Tissue-specific somatic mosaicism in spinal and bulbar muscular atrophy isdependent on CAG-repeat length and androgen receptor-gene expression level

The factors influencing the tissue-specific pattern of somatic mosaicism in GAG-repeat diseases have not yet been fully resolved. We performed a detai led analysis of the degree of somatic mosaicism in various tissues from 20 patients with spinal and bulbar muscular atrophy (SBMA), including 4 who we re deceased. The most outstanding feature was the prominent somatic mosaici sm observed in the cardiac and skeletal muscles, composed predominantly of postmitotic cells, and in the skin, prostate, and testis. The CNS tissues, liver, and spleen showed the least mosaicism. The tissue distribution of so matic mosaicism in patients with SBMA was markedly different from that in p atients with Huntington disease (HD) and from that in patients with dentato rubral-pallidoluysian atrophy (DRPLA). The degree of somatic mosaicism corr elated with the GAG-repeat number but not with age at examination. Furtherm ore, tissues with a higher mosaicism level corresponded well to those with a higher expression Level of androgen receptor protein. The tissue-specific pattern of somatic mosaicism related not only to cell composition with dif ferent cell turnover rates but to repeat size and gene expression levels, a nd postnatal cell division is unlikely to be a major cause of somatic mosai cism probably because of the relative stability of CAG repeat in SBMA.