Identification of fragile X pre-mutation carriers in the Chinese obstetric population using a robust FMR1 polymerase chain reaction assay: implications for screening and prenatal diagnosis

摘要

There is significant morbidity associated with fragile X syndrome. Unfortunately, most maternal carriers are clinically silent during their reproductive years. Because of this, many experts have put forward the notion of preconception or prenatal fragile X carrier screening for females. This study aimed to determine the prevalence of fragile X syndrome pre-mutation and asymptomatic full-mutation carriers in a Chinese pregnant population, and the distribution of cytosine-guanine-guanine (CGG) repeat numbers using a robust fragile X mental retardation 1 (FMR1) polymerase chain reaction assay. Methods: This was a cross-sectional survey in prospectively recruited pregnant women from a university hospital in Hong Kong. Chinese pregnant women without a family history of fragile X syndrome were recruited between April 2013 and May 2015. A specific FMR1 polymerase chain reaction assay was performed on peripheral blood to determine the CGG repeat number of the FMR1 gene. Prenatal counselling was offered to full-mutation and pre-mutation carriers. Results: In 2650 Chinese pregnant women, two individuals with pre-mutation alleles (0.08%, one in 1325) and one asymptomatic woman with full-mutation (0.04%, one in 2650) alleles were identified. The overall prevalence of pre-mutation and full-mutation alleles was 0.11% (1 in 883). Furthermore, 30 (1.1%) individuals with intermediate alleles were detected. In the 2617 women with normal CGG repeats, the most common CGG repeat allele was 30. Conclusions: The overall prevalence of pre-mutation and asymptomatic full-mutation carriers in the Chinese pregnant population was one in 883, detected by a new FMR1 polymerase chain reaction assay. New knowledge added by this study This study reports the prevalence of fragile X pre-mutation carriers in Chinese pregnant women. The prevalence of pre-mutation and asymptomatic full-mutation carriers was one in 883 and disproves the belief that carrier rates in Chinese are extremely low. Implications for clinical practice or policy Maternal fragile X carriers are not rare in a Chinese population. Women should be offered the option of carrier screening during the preconception period or prenatally. [Abstract in Chinese] Introduction Fragile X syndrome (FXS) is the second leading genetic cause of intellectual disability after Down syndrome, 1 affecting one in 4000 males and one in 8000 females. 2 The typical phenotypes include behavioural abnormalities, autism, cognitive impairment, and dysmorphism such as large protruding ears, elongated face, and macroorchidism in male patients. This syndrome is caused by a defective fragile X mental retardation 1 (FMR1) gene located on the X chromosome, where there is an unstable cytosine-guanine-guanine (CGG) trinucleotide repeat in the 5’ untranslated region. 3 Normally the number of CGG repeats is less than 44, but if it is more than 200 (full mutation), the FMR1 gene expression will be ‘shut down’ due to methylation of its promoter. The protein product, which is essential for normal neurodevelopment, is thus not produced. When the repeat number is between 55 and 200 (pre-mutation), 4 the FMR1 gene can be expressed but the repeat number is potentially expandable to full mutation during its transmission to the next generation. Such risk of expansion is increased with the size of the repeat number, and is close to 100% when the size of CGG repeats is 100 or more. In addition, pre-mutation carriers are at risk of developing fragile X–associated primary ovarian insufficiency (FXPOI) and fragile X–associated tremor/ataxia syndrome (FXTAS) in late adult life, although they are mentally normal. 5 6 Intermediate alleles are repeat numbers between 45 and 54, and individuals carrying theses alleles are at risk of expanding into pre-mutation but not into full mutation. 7 8 9 Because of the significant morbidity associated with FXS, and since most maternal carriers are clinically silent during their reproductive years, many experts have put forward the notion of preconception or prenatal fragile X carrier screening for females. 10 The prevalence of pre-mutation carriers will directly affect the efficacy and cost-effectiveness of screening, but this varies widely between different ethnic groups and countries. While it is well known that Caucasians and Jews have high carrier rates of 1 in 100-250, 10 many studies in Chinese populations report an extremely low carrier rate. 11 12 13 Among these studies, the largest included 10 046 newborn boys, but identified only six pre-mutation carriers (1 in 1674). 13 These studies, however, were limited by the fact that the screening methods used were polymerase chain reaction (PCR) assays that were not accurate or were unable to amplify long CGG repeats. 14 In addition, the screening of a low-risk Chinese pregnant population has not been studied. Recently, we have validated a new fragile-X-related–specific PCR assay that utilises a low-cost capillary electrophoresis instrument an