The DNA laboratory and neurological practice

Diagnosis of inherited metabolic disorders affecting the nervous system

Knowledge of the molecular causes for genetic diseases that affect the nervous system is rapidly expanding. Especially striking has been the finding in several autosomal dominant neurodegenerative disorders that unstable expansions of trinucleotide repeats are responsible for the genetic disorder and that the length of the repeat can be correlated with the age of onset and the severity of symptoms. Phenotypic heterogeneity in many disorders associated with enzyme deficiencies can often be linked to the amount of residual enzyme activity occurring with different gene mutations. Making a specific diagnosis of a neurological disorder associated with genetically determined metabolic defects requires access to a laboratory that can assist in arranging for appropriate testing to be carried out. In some disorders such as the aminoacidurias diagnostic metabolic studies can be performed in hospital clinical chemistry laboratories. In others, such as the lysosomal storage diseases, a laboratory that carries out special lipid analyses and white blood cell enzyme assays will be necessary. DNA mutational analyses are becoming commercially available for diagnosing many disorders such as mitochondrial diseases and those conditions associated with expanded trinucleotide repeats. It may be necessary to contact individual research laboratories when confronted with a disorder that has been newly discovered or that is very rare. A computerised directory of specialised laboratories that perform disease specific testing for genetic disorders should be useful in choosing the appropriate diagnostic or research laboratory.

Recent advances in muscular dystrophies and myopathies

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Investigation of peripheral neuropathy

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Neurology

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Amyloid and peripheral nervous system disease

The peripheral nervous system can be involved in the following amyloid deposition diseases. (1) Amyloid deposition composed of beta 2-microglobulin in patients on long term hemodialysis causing a carpal tunnel syndrome; (2) deposition of light chain immunoglobulin derived amyloid leading to polyneuropathy, carpal tunnel syndrome and autonomic nervous system involvement in patients with primary amyloidosis, or amyloidosis secondary to or associated with multiple myeloma, Waldenström's macroglobulinemia, non-Hodgkin's lymphoma, and solid neoplasms like hypernephroma; and (3) several types of heredofamilial amyloid polyneuropathies, which are mainly caused by a point-mutation in the transthyretin gene on chromosome 18. The clinical and biochemical features of these three groups will be discussed, with special attention for recently developed therapy. In clinical practice, amyloid polyneuropathy should be considered in case of familial occurrence of a polyneuropathy and when a patient presents with a polyneuropathy and a monoclonal gammopathy.

Clinical genetics in neurological disease

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