Fragile-X syndrome in east Finland: molecular approach to genetic and prenatal diagnosis.

Abstract

Fragile X is the most common inherited form of mental retardation, having an incidence of one in 1,250 males. The fragile X syndrome results from amplification of the CGG repeat found in the fragile X mental retardation-1-gene (FMR-1). This CGG repeat shows length variation in normal individuals and is increased significantly in both carriers and patients. Non-retarded carriers of both sexes can be detected reliably only by direct DNA analysis of the fra(X) mutation. Here we investigate fragile X families using the intragenic probes St.B 12.3 and St.B 12.3 XX (J-L Mandel). Fragile X syndrome could be verified in 51 families of the population of 900,000 in East-Finland during 1989-92. The index cases of the syndrome were found by the help of health care personnel, 463 relatives of these families consented to be tested for the mutation of the FMR-1 gene. Ninety-three relatives with the full mutation and 127 healthy carriers with the premutation of the fragile X syndrome were diagnosed. In addition, 28 decreased males were obligate carriers. During the year 1992 prenatal diagnosis for fragile X was carried out in nine pregnancies. For understanding of the occurrence of fragile X syndrome and for better prevention of this syndrome in next generations, systematic genetic counseling and carrier screening in these families is essential.