Fragile X syndrome is the most frequent form of inherited mental retardation and segregates as an X-linked dominant with reduced penetrance. Recently, we have identified the FMR-1 gene at the fragile X locus. Two molecular differences of the FMR-1 gene have been found in fragile X patients: a size increase of an FMR-1 exon containing a CGG repeat and abnormal methylation of a CpG island 250 bp proximal to this repeat. Penetrant fragile X males who exhibit these changes typically show repression of FMR-1 transcription and the presumptive absence of FMR-1 protein is believed to contribute to the fragile X phenotype. It is unclear, however, if either or both molecular differences in FMR-1 gene is responsible for transcriptional silencing. We report here the prenatal diagnosis of a male fetus with fragile X syndrome by utilizing these molecular differences and show that while the expanded CGG-repeat mutation is observed in both the chorionic villi and fetus, the methylation of the CpG island is limited to the fetal DNA (as assessed by BssHII digestion). We further demonstrate that FMR-1 gene expression is repressed in the fetal tissue, as is characteristic of penetrant males, while the undermethylated chorionic villi expressed FMR-1. Since the genetic background of the tissues studied is identical, including the fragile X chromosome, these data indicate that the abnormal methylation of the FMR-1 CpG-island is responsible for the absence of FMR-1 transcription and suggests that the methylation may be acquired early in embryogenesis.
Lowe's oculocerebrorenal syndrome (OCRL) is a human X-linked developmental disorder of unknown pathogenesis and has a pleiotropic phenotype affecting the lens, brain and kidneys. The OCRL locus has been mapped to Xq25-q26 by linkage and by finding de novo X; autosome translocations at Xq25-q26 in two unrelated females with OCRL. Here we use yeast artificial chromosomes with inserts that span the X chromosomal breakpoint from a female OCRL patient in order to isolate complementary DNAs for a gene that is interrupted by the translocation. We show that the transcript is absent in both female OCRL patients with X; autosome translocations and that it is absent or abnormally sized in 9 of 13 unrelated male OCRL patients with no detectable genomic rearrangement. The open reading frame encodes a new protein with 71% similarity to human inositol polyphosphate-5-phosphatase. Our results suggest that OCRL may be an inborn error of inositol phosphate metabolism.
Fragile X syndrome is the most common form of inherited mental retardation in man. The disease is associated with expansion in the number of tandem CGG trinucleotide repeats in the 5' untranslated region of the human FMR1 gene. Transmitting males, individuals who are unaffected carriers of the disease, show a moderate increase in the number of repeat units, while fully penetrant males show a major expansion in repeat number. Major expansion of the repeat in affected males is correlated with methylation of certain restriction enzyme recognition sites in the 5' CpG island containing the trinucleotide repeat in these patients. Phenotypic expression of the mutation appears to be due to transcriptional silencing of the FMR1 gene. We now report direct high resolution methylation analysis of the trinucleotide repeat and its flanking regions using ligation-mediated PCR genomic sequencing. We find the cytosine residue of all CpG dinucleotides examined within and surrounding the FMR1 trinucleotide repeat to be unmethylated in the DNA of normal male leukocytes and transmitting male lymphoblasts; these same cytosines are methylated in affected male lymphoblasts, in a somatic cell hybrid containing a fragile X chromosome from an affected male, and in a somatic cell hybrid containing a normal inactive X chromosome. The methylation pattern of the FMR1 5' CpG island in affected patients as determined by genomic sequencing is remarkably similar to that seen for the X-linked human phosphoglycerate kinase and hypoxanthine phosphoribosyltransferase gene 5' CpG islands on the inactive human X chromosome.(ABSTRACT TRUNCATED AT 250 WORDS)
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.