Skewing of X-inactivation ratios in blood cells of aging women is confirmed by independent methodologies

Abstract

Nonrandom X-chromosome inactivation (XCI), also known as skewing, has been documented in the blood cells of a significant proportion of normal aging women by the use of methylation-based assays at the polymorphic human androgen receptor locus (HUMARA). Recent data obtained with a new transcription-based XCI determination method, termed suppressive polymerase chain reaction (PCR), has shed controversy over the validity of XCI ratio results obtained with HUMARA. To resolve this disparity, we analyzed XCI in polymorphonuclear leukocytes of a large cohort of women aged 43 to 100 years with the use of HUMARA (n=100), a TaqMan single nucleotide polymorphism (SNP) assay (n=90), and the suppressive polymerase chain reaction (PCR) assay (n=67). The 3 methods yielded similar skewing incidences (42%, 38%, and 40%, respectively), and highly concordant XCI ratios. This confirms that the skewing of XCI ratio seen in blood cells of aging women is a bona fide and robust biologic phenomenon.

Figure 1

Correlation of XCI patterns obtained with the use of 3 different methods. (A) HUMARA assay versus TaqMan SNP assay at the IDS locus. For each individual, the %Sup measured by HUMARA was compared with the percentage of IDS allele obtained with the TaqMan SNP assay, which was quantitatively closer to the HUMARA value. Data from both assays were available for 90 women. (B) HUMARA assay versus suppressive PCR assay at the IDS locus. The %Sup obtained by HUMARA was compared with the percentage of IDS allele obtained with the suppressive PCR assay, which was quantitatively closer to the HUMARA value. Data from both assays were available for 67 women. (C) TaqMan SNP assay versus suppressive PCR assay. The percentage of IDS allele obtained with the TaqMan SNP assay was compared with that obtained with the suppressive PCR assay. Data from both assays were available for 60 women. Those aged 44 to 65 years are represented by ▵, whereas those older than 65 years are represented by ●.