Synthetic lethal interactions and convergent biological effects underlie the mutual exclusivity of MDS-associated spliceosomal gene mutations

RNA splicing factor mutations constitute the most common class of alterations in myelodysplastic syndromes (MDS). These occur as heterozygous mutations at restricted residues in SF3B1, SRSF2, and U2AF1 in a mutually exclusive manner. The mutual exclusivity of spliceosomal mutations suggests synthetic lethal and/or convergent biological effects of these mutations; however, there is currently no functional evidence supporting either of these possibilities. Here we report that spliceosomal mutations, despite imparting distinct alterations on gene expression and splicing, are negatively selected for when co-expressed in the same cell or in a homozygous state. Co-expression of these mutations results in additive, rather than synergistic, effects on RNA splicing mechanisms. Despite disparate global effects on splicing, aberrant splicing of distinct kinases by mutant SF3B1 and SRSF2 results in hyperactivated NF-κB signaling. These data identify convergent biological consequences of splicing factor mutations and the functional basis for the mutual exclusivity and heterozygous nature of these mutations. To understand the functional basis of this apparent mutual exclusivity of RNA splicing factor mutations, we generated mice to permit inducible heterozygous expression of two of the most common mutations in RNA splicing factors (the SF3B1K700E/+ and SRSF2P95H/+ mutations) simultaneously (Mx1-Cre Sf3b1K700E/+ Srsf2P95H/+)