Loss of cystathionine-β-synthase contributes to elevated OXPHOS, a vulnerability in Ara-C-resistant Myeloid Leukemia in Down syndrome

Myeloid leukemia associated with Down syndrome (ML-DS), as classified by WHO 2016, includes acute myeloid leukemia (AML) and myelodysplasia in children with DS. While ML-DS patients show high sensitivity to cytarabine (Ara-C)-based chemotherapy with better overall survival than non-DS AML patients, relapsed/refractory cases have dismal outcomes. This underscores the need to understand Ara-C-resistance mechanisms and develop effective therapies. The chromosome 21 gene, cystathionine-β-synthase (CBS), is significantly overexpressed in ML-DS cells. Overexpression of CBS leads to increased hydrogen sulfide (H₂S) production, which reduces complex IV activity and Oxidative Phosphorylation (OXPHOS). OXPHOS has been shown to play an important role in Ara-C resistance in non-DS AML. Thus, in this study, we investigated the role of CBS as a regulator of OXPHOS and Ara-C response. We found that Ara-C-resistant ML-DS cells have lower CBS activity. Overexpression of CBS in an Ara-C-resistant ML-DS cell line resulted in increased H₂S and Ara-C sensitivity and decreased both complex IV activity and OXPHOS. Knockdown of CBS in an Ara-C-sensitive ML-DS cell line increased OXPHOS and Ara-C resistance. However, complex IV activity decreased and H₂S production was unchanged, indicating that CBS regulates OXPHOS through both a H₂S-dependent and -independent mechanism. We further demonstrate that targeting both OXPHOS, using ONC213, and Apoptosis, using venetoclax, results in synergistic induction of cell death in Ara-C-resistant ML-DS cells. This study identifies CBS as a regulator of OXPHOS and Ara-C response, while the combination of ONC213 and venetoclax offers a promising therapeutic approach for relapsed/refractory ML-DS, addressing key vulnerabilities to improve patient outcomes.