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At least 19 recordsLinked to original sources
PubMed1993

Heritable unstable DNA sequences and hypermethylation associated with fragile X syndrome in Japanese families.

Fragile X syndrome, associated with the fragile site at Xq27.3 (FRAXA), is the most common form of familial mental retardation. The fragile X mutation has recently been characterized as a heritable unstable DNA sequence, p(CCG)n/p(CGG)n, in the FRAXA locus. In the present study, a correlation between fragile X-genotypes in the FRAXA locus and hypermethylation of an adjacent CpG island was examined in four Japanese families with fragile X syndrome. We show here that the heritable unstable DNA sequences in the fragile X chromosome usually increase in size when transmitted by female carriers, and that the degree of methylation in the CpG island correlated with the increased sizes of the unstable DNA sequences. When a hypermethylated full mutation was transmitted by a male to his daughters, both the size of the unstable DNA sequence and the degree of the methylation reduced to the premutation range. Our observations suggest that female meiosis has a greater potential for amplifying unstable DNA sequences and that amplified DNA sequences can be transmitted through germ cells, while male germ cells seem not to be able to tolerate highly amplified unstable DNA sequences.

Base Sequence↗
PubMed1996

Analysis of unstable DNA sequence in FMR1 gene in Polish families with fragile X syndrome.

The unstable DNA sequence in the FMR1 gene was analyzed in 85 individuals from Polish families with fragile X syndrome in order to characterize mutations responsible for the disease in Poland. In all affected individuals classified on the basis of clinical features and expression of the fragile site at X(q27.3) a large expansion of the unstable sequence (full mutation) was detected. About 5% (2 of 43) of individuals with full mutation did not express the fragile site. Among normal alleles, ranging in size from 20 to 41 CGG repeats, allele with 29 repeats was the most frequent (37%). Transmission of premutated and fully mutated alleles to the offspring was always associated with size increase. No change in repeat number was found when normal alleles were transmitted.

Alleles↗
PubMed1994

[Unstable DNA sequence and methylation in fragile X syndrome].

Fragile X syndrome is characterized as an inherited unstable DNA sequence: the increasement of (CGG)n copy number. It suffered from the inactivation of FMR-1 (fragile X mental retardation 1), which is inhibited by amplification of (CGG)n and methylation of CpG island. The (CGG)n repeat variation in normal Chinese population was detected by PCR with sequencing gel analysis. We also analysed the amplification of (CGG)n and methylation of CpG island at Xq27.3 from 15 individuals in 6 families by Southern hybridization. These results indicate that abnormal methylation of CpG island always occurs in Fra (X) patients together with large amplification of (CGG)n. The (CGG)n could be either stable or amplified in springs of carrier females. These suggest that there would be a new genetic mechanism dynamic mutation.

Base Sequence↗
PubMed1993

Prenatal diagnosis of fragile X syndrome by direct detection of the dynamic mutation due to an unstable DNA sequence.

The fragile X syndrome is the most common familial form of mental retardation. The mutation causing the syndrome is dynamic mutation due to an unstable DNA (CCG)n repeat localized at Xq27.3. We have previously reported a PCR procedure to prepare a diagnostic probe, pPCRfx1, which can be used to determine the genotype of fragile X mutation individuals by Southern blot analysis. In the present study, pPCRfx1 was applied to the prenatal diagnosis, using chorionic villus cells, of a fetus which was at risk of having fragile X syndrome. In the PstI assay, the Southern blot showed the typical pattern of a female carrier with the full mutation. Analysis of the DNA methylation patterns by EcoRI + EagI assay showed that the EagI restriction site was not methylated on the mutated X chromosome of chorionic villi, but the sites were totally methylated in the brain and other tissues of the fetus. Thus the fetus was diagnosed to be a heterozygous female carrier of the dynamic mutation involved in the fragile X syndrome.

Blotting, Southern↗
PubMed1992

Unstable DNA sequence in myotonic dystrophy.

A variable DNA sequence has been detected in patients with myotonic dystrophy. We set out to determine whether identification of this specific molecular defect would improve clinical management of patients and families with myotonic dystrophy. 127 affected patients who were studied had an expanded DNA fragment not seen in 73 normal controls. The increase in length of the fragment correlated broadly with disease severity, and we noted expansion of the sequence in successive generations of the same family. Progressive expansion of the affected gene provides a molecular explanation for an apparently earlier onset in successive generations (anticipation) in myotonic dystrophy and supports the role of an unstable repeat sequence as the basis of the defect. The specificity of this finding will assist in accurate diagnosis of myotonic dystrophy and genetic counselling of affected families.

Adolescent↗
PubMed1994

Structural organization, amplification, deletion and rearrangements of DNA sequences associated with an unstable region of the chromosome of Streptomyces coelicolor A3(2).

The chromosome of Streptomyces coelicolor A3(2) carries unstable DNA sequences hybridising with DNA sequences from an unstable chromosomal region of the related species S. lividans. These S. coelicolor sequences are nearly identical to those of S. lividans TK23 in organisation but differ from those of S. lividans 66 TK64 which harbours a tandem duplication of these sequences. Southern hybridisations using heterologous probes and S. coelicolor DNA cleaved with a variety of restriction enzymes permitted us to construct a partial restriction map of the unstable region of the chromosome of S. coelicolor. Genetic analysis shows that the unstable region yields distinguishable variants with several distinct DNA rearrangements.

