SKIDA1 Sustains MLL-ENL-Expressing Hematopoietic Stem and Progenitor Cells
Authors: Mendoza-Castrejon Jonny , Casey Emily B , Patel Riddhi M , Magee Jeffrey A. 
Published: November 2021

Abstract
Chromosomal translocations involving the MLL1 gene often drive infant acute myeloid leukemia (AML). MLL fusion proteins (e.g., MLL-ENL, MLL-AF9, MLL-AF10) activate self-renewal programs in hematopoietic stem and progenitor cells, ultimately leading to transformation. The high frequency of MLL1 rearrangements in infant leukemias suggests that neonatal progenitors are uniquely poised to transform in response to these mutations. Indeed, we have recently shown that MLL-ENL initiates AML more efficiently in neonatal progenitors than in adult progenitors (Okeyo-Owuor et. al. Blood Advances, 2019). This raises the question of whether MLL-ENL induces key effectors of transformation more efficiently in neonatal progenitors than in adult progenitors. We identified Ski/Dach Domain Containing 1 (Skida1) as a gene that is highly induced by MLL-ENL in neonatal, but not adult hematopoietic progenitors. SKIDA1 is also highly expressed in human pediatric AML, and this expression is largely restricted to leukemias with MLL1 rearrangements. These observations suggest that Skida1 may be a critical, neonate-specific effector of MLL-ENL driven leukemogenesis.
To test whether SKIDA1 plays a role in hematopoiesis and leukemogenesis, we generated a germline loss-of-function mouse allele. First, we tested whether loss of SKIDA1 perturbed hematopoiesis by flow cytometry. SKIDA1 deletion caused modest expansion of hematopoietic stem cells (HSCs) in neonates at post-natal day 0 (P0) and a modest decrease of hematopoietic committed progenitor cells (HPCs) at P14, but no changes in fetal or adult HSC/HPC numbers were observed. We then assessed HSC function by transplanting HSCs into lethally irradiated mice to assess their multi-lineage repopulating potential. Loss of SKIDA1 did not perturb HSC function at any stage of development, indicating a minor role in normal hematopoiesis. Next, we tested whether SKIDA1 modulates HSC or HPC numbers in the context of MLL-ENL expression. We crossed Skida1-/- mice to mice with a tetracycline inducible MLL-ENL transgene. The resulting mice (TetO-MLL-ENL; Vav1-Cre) express MLL-ENL specifically in hematopoietic cells beginning at embryonic day 10.5 (E10.5) in the absence of doxycycline, concordant with the onset of Vav1-Cre expression. When we induced MLL-ENL expression in Skida1 -/- mice, we observed near complete loss of HSCs and a severe reduction of HPCs. The reduction in HSC and HPC numbers in TetO-MLL-ENL; Vav1-Cre; Skida1 -/- neonates was far more severe than we observed in TetO-MLL-ENL; Vav1-Cre mice alone. Furthermore, this phenotype emerged between P0 and P14, indicating that it requires a neonatal context to manifest. This coincides with the developmental stage at which MLL-ENL most efficiently induces AML (Okeyo-Owuor et. al. Blood Advances, 2019). This data suggest that SKIDA1 helps sustain pre-leukemic MLL-ENL-expressing HSCs and HPCs shortly after birth. Ongoing studies will address the mechanism by which SKIDA1 regulates AML.
Our current work raises two important points. First, SKIDA1 appears to have a far more important role in sustaining MLL-ENL-expressing progenitors than normal progenitors, raising the prospect for an excellent therapeutic window. Secondly, our findings show that age-specific MLL-ENL targets can shape pre-leukemic hematopoiesis and potentially AML initiation. This may help explain why MLL1 rearrangements account for a higher percentage of infant and childhood leukemias than adult leukemias.

Disclosures
No relevant conflicts of interest to declare.