Distinct phenotypes within autosomal recessive ataxias not linked to already known loci.

Autosomal recessive ataxias represent a large group of neurodegenerative disorders characterized by progressive degeneration of central and peripheral nervous systems and a genetic heterogeneity. To analyse clinical, neurophysiological and nerve biopsy findings in 14 Tunisian unrelated families showing linkage exclusion to the known autosomal recessive ataxia loci, 20 Tunisian families with a total of 73 affected subjects were selected on the presence of a clinical phenotype associating a cerebellar ataxia with retained tendon reflexes on at least the index patient. A genetic linkage study was performed with markers spanning the Friedreich ataxia, Spastic ataxia of the Charlevoix-Saguenay, Autosomal recessive ataxia associated with isolated vitamin E deficiency, Ataxia with oculomotor apraxia, Infantile onset spinocerebellar ataxia, Ataxia with Hearing Loss and Optic Atrophy, AT, ATLD, Spinocerebellar ataxia with axonal neuropathy, Cayman ataxia, Cerebellar ataxia with mental retardation optic atrophy and skin abnormalities, Salla syndrome, Marinesco-Sjögren and the Childhood Spinocerebellar Ataxia loci. Out of the 20 families, 4 showed linkage to the spastic ataxia of the Charlevoix-Saguenay locus, one to the Friedreich ataxia locus and one to the Ataxia with oculomotor apraxia locus. Linkage to all tested loci has been excluded in the 14 remaining families. These families were divided into 3 groups according to tendon reflex status in lower limbs which appear as the most obvious distinguishing clinical sign between patients and families: Group A was characterized by brisk tendon reflexes in lower limbs, group B by a homogeneous feature of tendon reflexes with the absence of ankle reflexes and brisk knee reflexes and group C by variable features of tendon reflexes in lower limbs within the same family. Haplotype analysis and Lod score calculation did not show any evidence of linkage to the 16 known loci of cerebellar ataxias. Aim of this study was to reveal the vast clinical phenotypic variability in patients with autosomal recessive ataxia not linked to known loci. Data obtained indicate that detailed clinical and neurophysiological nerve investigations will be essential in order to pool patients within homogeneous subgroups for gene mapping.