Hematopoiesis is not clonal in healthy elderly women. Commentary

Clonality assays, based on X-chromosome inactivation, discriminate active from inactive alleles. Skewing of X-chromosome allelic usage, based on preferential methylation of one of the HUMARA alleles, was reported as evidence of clonal hematopoiesis in approximately 30% of elderly women. Using a quantitative, transcriptionally based clonality assay, we reported X-chromosome-transcribed allelic ratio in blood cells of healthy women consistent with random X-inactivation of 8 embryonic hematopoietic stem cells. Furthermore, we did not detect clonal hematopoiesis in more than 200 healthy nonelderly women. In view of the susceptibility of aging hematopoietic stem cells to epigenetic dysregulation, we reinvestigated the issue of clonality in elderly women. Forty healthy women (ages 65-92 years; mean, 81.3 years) were tested by a novel, quantitative polymerase chain reaction (qPCR) transcriptional clonality assay. We did not detect clonal hematopoiesis in any of the tested subjects. We also tested DNA from the same granulocyte samples using the methylation-based HUMARA assay, and confirmed previous reports of approximately 30% extensively skewed or monoallelic methylation, in agreement with likely age-related deregulated methylation of the HUMARA gene locus. We conclude that the transcriptionally based X-chromosome clonality assays are suitable for evaluation of clonal hematopoiesis in elderly women.