NONRANDOM X-INACTIVATION PATTERNS IN NORMAL FEMALES - LYONIZATION RATIOS VARY WITH AGE

The utility of X-inactivation based clonality assays for evaluation of human neoplasia is well-documented. However, excessive Lyonization is a potential limitation of these assays, because it mimics clonal deri vation of cells. The incidence of excessive Lyonization in healthy fem ales is controversial, with reported incidence varying from 4% to 33%, Several explanations have been offered for the observed variation, in cluding different criteria for excessive Lyonization, diversity of X-l inked clonality assays, small population sizes and more recently, tiss ue specificity of X-inactivation patterns. However, it is also possibl e that stem cell depletion, clonal hematopoiesis, or selection pressur es on blood cells results in an increased incidence of excessive skewi ng. If any of the latter is true, then the incidence of excessive skew ing should increase with age in blood cells. To test this hypothesis, we determined X-inactivation ratios at the human androgen receptor loc us of 295 normal females from three age groups: (1) neonates, (2) fema les 28 to 32 years old and, (3) females aged greater than or equal to 60 years. The incidence of skewing (allele ratios greater than or equa l to 3:1) was 8.6% (14 of 162) in neonates (P = .104 v 28 to 32);16.4% (11/67) in 28 to 32 y.o. (P = .0064 v greater than or equal to 60), a nd 37.9% (25 of 66) in women greater than or equal to 60 y.o. (P < .00 01 v neonates). When a more stringent criterion for skewing was used ( allele ratios greater than or equal to 10:1), the incidence was 1.9% ( 3 of 162) in neonates (P = .362 v 28 to 32), 4.5% (3 of 67) in 28 to 3 2 y.o. (P = .0022 v greater than or equal to 60), and 22.7% (15 of 66) in greater than or equal to 60 y.o. group (P < .0001 v neonates). Res ults have been confirmed at the phophoglycerate kinase locus for 48 he terozygous females. The incidence of excessive skewing increases with age. In neonates, the incidence is low and may correspond to true exce ssive Lyonization. Acquired skewing occurs with aging in normal female s and is present in 38% of females over the age of 60. Further studies are needed to determine whether acquired skewing is a consequence of stem cell depletion, true clonal hematopoiesis, growth advantage confe rred by parental-specific X-chromosomes, or other causes. These data p rovide an explanation for variation in reported incidence of excessive skewing in normal females. Furthermore, these findings suggest that a ny study of clonality using X-inactivation based assays should incorpo rate age-matched controls for Lyonization. (C) 1996 by The American So ciety of Hematology.