Myelodysplastic syndromes (MDS) are a heterogeneous group of incurable malignant blood cell diseases with largely unknown aetiology. Approximately 30% of MDS patients progress to acute myeloid leukaemia (AML). Mouse models that faithfully represent MDS are based on rare genetic abnormalities and currently represent only a very small subset of MDS patients.
We have discovered that homeobox A1 (HOXA1) mRNA is upregulated 2-fold in 50% of MDS patients, irrespective of karyotype (n=183 MDS, 17 controls, P<0.05). Human HOXA1 and mouse Hoxa1 (WT-Hoxa1) are expressed as two different spliceforms generated by alternative splicing within exon 1: a full-length form (Hoxa1-FL) and a truncated form (Hoxa1-T), which lacks the homeobox domain. Mutations in splicing machinery have been recently identified in up to 85% of MDS patients. We therefore hypothesised that deregulated HOXA1 spliceforms may contribute to MDS.
Retroviral overexpression of either WT-Hoxa1 (which generates both Hoxa1-FL and Hoxa1-T) or Hoxa1-T in murine bone marrow (BM) cells showed opposing effects, suggesting that Hoxa1-T may negatively regulate Hoxa1-FL. We therefore generated a mutant Hoxa1 (MUT-Hoxa1), which expresses normal Hoxa1-FL but not Hoxa1-T, by site-directed mutagenesis at the splice site of Hoxa1-T. We transplanted recipient mice with BM overexpressing MUT-Hoxa1, WT-Hoxa1 or empty vector control.
All recipients of WT-Hoxa1 or MUT-Hoxa1 BM developed features resembling human MDS including peripheral blood macrocytic anaemia and thrombocytopenia, dysplastic features in BM erythroid and megakaryocyte lineages and increased apoptosis in BM erythroid cells. Strikingly, recipients of MUT-Hoxa1 BM developed a more severe MDS phenotype that spontaneously progressed to AML.
In summary, altered HOXA1 spliceform expression likely contributes to MDS in up to 50% of patients. Our MUT-Hoxa1 and WT-Hoxa1 mouse models will therefore be highly valuable in identifying better therapies for a significant subset of MDS patients.