Comparison of X-chromosome inactivation patterns in multiple tissues from human females

Abstract

Background: X-chromosome inactivation (XCI) is the mechanism by which gene dosage uniformity is achieved between female mammals with two X chromosomes and male mammals with a single X chromosome, and is thought to occur randomly. For molecular genetic testing, accessible tissues (eg blood) are commonly studied, but the relationship with inaccessible tissues (eg brain) is poorly understood. For accessible tissues to be informative for genetic analysis, a high degree of concordance of genetic findings among tissue types is required.

Objective: To determine the relationship among multiple tissues within females at different ages (fetus to 82 years).

Methods: XCI patterns were analysed using the polymorphic androgen receptor (AR) gene assay. DNA was isolated from 26 different human females without history of malignancy, using 34 autopsy tissues representing the three embryonic germ layers.

Results: 33 of the 280 tissue samples analysed from 13 of the 26 females showed skewed XCI values (>80:20%). Average XCI value was not significantly different among the tissues, but a trend for increasing XCI variability was observed with age in blood and other tissues studied (eg the SD for all tissues studied for the 0-2 years group was 9.9% compared with 14.8% in the >60 years group). We found a significant correlation (r(s) = 0.51, p = 0.035) between XCI values for blood and/or spleen and brain tissue, and in most other tissues representing the three embryonic germ layers.

Conclusions: In our study, XCI data were comparable among accessible (eg blood) and inaccessible tissues (eg brain) in females at various ages, and may be useful for genetic testing. A trend was seen for greater XCI variability with increasing age, particularly in older women (>60 years).

Figure 1

Scatterplot of X-chromosome inactivation values for blood/spleen and brain (cerebrum) versus age. d, Days; w, weeks; y, years.

Figure 2

Scatterplot of X-chromosome inactivation values for blood/ spleen versus brain. Lines are +/−10% variation in allele inactivation to allow for technical variation. When both blood and spleen were available from an individual, the average of the X-chromosome inactivation values was taken. w, Weeks; y, years.

Figure 3

Mean difference in X-chromosome inactivation for germ layers by age group. Mean absolute value of the difference between X-chromosome inactivation value for the first (smaller) and second alleles was calculated for each individual. Mean (SD) were determined for each subject age group and for tissues representing each of the three germ layers.

Figure 4

Pie chart illustrating the proportion (%) of the age-group average SD, calculated from the variance of each individual within a group. Group mean variance in percentage X-chromosome inactivation of the first allele was calculated for all tissues from each individual and grouped by subject age, and the SD calculated from the individual variances.