The ongoing conundrum of MLL-AF4 driven leukemogenesis

MLL-AF4+ pro-B acute lymphoblastic leukemia (ALL) in infants represents an aggressive, high-risk type of childhood leukemia arising from prenatally acquired preleukemic t(4;11) chromosomal translocations. However, despite various reported attempts, accurately mimicking MLL-AF4–driven leukemogenesis in mice remains a difficult task. Enforced expression of the MLL-AF4 fusion protein in cord blood–derived human hematopoietic stem cells (HSCs) transplanted into immunodeficient mice either results in the development of malignancies that deviate from the pro-B ALL phenotype as observed in humans, or does not lead to neoplasia at all.²  Obviously, these discrepancies raise important questions: (I) Does MLL-AF4+ pro-B ALL in infants arise in CD34+ HSCs? (II) Are MLL-AF4 fusion proteins driving leukemogenesis on their own, or are cooperative genetic lesions required? (III) Do MLL-AF4 fusion proteins exert sufficient transforming capacity? For instance, Bursen et al recently showed that not MLL-AF4, but enforced expression of its reciprocal fusion protein AF4-MLL in murine HSCs or progenitor cells, induced ALL in mice without the requirement of MLL-AF4.³  In contrast, Tamai et al showed that enforced expression of MLL-AF4 in murine HSCs is sufficient to induce ALL, but demonstrated that the process of transformation is significantly accelerated by cooperative K-Ras mutations.⁴  Nonetheless, these experiments remain to be repeated in HSCs of human origin to appreciate the relevance of AF4-MLL and/or RAS activation in the development of MLL-AF4+ pro-B ALL. Moreover, the reciprocal AF4-MLL fusion transcript is present in the majority of, but not all, patients with MLL-AF4+ ALL,³  and RAS mutations are found in ∼25% of the cases.⁵  Hence, distinct mechanisms of transformation, as well as the involvement of yet unknown genetic events, cannot be ruled out.

Meanwhile, Dr Pablo Menendez and coworkers have been elegantly addressing the question of the cell of origin from which MLL-AF4+ pro-B ALL may arise.⁶  On the basis of their earlier observations that bone marrow–derived mesenchymal stem cells from patients with MLL-AF4+ pro-B ALL harbor and express the MLL-AF4 fusion gene,⁶  Menendez et al reasoned that this type of leukemia may well arise in prehematopoietic mesodermal or hemangioblastic precursors sprouting from differentiating hESCs. To test this hypothesis, this research group recently created a cellular system to study early hematoendothelial development in MLL-AF4–expressing hESCs. Interestingly, introducing MLL-AF4 expression in hESCs enhanced the specification of hemogenic precursors, but impaired further hematopoietic commitment in favor of an endothelial cell fate. Alas, MLL-AF4 expression alone appeared not sufficient to induce leukemia in hESC-derived hematopoietic cells.⁷  In the present study, Bueno et al¹  explored the impact of FLT3 activation on the hematopoietic fate of MLL-AF4–expressing hESCs. Patients with MLL-AF4+ pro-B ALL frequently display constitutive FLT3 activation, usually as a result of high-level FLT3 expression, or sporadically from activating mutations within the tyrosine kinase domain.⁸  Activated FLT3 positively affects several signal transduction pathways, all of which favor cell survival and proliferation, and supposedly provides (pre-)leukemic cells with a growth advantage and possibly with enhanced transforming capacity. Hence, FLT3 activation may well be an additional genetic event required for MLL-AF4–driven leukemogenesis. Interestingly, Bueno et al¹  show that in MLL-AF4–expressing hESCs, activated FLT3 is capable of abolishing hematopoietic differentiation, indeed suggesting a role for FLT3 activation in the development of MLL-AF4+ pro-B ALL. However, FLT3 activation did not seem sufficient to cooperate with MLL-AF4 in transforming hESC-derived hematopoietic cells.

Nonetheless, the presented MLL-AF4–expressing hESC model represents an intriguing experimental system that hopefully soon will also be used to explore the impact of other potential secondary oncogenic lesions. In the meantime, it remains important to keep searching for alternative target cells that may resemble the actual cell of origin, as well as additional (epi)genetic hits that potentiate MLL-AF4–driven leukemogenesis (eg, use of whole-genome sequencing approaches).