The genomic structure and expression of MJD, the Machado-Joseph disease gene

Machado-Joseph disease (MJD) is an autosomal dominant neurodegenerative dis order that is clinically characterized by cerebellar ataxia and various ass ociated symptoms. The disease is caused by an unstable expansion of the CAG repeat in the MJD gene. This gene is mapped to chromosome 14q32.1. To dete rmine its genomic structure, we constructed a contig composed of six cosmid clones and eight bacterial artificial chromosome (BAC) clones. It spans ap proximately 300kb and includes MJD. We also determined the complete sequenc e (175,330bp) of B445M7, a human BAC clone that contains MJD. The MJD gene was found to span 48,240bp and to contain 11 exons. Northern blot analysis showed that MJD mRNA is ubiquitously expressed in human tissues, and in at least four different sizes; namely, 1.4, 1.8, 4.5, and 7.5kb. These differe nt mRNA species probably result from differential splicing and polyadenylat ion, as shown by sequences of the 21 independent cDNA clones isolated after the screening of four human cDNA libraries prepared from whole brain, caud ate, retina, and testis. The sequences of these latter clones relative to t he MJD gene in B345M7 indicate that there are three alternative splicing si tes and eight polyadenylation signals in MJD that are used to generate the differently sized transcripts.