Base Sequence↗
PubMed1993

[DNA diagnosis of myotonic dystrophy in a family].

Myotonic Dystrophy (MyD) is the most common form of muscular dystrophy affecting adults. Recently a heritable unstable DNA sequence containing CTG repeat in the region associated with MyD was detected. We have analyzed DNA from the members of an MyD family using Southern hybridization and PCR method. Unaffected individuals of this family had no expansion of unstable DNA sequence after EcoRI and BglI digestion and had CTG repeats under 12, while affected individuals had more than 0.2 kb expansion of DNA sequence and had over 50 CTG repeats. We confirmed that the length of unstable MyD region correlated with severity of the disease in a family and that the diagnosis of MyD had been improved by development of DNA probes for the CTG repeats.

Adult↗
PubMed1996

Clinical implications of unstable DNA repeat sequences.

In this article we review the clinical and genetic features characteristic of a number of diseases recently explained by a novel genetic mechanism: unstable segments of the genome containing trinucleotide repeat sequences. Disorders identified to date are mostly progressive, and display unusual inheritance patterns such as anticipation. Anticipation is manifested as an earlier age at onset or a more severe phenotype in later generations of a family, and can be correlated to an increased repeat expansion size. Thus in later generations the disease onset can take place in childhood whereas affected individuals in earlier generations had only adult symptoms. Paediatric cases of typically adult disorders have been shown to be caused by exceptionally long repeat sequences. Anticipation has been observed in a number of disorders not yet identified at the molecular level. Such disorders could be caused by repeat expansions, and are presently subject to intense research efforts. If repeat sequence expansions are related to these disorders, the longest expansions should be seen in the childhood cases, making these the optimal cases to study. Various DNA-based methods have been developed for the detection of these mutations, making possible preclinical and prenatal diagnostics as well as detection of novel expansions.

Adult↗
PubMed1993

Direct diagnosis of myotonic dystrophy with a disease-specific DNA marker.

BACKGROUND: Myotonic dystrophy is the most common inherited form of muscular dystrophy affecting adults. Its symptoms are not confined to muscle, and variability in their nature and in the patient's age at their onset can make diagnosis difficult. A specific unstable DNA sequence associated with myotonic dystrophy has recently been identified. We describe the use of a DNA probe (p5B1.4) that can detect this mutation directly, improving the accuracy and speed of diagnosis. METHODS: We analyzed DNA extracted from the peripheral-blood lymphocytes of 112 unrelated patients with myotonic dystrophy and their families, using molecular genetic techniques. Southern blot analysis and amplification with the polymerase chain reaction were used to determine the extent of expansion of the unstable DNA sequence. RESULTS: Probe p5B1.4 allowed direct identification of the myotonic dystrophy mutation in 108 of the 112 unrelated patients. In three families for whom the clinical and genetic data obtained with linked probes were ambiguous, the probe identified persons at risk for symptoms of this disorder and demonstrated that a possible sporadic case of myotonic dystrophy was familial. In one of these families the size of the unstable myotonic dystrophy-specific fragment decreased on transmission to offspring, who remained asymptomatic. CONCLUSIONS: The diagnosis of myotonic dystrophy is improved by the use of a probe that detects directly the mutation responsible for this disorder.

Adolescent↗
PubMed1991

Hereditary unstable DNA: a new explanation for some old genetic questions?

Fragile X syndrome, associated with the fragile X chromosome, is the most common cause of familial mental retardation. The condition is characterised by a heritable DNA sequence that consists of an abnormal number of CCG repeats, and which is unstable in both mitosis and meiosis. We suggest that such heritable unstable DNA sequences could be present in other parts of the genome and that these might explain a number of genetic events that are not well understood in terms of classic genetic mechanisms. Such poorly explained observations include anticipation, incomplete penetrance, variable expression, and possibly imprinting, variegation, and multifactorial inheritance.

Chromosome Fragility↗
PubMed2001

Non-clonability correlates with genomic instability: a case study of a unique DNA region.

Instability of eukaryotic DNA in constructs propagated in prokaryotic hosts is a frequently observed phenomenon. With the exception of a very high A+T-content and the presence of multiple repetitions, no general rule at the basis of this phenomenon is actually known. The intergenic spacer located between the pi and alpha(D) chicken alpha-type globin genes is frequently deleted from recombinant phages and plasmids. Here we have cloned this DNA fragment using a specially designed bacterial strain (SURE competent cells, Stratogene). Comparative analysis of DNA of recombinant clones bearing deletions and clones containing the intact genomic DNA fragment has revealed two important DNA sequence motifs that contribute to the unclonability of eukaryotic DNA in prokaryotic cells. First, the similarity to bacterial transposons (i.e. the presence of repeats flanking a several kilobase DNA fragment) may cause the loss of the fragment during propagation of the recombinant DNA in E. coli. Second, a high content of rotationally correlated kinkable elements (TG*CA steps) may result in non-clonability of the DNA sequence. Interestingly, the latter type of "unclonable" DNA sequence motifs identified in the globin gene domain is unstable (frequently rearranged) also in the eukaryotic chromosome resulting in a local polymorphism. In the chicken domain of alpha globin genes this unstable DNA sequence seems to be partially protected by interaction with nuclear matrix proteins.

