Bert B.A. de Vries

Screening and Diagnosis for the Fragile X Syndrome among the Mentally Retarded: An Epidemiological and Psychological Survey

The fragile X syndrome is an X-linked mental retardation disorder caused by an expanded CGG repeat in the first exon of the fragile X mental retardation (FMR1) gene. Its frequency, X-linked inheritance, and consequences for relatives all prompt for diagnosis of this disorder on a large scale in all affected individuals. A screening for the fragile X syndrome has been conducted in a representative sample of 3,352 individuals in schools and institutes for the mentally retarded in the southwestern Netherlands, by use of a brief physical examination and the DNA test. The attitudes and reactions of (non)consenting parents/guardians were studied by (pre- and posttest) questionnaires. A total of 2,189 individuals (65%) were eligible for testing, since they had no valid diagnosis, cerebral palsy, or a previous test for the FMR1 gene mutation. Seventy percent (1,531/2,189) of the parents/guardians consented to testing. Besides 32 previously diagnosed fragile X patients, 11 new patients (9 males and 2 females) were diagnosed. Scoring of physical features was effective in preselection, especially for males (sensitivity .91 and specificity .92). Major motives to participate in the screening were the wish to obtain a diagnosis (82%), the hereditary implications (80%), and the support of research into mental retardation (81%). Thirty-four percent of the parents/guardians will seek additional diagnostic workup after exclusion of the fragile X syndrome. The prevalence of the fragile X syndrome was estimated at 1/ 6,045 for males (95% confidence interval 1/9,981-1/ 3,851). On the basis of the actual number of diagnosed cases in the Netherlands, it is estimated that >50% of the fragile X cases are undiagnosed at present.

Benign Hereditary Chorea of Early Onset Maps to Chromosome 14q

Benign hereditary chorea (BHC) is an autosomal dominant disorder characterized by an early-onset nonprogressive chorea. The early onset and the benign course distinguishes BHC from the more common Huntington disease (HD). Previous studies on families with BHC have shown that BHC and HD are not allelic. We studied a large Dutch kindred with BHC and obtained strong evidence for linkage between the disorder and markers on chromosome 14q (maximum LOD score 6.32 at recombination fraction 0). The BHC locus in this family was located between markers D14S49 and D14S1064, a region spanning approximately 20.6 cM that contains several interesting candidate genes involved in the development and/or maintenance of the CNS: glia maturation factor-beta, GTP cyclohydrolase 1 and the survival of motor neurons (SMN)-interacting protein 1. The mapping of the BHC locus to 14q is a first step toward identification of the gene involved, which might, subsequently, shed light on the pathogenesis of this and other choreatic disorders.

General overgrowth in the fragile X syndrome: variability in the phenotypic expression of the FMR1 gene mutation

The fragile X syndrome, which often presents in childhood with overgrowth, may in some cases show some diagnostic overlap with classical Sotos syndrome. We describe four fragile X patients with general overgrowth, all of whom are from families with other affected relatives who show the classic Martin-Bell phenotype. Molecular studies of the FMR1 gene in all cases showed the typical full mutation as seen in males affected by the fragile X syndrome. Endocrine studies were unremarkable, except in one case where there were raised levels of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-3 (IGFBP-3) These cases illustrate the clinical variability of the fragile X syndrome and the necessity of performing analysis of the FMR1 gene in mentally retarded patients presenting with general overgrowth.

Isolated postaxial polydactyly type B with mosaicism of a submicroscopic unbalanced translocation leading to an extended phenotype in offspring.

Postaxial polydactyly (PAP) is characterized by the presence of one or more extra ulnar or fibular digits or parts of it. PAP type B presents frequently as a skin tag on the hand(s). It is usually an isolated malformation, but in 6.6% it is associated with other congenital abnormalities, mostly well recognizable syndromes. We present a male with PAP‐B only and his daughter with an extended phenotype including mental retardation and minor dysmorphisms. Both share a cytogenetically balanced t(4;7)(p15.2;q35), present in mosaicism in the father. We found microdeletions associated with the breakpoints. The chromosomal regions described here have not been previously associated with the PAP‐B phenotype. We present the first case of an individual with isolated PAP‐B and a submicroscopic chromosome abnormality.

The Fragile X Syndrome: A model for mental retardation

Mental retardation is one of the most common disorders in man. Fragile X syndrome is the most frequent form of inherited mental retardation. The identification of the gene involved, the FMR1 gene, has uncovered the molecular basis for the inheritance pattern of fragile X syndrome. This review describes the clinical phenotype of the disease and gives an explanation for the differences seen in phenotypes between patients. A short description is given of our present day knowledge about the function of the gene.