This report describes spinal and bulbar muscular atrophy, X-linked 1 (SMAX1), a subtype of spinal muscular atrophy; SMAX1 exhibits X-linked recessive inheritance. SMAX1 is also known as Kennedy's disease. SMAX1 is caused by a trinucleotide CAG repeat expansion in exon 1 of the gene encoding the androgen receptor (AR), resulting in an expanded polyglutamine tract in the protein. AR is associated with multiple other diseases, including androgen insensitivity (OMIM:300068). There is no gene orthologous to androgen receptor in Drosophila.
Multiple UAS constructs of the human gene, Hsap\AR, have been introduced into flies, including wild-type and variants with differing numbers of CAG repeats. Some of the fly models characterize activated AR and require an AR ligand; in these experiments larvae are raised in the presence of dihydroxytestosterone.
Variant(s) implicated in human disease tested (as transgenic human gene, AR): Q58_Q80 (CAG)n EXPANSION; UAS-AR constructs with differing numbers of the pathological CAG repeat have been created.
[updated Jun. 2017 by FlyBase; FBrf0222196]
Spinal muscular atrophy (SMA) is characterized by progressive muscle weakness resulting from degeneration and loss of the anterior horn cells (i.e., lower motor neurons) in the spinal cord and the brain stem nuclei. Onset ranges from before birth to adolescence or young adulthood. Poor weight gain, sleep difficulties, pneumonia, scoliosis, and joint contractures are common complications. [From GeneReviews, Spinal Muscular Atrophy, pubmed:20301526 2016.07.11]
[SPINAL AND BULBAR MUSCULAR ATROPHY, X-LINKED 1; SMAX1](https://omim.org/entry/313200)
[ANDROGEN RECEPTOR; AR](https://omim.org/entry/313700)
Homozygous females may exhibit mild features related to the condition, including muscle cramps and occasional tremors; milder symptoms in females may be related to lower androgen levels. [from Genetics Home Reference, Spinal and bulbar muscular atrophy; 2016.01.20]
SMAX1 occurs primarily in men. Age at onset is usually in the third to fifth decade of life, but earlier involvement has been reported. The disorder is characterized by slowly progressive limb and bulbar (mouth and throat) muscle weakness with fasciculations, muscle atrophy, and gynecomastia (Harding et al., 1982; pubmed:6890989). [from OMIM:313200; 2016.01.20]
Steroid hormone receptors are ligand-activated transcription factors that affect cellular proliferation and differentiation in target tissues. [from UniProt:P10275; 2016.01.20]
The androgen receptor (AR) protein belongs to the class of nuclear receptors called activated class I steroid receptors, which recognize DNA sequences classified as canonical androgen response elements (AREs). The major domains of AR include N- and C-terminal activation domains, a ligand-binding domain, and a polyglutamine tract (Callewaert et al., 2003; pubmed:12788064). [from OMIM:313700; 2016.01.20]