Supplementary Material for: The Frequency of Huntington Disease and Huntington Disease-Like 2 in the South African Population

Background: Huntington disease (HD) has most recently been estimated to affect between 10.6 and 13.7 per 100,000 individuals in European populations. However, prevalence is known to differ geographically. In South Africa, the only published estimates are from a survey performed in the 1970s, an era when the disease was believed to be rare or absent in black individuals and molecular confirmation was absent. The disease phenotype in South Africa is currently attributable to mutations in both the huntington and junctophilin-3 genes, which underlie the well-known HD and the rarer HD-like 2 (HDL2) respectively. This study aimed at providing improved minimum estimates of disease frequency in South Africa, based on molecular genetic testing data. Methods: A review of all testing records for HD and HDL2 over a 20-year period was undertaken. HDL2 is virtually indistinguishable on clinical features, thus necessitating its inclusion. Results: Based on molecular diagnostic records, minimum estimates of disease frequency are: 5.1, 2.1 and 0.25 (per 100,000 individuals) for the white, mixed ancestry and black population groups respectively. Conclusion: Although ascertainment remains incomplete, these minimum estimates suggest that disease frequencies are significantly higher than those previously reported in South Africa.