Genetic evidence now firmly supports the "out of Africa" hypothesis for human migration, with people dispersing from Africa toward Europe and, largely separately, toward Asia. These 3 major populations are genetically distinct, with those of African descent having the greatest genetic diversity and those of Asian and European descent having less diversity, with origins from smaller groups.1 Nearly all diseases have been clinicopathologically defined in white populations, and yet this group has the least genetic diversity, limiting such definitions.1 We have functioned with the implicit assumption that diseases will be clinically similar in all populations. However, the article by Lu and colleagues2 in this issue of the ARCHIVES offers further evidence that this pervasive but unlikely supposition is not correct, by confirming previous studies that spinocerebellar ataxia type 2 (SCA2) is a cause of familial parkinsonism and can be identical to typical Parkinson disease (PD) symptomatically.1- 7 Their work also reveals that, in their series of ethnic Chinese, SCA2 is responsible for almost 10% of familial parkinsonism cases. Thus, while SCA2 might be a rare cause of PD in whites, it causes a considerable minority of cases in Taiwan. This article, along with prior literature,3- 7 emphasizes how genetic diseases have a spectrum of phenotypes ranging beyond the classically defined forms.
Gwinn-Hardy K. When Is Ataxia Not Ataxia?. Arch Neurol. 2004;61(1):25-26. doi:10.1001/archneur.61.1.25