Early Diagnosis of TTR Amyloidosis by Use of Molecular Biology

Diagnosis, Idiopathic Progressive Neuropathy, Polyneuropathy

Peripheral neuropathies are diseases that affect the nervous system outside the brain and spinal cord, their prevalence is 1% in the general population, the causes are extremely varied with more than 200 identified causes; the main ones are diabetes, excessive alcohol consumption and chemotherapy. They may be sometimes disabling but generally preserve autonomy. Transthyretin amyloidosis is a rare multisystematic hereditary disease with autosomal dominant transmission. They present usually as a peripheral neuropathies (FAP). They are due to a point mutation of the transthyretin gene (chr 18q). FAP is secondary to endoneurial amyloid deposits and are characterized by a slowly progressive sensory, motor and autonomic. FAP is the most severe hereditary polyneuropathy of the adult are irreversible and fatal within 5 to 12 years from onset. Most frequent mutation of TTR gene is located on the second exon; but more than 100 mutations have been reported. Prevalence of FAP is 1 per 1 million inhabitants. They have been reported until 1990s' in four endemic areas North of Portugal, Sweden, Japan and Majorca. In these areas, diagnosis is facilitated because of the stereotypical presentation : a length-dependent polyneuropathy with predominant involvement of thermal and pain sensations and autonomic dysfunction, early onset in the third decade and a predominant Met30 TTR mutation. Positive family history is frequent 85% (one of the parents is affected). Diagnosis requires detection of TTR mutation by molecular biology (blood sample) and characterization of amyloid deposit on labial salivary gland biopsy.

Study of the TTR gene by complete sequencing; search for amyloidogenic mutations of the TTR gene (according to the site http: // amyloidosismutations.com / mut-attr.php) in the laboratory of molecular biology of the CHU BICÊTRE (APHP) managed by Pr Anne Mantel. Preselection of the cases to be tested among the cases of peripheral neuropathies of indetermined cause referred via the network Cornamyl of which the reference centers of the neuromuscular diseases are belonging . Currently, FAP is a worldwide disease. Diagnosis of TTR-FAP is extremely difficult and usually delayed by 4 years in non endemic areas for many reasons : * positive family history are lacking in 50% of cases (sporadic forms). * incomplete ability of biopsies to characterize amyloid deposits. * clinical presentation is varied and may mimick many types of rare peripheral neuropathies: CIDP, axonal idiopathic polyneuropathy, upper limb neuropathies, recurrent carpal tunnel syndrome after surgery, ataxic neuropathy, motor neuropathy. Conversely to endemic areas, look for V30M mutation is not enough to exclude TTR-FAP, TTR gene sequencing is required. With the help of the french network for FAP (Cornamyl), cases have been identified in 81/100 geographical departments, with a wide genetic heterogeneity (41 mutations reported) ; age of onset is late: 75% after 50 yo.

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