Very low frequencies of human normal CD34+ haematopoietic progenitor cells express the Wilms' tumour gene WT1 at levels similar to those in leukaemia cells

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Abstract
Summary. The Wilms' tumour gene, WT1, is expressed at high levels in leukaemia cells and plays an important role in leukaemogenesis. WT1 is also expressed in human normal CD34+ bone marrow (BM) cells at about 100 times lower levels than in leukaemia cells. To identify and characterize WT1‐expressing cells in CD34+ BM cells, they were sorted into single cells and analysed for WT1 expression using two kinds of single‐cell reverse transcriptase polymerase chain reaction (RT–PCR) methods. Using the semiquantitative single‐cell polyA‐PCR + sequence‐specific (SS)‐PCR method, WT1 expression was detected in four (1·3%) out of 319 CD34+ BM single cells. To confirm the above results, a single‐cell nested sequence‐specific (NSS)‐RT–PCR method that was less quantitative but more sensitive than the polyA‐PCR + SS‐PCR method was also performed, and WT1 expression was detected in 15 (1·1%) out of 1315 CD34+ BM single cells. In total, WT1 expression was found in 19 (1·2%) out of 1634 CD34+ BM single cells. No significant differences in the frequencies of WT1‐expressing cells were found between CD34+CD38– and CD34+CD38+ BM single cells. Furthermore, WT1‐expressing CD34+ BM single cells expressed WT1 at levels similar to those in K562 leukaemia single cells. Analysis of lineage‐specific and cell cycle gene expression in WT1‐expressing CD34+ BM single cells showed that the WT1 gene could be expressed in both uncommitted, dormant CD34+CD38– and lineage‐committed, proliferating CD34+CD38+ BM cells. Our results could indicate that these WT1‐expressing CD34+ BM cells were normal counterparts of leukaemia cells.