Molecular analysis of the (CGG)n expansion in the FMR-1 gene in 59 Spanish fragile X syndrome families

Abstract

The fragile X mental retardation syndrome is caused by an expansion of a trinucleotide repeat (CGG)n in the FMR-1 gene. Molecular genetic study of fragile X provides accurate diagnosis and facilitates genetic counseling in families with affected members. We present here the molecular study of 59 Spanish fragile X syndrome families using probe StB 12.3 and the polymerase chain reaction (PCR) of the (CGG)n repeat sequence of the FMR-1 gene. The results obtained have allowed us to characterize 455 individuals, including eight prenatal diagnoses. The clinical diagnosis of fragile X in 89 affected males was confirmed, 137 female carriers were identified (48 of whom were mentally retarded), 176 individuals "at risk" were found not to have the expansion, and 12 cases of normal transmitting males (NTM) were detected. In the sample studied, no de novo mutations were detected, nor any mutation different from that described for the (CGG)n expansion. One nonmentally retarded male was detected as having an unmethylated CpG island for the FMR-1 gene, but with more than 200 CGG repeats (high functioning male). The analysis of the (CGG)n repeat in 208 normal chromosomes gave an allele distribution similar to that in other Caucasoid population groups, with alleles of 29 and 30 CGG repeats accounting for 46% of the chromosomes. The combination of Southern analysis and PCR of the (CGG)n repeat is highly efficient for diagnosis, compared with cytogenetic techniques, especially in the detection of female carriers, NTMs, and prenatal diagnosis, enabling accurate genetic counseling to be provided in all cases.

Fragile X founder effect and distribution of CGG repeats among the mentally retarded population of Andalusia, South Spain

“…Based on these results, we considered that non-fragile-X individuals were representative of the normal population for the FRAXA locus, although a larger number of X chromosomes from our normal population should be studied. Considering the range between 5 and 52 triplets as normal, our sample contained 90% of small alleles (< 35 CGG) and 10% of large alleles (>35 CGG), in agreement with a previous report (Milá et al, 1994).…”

“…In addition to all the previously described DXS548 alleles, we detected a new allele which we called "allele 10" (17 CA), following the terminology recommended by Macpherson et al (1994) (Figure 2). The frequencies of DXS548 alleles in our sample show a slightly higher genetic diversity than in other populations, which probably reflects Spain's more heterogeneous genetic background, as it has been previously reported for other loci (Bertrantpetit and Cavalli-Sforza, 1991;Cavalli-Sforza and Piazza, 1993;Chillon et al, 1994;Milá et al, 1994;Milá, 1997). In the non-fragile X chromosomes, we observed that the most frequent DXS548 allele was allele 7 (20 CA, 47%), followed by allele 6 (21 AC, 23%) (Table 1).…”

Distribution of Two X-Linked Trinucleotide Polymorphisms in Greece

“…To determine the clinical importance of this observation, a population-based study for the identification of fragile X families in Crete is necessary. Based on our results and those of other groups, the Hellenic population overall is rather similar to other European populations [15, 16, 17]. The mean repeat number in the FMR-1 gene is 30 and the mean in the AR gene is 21 (tables 1, 2).…”

Screening for Expanded Alleles of the FMR1 Gene in Blood Spots from Newborn Males in a Spanish Population

“…Previous studies have indicated that the most common alleles vary of few repeats in different populations including 28 CGG repeats, 20 29 CGG repeats in Taiwan, 27 30 CGG repeats, 28 29 and 30 CGG repeats in Spain, 29 and 29 CGG repeats in a Chinese population. 30 However, it should be noted that small differences among studies (1-2 CGG repeats) may be a consequence of experimental errors in various labs in the absence of single repeat precision and sequenced CGG standards.…”