Animals↗
PubMed1995

Unstable triplet repeat sequences: a source of cancer mutations?

Numerous mutations have been related to various types of cancer. Short tandem repeats (STRs) are repetitive DNA elements that are often polymorphic in normal populations. Triplet repeat expansion has been related pathogenetically to six diseases: fragile X syndrome, fragile X E syndrome, spinobulbar muscular atrophy, myotonic dystrophy, Huntington's disease, and spinocerebellar ataxia type 1. The characteristics of the GC-rich repeat expansion are diverse and result in profound changes in phenotype, sometimes within a single generation in affected families. We expect that simple repeat expansion will cause some cancers based on our knowledge of these unstable DNA sequences in the previously mentioned genes. This may occur by alteration of tumor suppressor gene expression, alteration in coding features of proteins, or change in bystander oncogene expression such as that which occurs with DNA methylation. The demonstrated meiotic instability could link this mechanism of mutation of familial cancer syndromes. The recent discovery of STR instability at multiple sites in hereditary nonpolyposis colon cancer suggests sequence instability may be a factor in cancer progression. Continued identification of candidate genes containing triplet repeats should allow a ready testing of the hypothesis that unstable simple repeat sequences can cause cancer.

Base Sequence↗
PubMed1992

Fragile X syndrome: molecular analysis reveals a new mechanism of mutation in human genetic diseases.

The fragile X syndrome belongs to the most common genetic diseases and has a prevalence of one in every 2000 children. The syndrome is named after the fragile site in q27.3 on the X chromosome. The molecular cloning of the DNA containing the fragile site has resulted in the identification of a heritable unstable DNA sequence revealing a new mechanism of mutation in human genetic disorders. This DNA sequence significantly facilitates the diagnosis and provides a rapid method for carrier detection and prenatal diagnosis. The unstable element is located within a candidate gene, FMR1. The FMR1 protein is not made in fragile X patients and nothing is known about its function. We will have to await studies on this protein to be able to understand the variable phenotype of this disease.

Chromosome Fragile Sites↗
PubMed1995

Myotonic dystrophy: correlation of clinical symptoms with the size of the CTG trinucleotide repeat.

An unstable DNA sequence of a gene encoding a protein kinase has been identified as the molecular basis of myotonic dystrophy. The correlation between different symptoms of myotonic dystrophy and the size of this unstable base triplet (CTG)n repeat was investigated in 14 patients. DNA was prepared from whole blood by standard procedures. Detailed clinical, psychological, electrophysiological (quantified measurement of myotonia, electrocardiography) and other laboratory examinations (muscle biopsy in 4 patients, slit lamp examination) were performed. Triplet size correlated significantly with muscular disability and inversely with age at onset of the disease. A greater frequency of mental and gonadal dysfunction could be observed in patients with a larger repeat size. Other symptoms, however, such as cataract, myotonia, gastrointestinal dysfunction and cardiac abnormalities were not correlated with repeat size. Somatic mosaicism with different amplification rates in various tissues might be one possible explanation for the variable phenotypes. Furthermore, other factors such as different expression of the myotonic dystrophy gene might contribute to the clinical variability of the disease at a given triplet size.

Adult↗
PubMedDate not supplied

Methylation and mutation patterns in the fragile X syndrome.

Chromosomes carrying the mutation causing the fragile X [fra(X)] syndrome have been shown to have an unstable DNA sequence close to or within the fragile site. The length variation is located within a DNA fragment containing a CGG trinucleotide repeat which is unstable in both mitosis and meiosis. We have used the probe StB12.3 from the region to analyze the mutations and the methylation patterns in 21 families segregating for the fra(X) syndrome. Among 40 fra(X) males all showed an abnormal pattern. The normal 2.8 kb band was absent in 36 individuals and replaced by a heterogeneous smear of larger size. The remaining four were shown to be "mosaics" with the presence of both mutated, unmethylated and mutated, methylated fragments. We found four normal transmitting males, one which was a great-grandson of another normal transmitting male indicating that the pre-mutation can remain stable through two meioses in the female. In nine fra(X) positive females the abnormal pattern consisted of a smear, usually seen in affected males, in addition to the normal bands. Five of these females were mentally normal. Of clinical importance is the prediction of mental impairment in females. We suggest that this is not made by the detection of the full mutation alone, but rather by the degree of methylation of the normal X chromosome. Our results suggest that difference of clinical expression in monozygotic twins may be correlated with difference in methylation pattern. Six out of 33 fra(X) negative females at risk were diagnosed as carriers. Our observations indicate that molecular heterogeneity is responsible for variable expression of the fra(X) syndrome in both males and females.

DNA Mutational Analysis↗