Análise de haplótipos intragênicos ao gene MJD1 em pacientes com doença de Machado-Joseph

Machado-Joseph disease (MJD), or spinocerebellar ataxia type 3 (SCA-3), is an autossomal dominant neurodegenerative disorder originally described in families of Portuguese-Azorean ancestries. Generalized incoordination of gait, speech and limb movements can be present in affected MJD patients.The disease onset occurs in general between 30 and 40 years of age, and symptoms progress slowly. Survival after onset is between 14 to 17 years. The disease-associated gene, named MJD1, was identified in 1994 and is located on chromosome 14. This gene is characterized by a CAG nucleotide repeat in the 5 end of exon 2. Number of repeats is polymorphic in the population; normal alleles vary from 12 to 37 CAG repeats, while expanded alleles vary between 61 to 84 repeats. A recent worldwide haplotype study demonstrated two different haplotypes in families of Azorean background which are specific to the island of origin. In families from the mainland of Portugal, both Azorean haplotypes can be found. The majority of the nonPortuguese families also share the same intragenic haplotype carried by families coming from Flores Island, but at least three others haplotypes can be determined. So far, the strongest hypothesis states that the spread of an original founder mutation is directly linked to the Portuguese-Azorean immigration. In the present study, we tested this hypothesis by linkage-disequilibrium analysis of three intragenic single-base-pair polymorphisms (ATG/GTG, CGG/GGG, TAA/TAC). Detection of the 669 polymorphism was carried out by PCR, followed by SSCP analysis and the confirmation of sequence variation was obtained by direct sequencing analysis. Detection of both 987 polymorphism and 1118 polymorphism was performed by allele-specific PCR. The results obtained indicated that the A-C-A intragenic haplotype was associated to the disease allele in the majority of patients (92%). These results are in agreement to previous studies. This indicates that a single MJD mutation was introduced in various populations, followed by a local founder effect; and this mutational event is probably very old. Based on findings presented here and comparing to data generated by the worldwide haplotype study, we can speculate that the origin of MJD associated-mutation in patients from South Brazil comes from Flores Island.