Twelve families with fragile X(q27)

High functioning fragile X males: demonstration of an unmethylated fully expanded FMR-1 mutation associated with protein expression

Fragile X (fra(X)) males with a standardized IQ score of 70 or higher represent a high functioning (HF) or nonretarded fra(X) male group. This group, which does not include nonpenetrant males, has received little research attention to date. Of 221 fra(X) males who had been evaluated through The Children's Hospital in Denver since 1981 and had completed cognitive or developmental testing, 29 (13%) were high functioning by the above definition. We found that HF males on the whole had a lower cytogenetic score and were younger than retarded fra(X) males, but there was no difference between these two groups in the number of typical fra(X) physical manifestations present. FMR-1 DNA testing was performed on 134 fra(X) males and methylation status was determined for 51 of these. A greater percentage of HF males had a mosaic pattern or an incompletely methylated full mutation than did retarded males. A unique DNA pattern, an unmethylated fully expanded mutation, was discovered in 3 of the highest functioning fra(X) males. Protein studies performed on 2 of these males demonstrated the presence of FMR-1 protein, albeit at lower levels than normal. FMR-1 protein was not present in retarded fra(X) males. Significant FMR-1 protein expression may be responsible for higher cognitive functioning in the 2 males with unmethylated fully expanded mutations compared to retarded fra(X) males.

Between-generation differences in ascertainment and penetrance: relevance to genetic hypotheses in fragile X

Between-generation differences in ascertainment were examined in 54 extended fragile X pedigrees, where all available members were clinically, psychometrically, and cytogenetically investigated. In 24 families a diagnosis was verified by molecular characterization using the pfxa3 fragile X-specific probe. We found considerable differences between generations in relative proportions of affected fragile X subjects versus 'non-penetrant' carriers. We also found deviation in the segregation ratio in unbiased samples of relatives in pedigrees. We claim that these irregularities are influenced by different rates of ascertainment, depending on the clinical expression of the condition (penetrance) and the fertility of fragile X individuals in a pedigree, as well as by the thoroughness of clinical investigation in individual families. Penetrance and fertility were estimated in fragile X females assessed by psychometric tests, and they were compared with earlier estimates based on a subjective judgement of their intellectual status. We suggest that the standard correction for ascertainment bias, such as has been applied in segregation analysis of this condition, is not sufficient to adjust for all types of bias.

Recurrence risks for relatives in families with an isolated case of the fragile X syndrome

The proportion of sporadic cases among affected males with fragile X-related mental impairment was reinvestigated in a new sample of family data and compared to previous studies. It was found that the estimate has increased over time from 0 in the original study to 0.24 in the present study. This difference indicated that the correction used for the ascertainment of families in the original study may not have been adequate and that the suggestion that all mothers of affected males are obligate carriers may be wrong. Based on this new information, recurrence risks for relatives in a family with an isolated case of the fragile X or Martin-Bell syndrome were calculated under different assumptions in order to investigate the effect of 1) the knowledge of the phenotype of ancestors of the proband, 2) the dependence of expression of the mutation on the sex of the carrier parent, 3) the value of the penetrance of mental impairment (MI), and 4) the equality of mutation rates in egg and sperm. The assumptions made for modelling the mutational process had the greatest effect on the recurrence risk in sibs of an isolated case, whereas small differences in penetrance parameters and assumptions based on whether the ancestors were known to be normal or of unknown phenotype made little difference. Recurrence risks for the sibs and first cousins of an isolated case calculated under different assumptions are presented.

Investigation of the segregation of the fragile X mutation in daughters of obligate carrier women

Two reports have suggested that over 50% of the offspring of obligate carrier women receive the mutation for the fra(X) or the Martin-Bell syndrome [Webb et al, 1986; Fryns, 1984]. Such a segregation distortion is difficult to assess for the fra(X) syndrome because of incomplete penetrance, variable expression and probable ascertainment biases. We have attempted to evaluate this possible segregation distortion in daughters of obligate carriers in a large sample of sibships ascertained in a survey of New South Wales, Australia. We used two definitions of expression: 1) presence of fra(X) positive cells if daughters were tested cytogenetically, and 2) mental impairment if daughters were not tested cytogenetically. The segregation frequency was estimated in different types of sibships of obligate carriers based on the way they were ascertained. This was done in order to have an internal check on possible ascertainment biases. Among the 189 cytogenetically tested daughters, 81 were fra(X) positive. Among the 97 untested daughters, 24 were mentally impaired in some way. Therefore, the segregation frequency as defined by fra(X) expression and/or mental impairment was 37%. Thus, no evidence was detected for segregation distortion. These data were significantly different than those collected by Webb et al [1986] and scored by the same method as the present data set.

Fragile X syndrome

The fragile X syndrome is the most common inherited form of mental retardation known. Its phenotype includes large or prominent ears, macroorchidism, and characteristic behavioral problems. It has attracted the interest of cytogeneticists and molecular biologists because of its characteristic fragile site on the X chromosome. It has puzzled geneticists because of its unusual inheritance pattern involving nonpenetrant males. This syndrome has also spearheaded an appreciation of cytogenetic abnormalities in the etiology of all degrees of developmental delay.

The frequency of the fragile X chromosome among schoolchildren in Coventry

A population study has been carried out among schoolchildren in the City of Coventry in order to ascertain the frequency of mental retardation associated with the fragile X chromosome. The prevalence of the fragile X mental retardation syndrome in the 11 to 16 year age group (the age of greatest ascertainment) was about 1.0 per 1,000 and therefore indicates that the syndrome is a major cause of mental retardation.