Homeodomain-driven oncogenic diversion to a TCRγδ phenotype in T-cell Acute Lymphoblastic Leukemia.

T-cell acute lymphoblastic leukemia (T-ALL) results from the malignant transformation of thymocytes blocked in their differentiation. Surface expression of the γδ T-cell receptor (γδTCR) is surprisingly frequent in T-ALLs, questioning the susceptibility of the γδ-lineage to leukemogenesis. Among 1233 T-ALL phenotyped in our center, 33% (n=403) expressed a TCR, of which 47% (n=191 (113 adults, 78 children)) were positive for γδTCR (γδTCR+). By integrating oncogenetic, immunogenetic, phenotypic and bulk transcriptomic data, we were able to delineate two distinct γδTCR+ T-ALL subtypes, with likely distinct physiological counterparts. The first (75% of cases) was characterized by ectopic expression of homeodomain-containing oncogenes (HD+), Vβ-Jβ rearrangements, phenotypic (including surface pre-TCRα chain) and transcriptional profiles reminiscent of cortical thymocytes, and was therefore termed cortical-like γδ T-ALLs. Transduction of murine T-cell progenitors and human CD34+ cells with HOXA9 or TLX3 (HD+ oncogenes) led to a differentiation bias toward γδTCR expressing thymocytes. The second (26%) was negative for these features, exhibited phenotypic and transcriptional profiles reminiscent of γδ thymocytes and was therefore termed bona fide γδ T-ALLs. These findings were validated in the COGAALL0434 cohort. Although bona fide γδ T-ALLs were enriched for early T-cell progenitor (ETP)-like and KMT2A-rearranged cases, they mostly eluded the phenotypic definition of ETP-ALLs. Similar to the ETP-like subtype, bona fide γδ T-ALLs were associated with a poor initial response to chemotherapy, but were sensitive to the BCL2-inhibitor venetoclax. Our results reveal developmental heterogeneity behind γδTCR expression in T-ALLs, and suggest that overrepresentation of this subtype reflects αβ-lineage commitment repression by HD+ oncogenes.