Diseases, Disorders information » Fragile X Syndrome

Myotonic Dystrophy

Disorders — Mon, 20 Jul 2009 16:32:41 +0000

Myotonic Dystrophy
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Myotonic dystrophy is the most common form of adult muscular dystrophy, with a prevalence of 2-14 per 100,000 individuals. This autosomal dominant condition is manifested by myotonia, cardiac arrhythmias, cataracts, male pattern baldness, male infertility (hypogonadism), and other associated endocrinopathies. The age of onset and severity of the disease show extreme variation, both within and between families. The rare congenital form of myotonic dystrophy is associated with profound newborn hypotonia and mental retardation. Such children are invariably the offspring of affected mothers. The mothers frequently have mild or subclinical features of myo-tonic dystrophy. Complications of pregnancy in women with myotonic dystrophy include polyhydramnios, decreased fetal movements, intestinal pseudoobstruction, and approximately 15% perinatal mortality. The myotonic dystrophy gene has been cloned, and an unstable DNA sequence specific for the disease has been characterized. Female viagra online at discount canadian pharmacy. Detection of an enlarged DNA fragment due to the expansion of CTG repeat with the myotonic dystrophy gene can be used for direct DNA diagnosis in affected individuals and persons at risk. Furthermore, there is a strong correlation between the length of fragment expansion and the degree of disease severity in gene carriers. The gene containing the repeat has been designated motolin protein kinase. Prenatal diagnosis is available.

Huntington’s Disease
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Huntington’s disease is an autosomal dominant disorder that affects about 1 per 100,000 individuals. This progressive neurodegenerative disorder is characterized by motor disturbances, cognitive loss, and psychiatric manifestations. Affected individuals develop a distinctive choreic movement disorder that has an insidious onset in the fourth to fifth decade of life and gradually worsens over a period of 10-20 years until death. A juvenile form of Huntington’s disease exists, with a preponderance of paternal transmission of the disease. The gene (designated IT15) contains a polymorphic CAG repeat sequence. Unaffected individuals usually have about 11-24 CAG repeats, whereas patients with Huntington’s disease have 42-86 CAG repeats. The length of the repeat appears to correlate with the age of onset and severity of the disease. Although prenatal diagnosis is technically possible for this adult-onset disease, genetic counseling is complex and should be carried out in the context of a multidisciplinary team.
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Fragile X Syndrome

Disorders — Fri, 17 Jul 2009 15:24:31 +0000

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The fragile X syndrome is the most common inherited form of mental retardation. In addition to moderate to severe mental retardation, affected males have facial dysmorphic features, macroorchidism, and a folate-sensitive fragile site on the X chromosome, at band Xq27.3. The female heterozygotes may also exhibit mental retardation. The responsible gene (FMR-1) has been cloned, and it includes the repeated CGG (cytosine-guanine-guanine) sequence that causes the fragile X mutation. Normal individuals have 2-50 CGG repeats at this locus, with an average of about 30 repeats. Alleles with more than about 50 repeats, including those identified in a normal family, are meiotically unstable. Individuals with no significant phenotypic abnormalities, but who are at high risk for having an affected offspring, have triple repeats ranging between 50 and 200. Such individuals are said to have a premutation, because it has a high probability of expanding to a full mutation in the next generation. Individuals with more that 200 CGG repeats are affected with the fragile X syndrome. The passage from premutation to full mutation status occurs only with transmission from the mother. Expanded CGG repeats appear to cause abnormal methylation of the FMR-1 gene, which, in turn, causes a reduction in gene transcription, resulting in disease. Molecular testing for clinical diagnosis, cartier detection, and prenatal diagnosis of the fragile X syndrome is available and is proving more reliable than cytogenetic testing. Results from chorionic villus sampling should be interpreted with caution because the methylation status of the FMR-1 gene is rarely established in chorionic villi at the time of sampling. Chorionic villus sampling, although a standard technique for prenatal diagnosis, may lead to a situation where follow-up amniocentesis is necessary to resolve an ambiguous result.

Kennedy’s Disease

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Kennedy’s disease (spinal and bulbar muscular atrophy) is an X-linked recessive disorder characterized by motor neuron degeneration that starts in adulthood and leads to progressive muscular weakness of the upper and lower extremities and late onset of mild androgen insensitivity. Affected males have reduced fertility and gynecomastia; female carriers have few or no symptoms. A mutation in the N terminus of the androgen receptor gene involving CAG repeats has been shown to be the cause of this disease. Unaffected individuals have 13-30 CAG repeats in the first exon (ie, transcribed region) of this gene; individuals with Kennedy’s disease have 40-62 repeats.