Loss Of BTG1 Function Promotes ETV6-RUNX1-Mediated Leukemic Transformation

Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy and is characterized by the acquisition of recurrent genetic aberrations, which include chromosomal translocations, submicroscopic deletions and point mutations. The t(12;21) ETV6-RUNX1 translocation is present in about 25% of all pediatric B-cell precursor ALL (BCP-ALL) cases and represents an early genetic event, which can already be detected in utero. However, ETV6-RUNX1 expression is insufficient to induce leukemia and requires other cooperating genetic lesions for BCP-ALL to develop.

Using extensive genomic profiling of pediatric BCP-ALL we and others have demonstrated that several recurrent gene deletions can be found in ETV6-RUNX1-positive leukemia. These include focal deletions affecting the B-cell translocation gene 1 (BTG1), which is a member of the BTG/Tob family of anti-proliferation genes. BTG1 deletions are present in 9% of all BCP-ALL cases, but appear to be specifically enriched (20%) in ETV6-RUNX1-positive leukemia. BTG1 protein displays no intrinsic enzymatic activity but may act by recruiting effector molecules like protein arginine methyltransferase 1 (PRMT1) to specific transcription factors, including RUNX1 and ATF4.

To investigate whether loss of BTG1 function cooperates with ETV6-RUNX1 in leukemic transformation we developed an in vitro transformation assay. To this end, primitive fetal liver progenitors (FLPs) were purified as c-Kit⁺ Ter119^- cells from fetal livers of embryonic day 13.5 (E13.5) of C57Bl6/J wild-type and Btg1^-/- mice. After transduction with control pMSCV-IRES-GFP (pMIG) and pMIG-ETV6-RUNX1 retrovirus, cells were serially replated in methylcellulose or liquid culture in the presence of cytokines SCF, FLT3L and IL-7. We observed a proliferative growth advantage of ETV6-RUNX1 over control virus, and in BTG1-deficient FLPs as compared to wild-type FLPs. Notably, the proliferative advantage of BTG1-deficient FLPs was even further enhanced by co-expression of ETV6-RUNX1. By immunoprecipitation from FLPs, we could demonstrate that endogenous PRMT1 binds to ETV6-RUNX1, and this interaction is lost in BTG1-deficient FLPs, arguing that BTG1 is required for the interaction between PRMT1 and ETV6-RUNX1.

In summary, our data indicate that loss of BTG1 function promotes leukemic transformation induced by oncogenic fusion protein ETV6-RUNX1, which implies that BTG1 gene deletions can act as a cooperating event in ETV6-RUNX1-driven leukemogenesis.

No relevant conflicts of interest to declare.