No evidence that skewing of X chromosome inactivation patterns is transmitted to offspring in humans

Abstract

Skewing of X chromosome inactivation (XCI) can occur in normal females and increases in tissues with age. The mechanisms underlying skewing in normal females, however, remain controversial. To better understand the phenomenon of XCI in nondisease states, we evaluated XCI patterns in epithelial and hematopoietic cells of over 500 healthy female mother-neonate pairs. The incidence of skewing observed in mothers was twice that observed in neonates, and in both cohorts, the incidence of XCI was lower in epithelial cells than hematopoietic cells. These results suggest that XCI incidence varies by tissue type and that age-dependent mechanisms can influence skewing in both epithelial and hematopoietic cells. In both cohorts, a correlation was identified in the direction of skewing in epithelial and hematopoietic cells, suggesting common underlying skewing mechanisms across tissues. However, there was no correlation between the XCI patterns of mothers and their respective neonates, and skewed mothers gave birth to skewed neonates at the same frequency as nonskewed mothers. Taken together, our data suggest that in humans, the XCI pattern observed at birth does not reflect a single heritable genetic locus, but rather corresponds to a complex trait determined, at least in part, by selection biases occurring after XCI. IntroductionIn mammals, dosage compensation of X-linked genes in females is achieved by the transcriptional silencing of 1 of the 2 X chromosomes during early development, a process known as X chromosome inactivation (XCI) (1). Once XCI has occurred, the heterochromatin-enriched inactive X chromosome is stably transmitted to each daughter cell through subsequent mitoses. In human and mouse embryos, either of the 2 X chromosomes (paternal or maternal) can be inactivated. As a consequence, females from both species present a mosaic pattern of 2 cell lines, one expressing maternal X-linked genes and the other expressing paternal X-linked genes. The XCI ratio (or XCI pattern) corresponds to the relative proportion of each cell line. This ratio can be determined by the analysis of expression (2), transcription (3), or differential methylation (4) of polymorphic X-linked genes, such as the human androgen receptor (HUMARA) gene (5). Significant deviation from the theoretical 1:1 ratio between the 2 parental alleles is called skewing. As it was first suggested by pioneers of X-inactivation analysis, including Fialkow (6) and Vogelstein (7), in most studies, skewing and extreme skewing are arbitrarily defined as greater than or equal to 75% and greater than or equal to 90% of cells expressing the same X chromosome, respectively.Skewed patterns of XCI were initially found in female carriers of specific X-linked mutations (8, 9) where skewing resulted from

Chronic hemolytic anemia is associated with a new glucose-6-phosphate dehydrogenase in-frame deletion in an older woman

“…2), showed a similar PCR amplification amount of two HUMARA alleles (278-and 287-bp) in accordance to a balanced pattern of XCI (Fig. 2C-1), with a Psup score of 40%, according to the equation reported by Bolduc et al [10]. Thus, patient HUMARA data was not in keeping with the blood G6PD mRNA studies which point to a marked increase expression of the mutant allele, accounting for the late development of her chronic hemolytic anemia.…”

“…However, data from Bolduc et al shows that the skewing extent varies by tissue type, although maintaining the direction (toward maternal or paternal allele) across tissues, in accordance with the concept that the primary XCI occurs during early development, prior to the splitting of the inner cell mass and to tissue specification [10].…”

“…The allele's size and area under the curve (AUC) was measured with the software ALFwin™ Fragment Analyser 1.00 (Amersham Pharmacia Biotech). The ratio of the active/inactive X chromosome was estimated according the equation defined by Bolduc et al [10]:…”

“…A Psup score equal or greater than 75% or equal or less than 25% indicates skewing. The Psup specifies the direction of skewing: Psup N50% indicates the X chromosome bearing the superior allele is predominant, while Psup b50% indicates the opposite [10].…”

“…Deviation from the theoretical 1:1 ratio between the 2 parental alleles is called skewing. Several reports using the HUMARA assay show that the incidence of skewing is relatively low at birth and in adult non-hematopoietic tissues, but higher and agedependent in hematopoietic cells, with 30-40% of healthy elderly women reported to have skewing (greater than 75% expression of one parental X chromosome) [5][6][7][8][9][10][11][12]. T lymphocytes show less evidence of skewing with age than neutrophils, presumably reflecting the neutrophils' short half-life, whereas T lymphocytes are long-living cells, and in consequence, some T cells are produced near the time of study but others are derived from earlier stem cells [11][12][13].…”