X‐chromosome inactivation patterns using HPRT and PGK polymorphisms in haematologically normal and post‐chemotherapy females

Abstract

Clonal analysis using X-chromosome inactivation patterns of the hypoxanthine phosphoribosyl transferase (HPRT) and phosphoglycerate kinase (PGK) genes has been used in the assessment of haematological malignancies. In many analyses normal haemopoietic tissue or constitutive DNA from each individual has not been readily available and it has been assumed that when the ratio of the percentage of the two alleles remaining after differential methylation enzyme digestion is greater than 3/1 then an oligoclonal/monoclonal population of cells is present. We report here in a study of 65 haematologically normal females informative for HPRT or PGK that 15 (23%) had a skewed Lyonization pattern with clonal analysis ratios greater than 3/1. No correlation was observed between age and the X-chromosome inactivation pattern to suggest the detection of pre-malignant clonal states. The incidence was similar in 23 females who had received conventional combination chemotherapy, with six women (26%) having a ratio greater than 3/1. In the majority of individuals therefore, conventional chemotherapy does not appear to alter the patient's X-chromosome inactivation pattern. These results indicate that caution must be exercised in interpreting the results of clonal analysis of any given individual.

X-inactivation analysis in the 1990s: promise and potential problems

“…Vogelstein et al 32 found only three out of 81 (3.7%) normal females to have NRXI using the phosphoglycerate kinase (PGK) and hypoxanthine phosphoribosyl transferase (HPRT) Xinactivation probes. Recently, Gale et al 29 found significant NRXI in blood-derived cells in 23% of normal females using PGK and HPRT probes and 22% of females using M27␤, 30 which has been confirmed in numerous other studies. [33][34][35][36] Discordance in the incidence of NRXI was initially explained by the diversity of assays used, the different criteria for NRXI, and the small population sizes.…”

“…Although criteria for NRXI are arbitrary, an allelic Review L Busque and DG Gilliland 129 ratio у3:1 (which corresponds to the expression of 75% of one allele) has been widely accepted in the literature. 13,22,[29][30][31][32] Determination of the incidence of NRXI should be straightforward since it can be simply estimated by using X-inactivation assays in healthy females. Surprisingly, data obtained in the last 20 years by different investigators have been contradictory.…”

“…5,29 In this model, the variance of the distribution of X-inactivation patterns in females follows a binomial distribution where the deviation from the mean is inversely proportional to the number of cells present at the time of X-inactivation. For example, if the inactivation occurs when there are only four cells present, the incidence of skewing (ratio у3:1) will be 37%.…”

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

“…In neonates, the median value of this score was 59.5% in epithelial cells and 62.2% in blood cells, while in mothers, it was 60.9% and 65.7%, respectively. In accordance with the literature (42)(43)(44)(45), skewing was defined as equal to or greater than 75% of the predominant allele. Using this criterion, 7.6% (33 of 436)…”

“…Skewing is said to be present when the percentage of the predominant allele exceeds 74% (i.e., is ≥75%). This arbitrary definition has been used by a number of other groups (69)(70)(71)(72). A percentage of predominant allele between 90% and 100% is considered extreme skewing.…”