Accumulation of an intron-retained mRNA for granulocyte macrophage-colony stimulating factor receptor common beta chain in neutrophils of myelodysplastic syndromes.

We recently identified a reduction in the neutrophil surface expression of common beta chain (beta c) of the receptor for granulocyte macrophage-colony stimulating factor (GM-CSF) in the patients with myelodysplastic syndromes (MDS). To determine the etiology of the impaired beta c expression, beta c mRNA from neutrophilic granulocytes of MDS patients and healthy controls was analyzed by a combination of direct reverse transcriptase-polymerase chain reaction-based single-strand conformational polymorphism and sequencing. Nine different beta c transcripts were detected, but none was specific for MDS. However, one of the transcripts (beta c79) containing a 79-base intron insertion between exons V and VI was significantly increased in MDS. This 27-kd isoform consisted of the beta c N-terminal 182 amino acids followed by a new 84-amino-acid sequence. beta c79 was overexpressed in all MDS subtypes. No genomic mutations were detected within the intron or at the intron/exon boundaries. The isoform is predominantly located in the cytoplasm by Western blot analysis and was unable to generate high-affinity binding sites or transduce a signal for proliferation when coexpressed with the receptor for human GM-CSF alpha chain. Our study suggests that the accumulation of the abnormal beta c transcripts with intron V retention results in the reduction in cell-surface expression of beta c observed in MDS.