Ataxias on the march from Quebec to Tunisia

In this issue of Neurology , Mrissa et al.1 add a new twist to the long list of recent discoveries in the ataxias. They examined a large family with a recessive early-onset ataxia that resembled Friedreich ataxia (FRDA) but differed from it by the preservation of tendon reflexes. Although some FRDA alleles (those caused by relatively short GAA expansions in the frataxin gene) do not lead to lost tendon reflexes (Friedreich ataxia with retained reflexes [FARR]), genetic linkage analysis quickly excluded linkage of the disease mutation to the FRDA locus on human chromosome 9q13. Another recessive ataxia characterized by vitamin E deficiency (AVED) and caused by a mutation in the α-tocopherol transfer protein was excluded as well. It may be surprising at first that the authors also examined linkage to dominant ataxia loci. However, consanguineous marriages and a high frequency of mutant alleles in the population can produce a pseudo-dominant inheritance pattern. When an affected homozygote mates with an unaffected heterozygote, 50% of the offspring will be affected. After the authors had excluded linkage to known dominant spinocerebellar ataxia (SCA) loci, Mrissa et al. embarked on a search of the entire human genome for …