Detection And Analysis Of Variation Of CGG Repeats Located In The FMR-1 Gene In Normal Chinese Population And Individuals In Four Mental Retardation Families

Fragile X syndrome (FXS) is the most common form of inherited mental retardation after Down syndrome. It affects approximately 1 in 4000 males and 1 in 8000 females in the general population. FXS is characterized by X-linked mental retardation with additional features such as a long face with large protruding ears, macroorchidism, and eye-gaze avoidance etc. FXS is associated with a fragile site, designated FRAXA (Fragile site, X chromosome, A site), at Xq27.3 near the end of the long arm.FXS is mainly caused by a kind of dynamic mutation, the expansion of a CGG repeat located in the 5'-untranslated region (5'-UTR) of the FMR1 (fragile X mental retardation) gene. In most affected individuals, CGG repeats are massively expanded over 200 repeats (full mutation) and become abnormally h ypermethylated, w hich r esult i n t he s ilence o f t he FMR1 gene. The absence of the FMR1 gene product, fragile X mental retardation protein (FMRP), is believed to be responsible for the typical physical and mental characteristics of the fragile X syndrome. Alleles with between 43 and 200 CGG repeats are called permutation. They are generally unmethylated with normal transcript and protein level, but are extremely unstable during transmission to next generation. However, mosaicism of both CGG repeatlength and methylation status exists in fragile X patients. Other mutations of the FMR1 gene, s uch a s d eletions a nd p oint m utations a mong p atients w ith usual phenotype but without fragile site expression, are also identified.To detect the patients and carriers of the fragile X syndrome, various analytic methods could be used. One is cytogenetic diagnosis to observe the expression of the fragile site FRAXA located at Xq27.3. Southern blot can be performed to analyze the expansion of the CGG repeat as well, while the FMRP antibody test allows rapid detection of male patients. Given limitation existing in all those methods, PCR analysis, an efficient method for rapid screening and diagnosis, has been used more and more.In the normal population, the number of CGG repeats in FMR1 is highly polymorphic with the variation ranging from 6 to 54. Such statistical data are obtained from Caucasia people. The population studys about CGG repeats in FMR1 have been reported on S outhem C hinese i n T aiwan, Hongkong and Chinese Mainland. Therefore in order to detect the carriers in the population and FXS patients with full mutation exactly, it will be very helpful to study the variable distribution of CGG repeats of FMR1 in Chinese.PCR-based analysis method was carried out in the following study. DNA amplification by PCR in CGG repeats were performed on 154 people as normal control (73 males and 81 females) and 23 members from four X-linked mental retardation (XLMR) families. After electrophoresed on a 6%-denaturing polyacrylamide gel (Acr:Bis=19:l), genotypes of the objects were determined by analyzing PCR products. 19 different alleles were detected inthe normal population with the CGG repeats varying from 12 to 40. The most common allele of CGG repeats was n=29 and its frequency was 16.2%. Abnormal expansion in CGG repeats of FMR1 were not detected on 23 members of the XLMR families. Unstable transmission within the CGG repeats less than 43 were found in the detection. It excluded the expansion of CGG repeats in FMR1 gene to be the cause of the 7 patients of the XLMR families. The study on variable distribution of FMR1 CGG repeats in Chinese people set up a simple and effective method for screening and gene diagnosis for FXS.