The X Chromosome and Recurrent Spontaneous Abortion: The Significance of Transmanifesting Carriers

If, as we have argued, a subset of female carriers of X-linked recessive lethal traits are genetically predisposed to spontaneous abortion, and if such carriers can reliably be ascertained via their skewed pattern X chromosome inactivation, it becomes possible to test the hypothesis that X-linked recessive lethal traits are a significant cause of RSA in the general population. Intrinsic to this argument are the assumptions that the trait is cell autonomous—that is, that it causes death or growth disadvantage to the cells with the mutant X active and that hemizygous males survive at least until the pregnancy is clinically observable through a positive bHCG test.To test this hypothesis, we have initiated a case-control study wherein we compare the frequency of highly skewed X chromosome inactivation in women with two or more unexplained spontaneous abortions to the frequency in female controls (Lanasa et al. 1998xSee all References1998). The women characterized with idiopathic RSA have undergone a complete evaluation to rule out any of the known causes of RSA described above (Stephenson 1996xFrequency of factors associated with habitual abortion in 197 couples. Stephenson, MD. Fertil Steril. 1996; 66: 24–29Abstract | Full Text PDF | PubMedSee all References1996). The controls are women from the same demographic region, with no known history of spontaneous abortion; furthermore, the cases and controls are age-distribution matched, so that the distribution of ages between the two groups is the same. Defining skewed X inactivation as preferential use of one X chromosome in ≥90% of peripheral leukocytes, we have found 7 (14.6%) of 48 to have skewed X inactivation. In contrast, only 1 (1.5%) of 68 control females exhibit this extent of nonrandom X inactivation. This finding is statistically significant, with P < .01 (Fisher's exact test, one-tailed).The frequency of nonrandom X inactivation is somewhat lower (1.5%) in our control group than has been reported previously. Other groups have estimated that the frequency of skewed inactivation (at the level of ≥90% silencing of one copy of the chromosome) as 3.2% (Gale et al. 1997xAcquired skewing of X-chromosome inactivation patterns in myeloid cells of the elderly suggests stochastic clonal loss with age. Gale, RE, Fielding, AK, Harrison, CN, and Linch, DC. Br J Haematol. 1997; 98: 512–519Crossref | PubMedSee all References1997) or 3.5% (Plenge et al. 1997xA promoter mutation in the XIST gene in two unrelated families with skewed X-chromosome inactivation. Plenge, RM, Hendrich, BD, Schwartz, C, Arena, JF, Naumova, A, Sapienza, C, Winter, RM et al. Nat Genet. 1997; 17: 353–356Crossref | PubMed | Scopus (189)See all References1997) in women of the same age in the population at large. Although case-control comparisons across studies may be perilous, it is interesting to note that, even when these higher estimates of skewing frequency are used for the control group, the frequency we observe in our group of RSA-affected women remains significant at the level of P < .05 (Fisher's exact test, one-tailed).Although the results presented here are clearly preliminary, it is interesting to speculate about the frequency of X-linked recessive lethal traits in the general population. Given a frequency of idiopathic RSA of 1 in 250 in the general population, and an affection rate of ∼1 in 7 in our case population, the population prevalence of X-linked lethals leading to RSA could be as high as 1 in 1,750. In fact, our ascertainment methodology will miss a large number of carriers, since, on average, a carrier would have to become pregnant five times to show two spontaneous abortions. Furthermore, there is great selective pressure against such traits. As X-linked recessive lethal traits can be passed on only to daughters, the carrier frequency should be halved in each generation. If 1 in 1,750, in fact, approximates the carrier frequency, then the new mutation rate must be 1 in 3,500. Since the highest-known single-gene–mutation rate is that of dystrophin at 1 in 10,000, a mutation rate of 1 in 3,500 indicates extensive genetic heterogeneity. This is consistent with the hypothesis that there are a significant number of vital genes on the X chromosome.RSA is a major women's health concern. As the application of molecular genetics to RSA advances, it will be possible to begin characterizing those genes that cause spontaneous abortion in the recessive state. The X chromosome inactivation assay affords a methodology by which female carriers of X-linked recessive lethal defects can be identified. Over time, then, by assembling familial pedigrees, the individual causative genes can be identified. The X-inactivation assay should become an important diagnostic tool in the clinical evaluation of women with RSA, as secondary skewed X inactivation will be the common denominator by which carriers of X-linked recessive lethal traits can be identified.