The Huntington's disease-like syndromes: what to consider in patients with a negative Huntington's disease gene test.

Huntington's disease (HD), which is caused by a triplet-repeat expansion in the IT15 gene (also known as huntingtin or HD), accounts for about 90% of cases of chorea of genetic etiology. In recent years, several other distinct genetic disorders have been identified that can present with a clinical picture indistinguishable from that of HD. These disorders are termed Huntington's disease-like (HDL) syndromes. So far, four such conditions have been recognized, namely disorders attributable to mutations in the prion protein gene (HDL1), the junctophilin 3 gene (HDL2), and the gene encoding the TATA box-binding protein (HDL4/SCA17), and a recessively inherited HD phenocopy in a single family (HDL3), the genetic basis of which is currently poorly understood. These disorders, however, account for only a small proportion of cases with the HD phenotype but a negative genetic test for HD, and the list of HDL genes and conditions is set to grow. In this article, we review the most important HD phenocopy disorders identified to date and discuss the clinical clues that guide further investigation. We will concentrate on the four so-called HDL syndromes mentioned above, as well as other genetic disorders such as dentatorubral-pallidoluysian atrophy, neuroferritinopathy, pantothenate-kinase-associated neurodegeneration and chorea-acanthocytosis.