UCLA

Hematopoietic stem cell (HSC) transplantation can cure life-threatening blood disorders such as leukemias and inherited blood diseases. However, shortage of HLA-matched donors, which disproportionally affects minorities and patients of mixed ethnic backgrounds, limits the number of patients that can be treated. Ex vivo expansion or de novo generation of transplantable human HSCs has not been successful due to poor understanding of the basic biology underpinning key HSC traits - self-renewal, engraftment and multi-lineage differentiation ability (together referred to as “stemness”). Previous studies identified MLLT3 as a key regulator of stemness in human HSCs whose expansion declines in culture and differentiation, and demonstrated that maintaining MLLT3 expression in culture expands transplantable HSCs. The focus of this is the characterization of a truncated isoform of MLLT3 . Analysis of RNA-seq data and epigenetic marks associated with the MLLT3 gene in human HSCs revealed a second TSS linked to a novel MLLT3 isoform (MLLT3-S), which encodes a truncated protein that can interact with known MLLT3 protein partners such as the Superelongation Complex (SEC) and Dot1L, but is unable to bind chromatin. MLLT3-L and MLLT3-S expression has opposing effects on gene expression and expansion of human HSCs in culture, suggesting distinct but complementary roles for the two isoforms. However, both isoforms of MLLT3 are necessary for proper HSC function. The MLLT3-S enhancer is accessible prior to its induction in fetal liver (FL) HSCs during their maturation, and coincides with downregulation of IGFBP2 expression. IGFBP2 is typically downregulated during HSC maturation, but its renewed expression is required for HSC proliferation in culture and MLLT3-L driven HSC expansion. The interplay between long and short isoforms of MLLT3 in human HSCs may provide a mechanism by which mature HSCs balance between expansion and maintenance modes.