The Significance of Fragile X Mental Retardation Gene 1 CGG Repeat Sizes in the Normal and Intermediate Range in Women With Primary Ovarian Insufficiency

Abstract

Primary ovarian insufficiency (POI) refers to the cessation of menses before the age of 40 years and occurs in 1% to 2% of women. The specific cause of POI is unknown in the majority of cases. Known causes include gene mutations of fragile X mental retardation 1 (FMR1), a gene located on the X chromosome. This gene codes for a protein essential for normal cognitive development and female reproductive function. FMR1 contains a CCG trinucleotide segment in an untranslated region of its DNA that is repeated less than 45 times in normal individuals (normal range of repeats). Mutation of FMR1 results in abnormal expansion of an unstable CGG triplet, leading to impaired cognitive and reproductive function. Between 55 and 200 CCG repeats is called premutation; affected individuals with premutation are at risk for POI and further expansion of repeats in subsequent generations. People with 45 to 54 CCG repeats (intermediate range of repeats) are considered to be at borderline risk for POI and further expansion of repeats. This case-control study was designed to determine whether the risk of POI is associated with CGG repeat length of normal and intermediate range (up to 55 repeats) in a large cohort of Dutch women with idiopathic POI. The second aim of the study was to examine a possible association between the number of CGG repeats and age at first manifestation of POI as a measurement for the severity of POI. The study population was composed of 375 well-phenotyped Dutch women diagnosed with POI; control subjects were 3368 women with natural menopause at 40 years or older. The FMR1 CGG repeat number was determined by polymerase chain reaction amplification of DNA extracted from blood samples drawn from each patient. Fisher exact test was used to assess the prevalence of intermediate-size CGG repeats in POI cases and control subjects. Analysis of variance tested differences in mean CGG repeat lengths on alleles 1 and 2 between POI cases and control subjects. Allele 1 has the lowest triple repeat number, and allele 2 has the highest triple repeat number. There was no significant difference between POI cases and control subjects in the frequency of intermediate-size CGG repeats on allele 2 (POI 2.7 vs control subjects 3.8%; the odds ratio was 0.72, with a 95% confidence interval of 0.38–1.39; P = 0.38). Linear regression analysis showed no association between the number of CGG repeats and age at first POI manifestation (β = 20.019, P = 0.72). These data suggest that intermediate-size CGG repeats are not associated with risk of POI. Therefore, evaluation of normal and intermediate FMR1 repeat size appears to be of little or no value in the diagnostic workup of women affected by POI or in the prognostic assessment of women at risk of developing POI. Reasons for caution in interpreting these results are as follows: (1) FMR1 CGG repeat lengths in POI cases and control subjects were genotyped in 2 different laboratories; technical differences could have affected the results; (2) distributions of CGG repeats could vary among ethnic populations; (3) women with primary amenorrhea (n = 17) were included in the POI group. The last 2 possible limitations were unlikely because exclusion of data from nonwhite women or women with primary amenorrhea did not affect the results.