X-chromosome inactivation patterns in females with Prader-Willi syndrome

Abstract

Prader-Willi syndrome (PWS) is a complex neurodevelopmental disorder caused by loss of paternally expressed genes from the 15q11-q13 region generally due to a paternally-derived deletion of the 15q11-q13 region or maternal disomy 15 (UPD). Maternal disomy 15 is usually caused by maternal meiosis I non-disjunction associated with advanced maternal age and after fertilization with a normal sperm leading to trisomy 15, a lethal condition unless trisomy rescue occurs with loss of the paternal chromosome 15. To further characterize the pathogenesis of maternal disomy 15 process in PWS, the status of X-chromosome inactivation was calculated to determine whether non-random skewing of X-inactivation is present indicating a small pool of early embryonic cells. We studied X-chromosome inactivation in 25 females with PWS-UPD, 35 with PWS-deletion, and 50 controls (with similar means, medians, and age ranges) using the polymorphic androgen receptor (AR) gene assay. A significant positive correlation (r = 0.5, P = 0.01) was seen between X-chromosome inactivation and age for only the UPD group. Furthermore, a significantly increased level (P = 0.02) of extreme X-inactivation skewness (>90%) was detected in our PWS-UPD group (24%) compared to controls (4%). This observation could indicate that trisomy 15 occurred at conceptus with trisomy rescue in early pregnancy leading to extreme skewness in several PWS-UPD subjects. Extreme X-inactivation skewness may also lead to additional risks for X-linked recessive disorders in PWS females with UPD and extreme X-chromosome skewness.

FIG. 1

X-inactivation analysis by genotyping of a polymorphic region in the AR gene (CAG repeat) using methyl sensitive restriction enzyme (HpaII) method. Genotyping of undigested genomic DNA (top) and HpaII digested DNA (bottom) are shown. The peak representing the DNA from the active (unmethylated) X-chromosome allele would be digested by the methyl sensitive enzyme and reduced in size. If non-random X-inactivation skewness is present the peak height would differ after digestion between the two alleles representing each X-chromosome (methylated-inactive and unmethylated-active). Examples of randomly and extremely skewed (>90%) X-inactivation are shown.

FIG. 2

Distribution of patterns of X-inactivation in 50 controls, 25 PWS-UPD, and 35 PWS-deletion females. X-inactivation patterns were divided into four categories (i.e., extremely, highly, moderately, and randomly skewed). A significantly greater percentage (Chi-square test; P < 0.05) of extremely skewed X-inactivation (>90%) was detected in the PWS-UPD group (24%) compared with controls (4%) shown with asterisks.