Sex chromosome aneuploidies among men with systemic lupus erythematosus

Abstract

About 90% of patients with systemic lupus erythematosus (SLE) are female. We hypothesize that the number of X chromosomes, not sex, is a determinate of risk of SLE. Number of X chromosomes was determined by single nucleotide typing and then confirmed by karyotype or fluorescent in situ hybridization in a large group of men with SLE. Presence of an sry gene was assessed by RT-PCR. We calculated 96% confidence intervals using the Adjusted Wald method, and used Bayes' theorem to estimate the prevalence of SLE among 47,XXY and 46,XX men. Among 316 men with SLE, 7 had 47,XXY and 1 had 46,XX. The rate of Klinefelter's syndrome (47,XXY) was statistically different from that found in control men and from the known prevalence in the population. The 46,XX man had an sry gene, which encodes the testes determining factor, on an X chromosome as a result of an abnormal crossover during meiosis. In the case of 46,XX, 1 of 316 was statistically different from the known population prevalence of 1 in 20,000 live male births. A previously reported 46,XX man with SLE had a different molecular mechanism in which there were no common gene copy number abnormalities with our patient. Thus, men with SLE are enriched for conditions with additional X chromosomes. Especially since 46,XX men are generally normal males, except for infertility, these data suggest the number of X chromosomes, not phenotypic sex, is responsible for the sex-bias of SLE.

Figure 1

B plots of the X chromosome in a 46,XY man (A), a 47,XXY man (B) and a 46,XX man (C). In these plots, the fluorescence of the B allele is plotted over the total (A+B allele) fluorescence at all SNPs along the X chromosome. Thus, at any given SNP, with homozygous BB genotype the y-axis value is 1.0, with homozygous AA genotype the y-axis value is 0.0, and with heterozygous AB gentotype the y-axis value is 0.5. Thus, 47,XXY (B) and 46,XX (C) men show numerous SNPs with a 0.5 value indicating the presence of two X chromosomes. At about base position 90 mb the 46,XY shows apparent heterozygocity. In fact, these are not true SNPs, instead being homologous sequence between this point of X and the distal p arm of Y at which there are a single nucleotide differences between the two sex chromosomes. Thus, 46,XY have a heterozygous pattern on the B allele plots. Klinefelter’s syndrome men have a triplicated pattern here (0.0. 0.33, 0.67, and 1.0 B allele frequencies). The 46,XX man also has the triplicated pattern at this point on the X chromosome because a portion of the distal Y chromosome p arm is present. At the far right is sex chromosome FISH of these same individuals.

Figure 2

The androgen receptor gene (AR) on the X chromosome (labeled Chr. X), the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) on chromosome 12 (labeled Chr. 12) and the sry gene on the Y chromosome (labeled Chr. Y) were expanded by rtPCR as previously reported (21). Results are shown for the 46,XX man with SLE. A 1:1 ratio of AR:GAPDH throughout the PCR reaction indicates the presence of an equal number of copies of these two genes, that is, two X chromosomes and two chromosomes 12. Expansion of sry indicates the presence of this gene, which encodes the testes determining factor. This experiment was repeated three times with similar results each time.

Figure 3

Diagram of a normal X chromosome (A) as well as the abnormal X chromosome found in the 46,XX man with SLE reported in this report (B) and the previously reported (41)46,XX male with SLE (C).