-ALL model expressing transgenic human MYC (hMYC) regulated by a lymphoblast-specific promoter, rag2. Prior work in zebrafish and human T-ALL found low OSR2 levels in ~95% of T-ALL. Based on this, we then used RNA-seq to analyze 10 hMYC zebrafish T-ALL, confirming low-to-absent osr2 in all 10 T-ALL relative to wild-type (WT) T cells. We further confirmed decreased osr2 expression by qRT-PCR of additional T-ALL and WT thymocytes. We hypothesized that if OSR2 suppresses T-ALL, impaired zebrafish Osr2 function might increase T-ALL incidence and shorten latency. To test this, we bred osr2-mutant fish to rag2:hMYC transgenic animals to create three genotypes: heterozygous osr2-mutant (osr2^het) fish, heterozygous hMYC (hMYC ^het) fish, and compound-heterozygote (osr2^het;hMYC^het) fish. We screened these genotypes for T-ALL incidence by serial fluorescence microscopy, with T-ALL subsequently confirmed by fluorescence-based flow cytometry. By 7 months of age, we found 9/18 (50%) of double-heterozygous fish developed T-ALL, compared to 0/7 hMYC^het fish (p = 0.026); osr2^het fish also did not develop T-ALL. Together, our findings suggest osr2 allelic loss accelerates MYC-driven T-ALL, supporting our hypothesis that osr2 is a T-ALL tumor suppressor.