Overexpression of wild-type androgen receptor in muscle recapitulates polyglutamine disease

Abstract

We created transgenic mice that overexpress WT androgen receptor (AR) exclusively in their skeletal muscle fibers. Unexpectedly, these mice display androgen-dependent muscle weakness and early death, show changes in muscle morphology and gene expression consistent with neurogenic atrophy, and exhibit a loss of motor axons. These features reproduce those seen in models of Kennedy disease, a polyglutamine expansion disorder caused by a CAG repeat expansion in the AR gene. Kennedy disease ͉ neuromuscular ͉ skeletal muscle ͉ spinal and bulbar muscular atrophy ͉ axonopathy A polymorphism in exon 1 of the androgen receptor (AR) gene, consisting of a variable number of glutamine (Q) repeats, affects AR function. Very long polyglutamine repeat (polyQ) tracts are associated with a progressive neuromuscular disease known as Kennedy disease (KD, or spinal bulbar muscular atrophy) (1). The etiological mechanism mediating polyQ toxicity is uncertain, but is generally thought to confer novel toxic functions to the protein, because expansion of polyQ tracts beyond 40 repeats in other proteins also cause neurodegenerative disease, including Huntington's disease (HD), and several autosomal dominant forms of spinocerebellar ataxia (SCA) (2). Histopathological studies of KD patients suggest ''neurogenic'' responses to denervation, and the etiology of this disease is therefore generally thought to begin with motoneuron pathology (3).Explicit mouse models of KD, in which polyQ AR alleles containing 60 CAG repeats or more are expressed, develop a disease phenotype that includes a marked reduction in body weight, kyphosis, and striking deficits in muscle strength and motor coordination (4-8). Androgen dependence, motoneuron and muscle pathology, and/or inclusions containing AR are also observed in these models (4,5,7,8). Our studies of AR in skeletal muscle (9, 10) led us to generate transgenic (Tg) mice in which AR is overexpressed solely in skeletal muscle fibers using an expression cassette containing the human skeletal ␣-actin (HSA) promoter. We discovered a striking phenotypic resemblance between these HSA-AR mice and mouse models of KD. This similarity is surprising given that the Q repeat in this AR transgene comprises only 22 glutamines and is expressed exclusively in skeletal muscle fibers and not in motoneurons. Results HSA Promoter Drives Transgene Expression Exclusively in SkeletalMuscle Fibers. We first validated our HSA expression cassette by generating HSA-LacZ (LacZ ϭ ␤-galactosidase gene) reporter mice [supporting information (SI) Fig. 5A]. Consistent with other expression cassettes containing the HSA promoter (11, 12) these reporter mice express ␤-gal specifically in skeletal muscle fibers, starting at embryonic day 9.5-10.5, with no detectable expression in other tissues, including the heart, viscera, fat or spinal cord ( SI Fig 5 B and C). We also created Tg mice in which a rat WT AR cDNA is driven by this same HSA expression cassette (SI Fig 6A), resulting in selective overexpression of AR in skeletal muscle fiber...

The role of androgen receptors in the masculinization of brain and behavior: What we've learned from the testicular feminization mutation

“…Thus there is only a single amino acid difference between wt and Tfm AR protein in rats resulting in expression of a normal sized but dysfunctional AR protein in Tfm rats. On the other hand, the mutation in Tfm mice introduces a premature stop codon, resulting in a shortened transcript and essentially no AR protein (He et al, 1991;Monks et al, 2007). Due to differences in the nature of these mutations, Tfm rats have some residual sensitivity to androgens through ARs, although it is greatly reduced (Yarbrough et al, 1990), while Tfm mice have virtually no sensitivity to androgen through ARs, as in CAIS women (Drews, 1998).…”

Kennedy disease (X-linked recessive bulbospinal neuronopathy): A comprehensive review from pathophysiology to therapy

“…Histopathological analysis of muscle tissues from transgenic and knock-in mice expressing polyQ-AR revealed both neurogenic and myopathic features [59] [49] [60] [33]. It is noteworthy that in the knock-in mouse model of SBMA, muscle pathology is evident prior to the onset of spinal cord pathology [61]; strongly supporting the view that muscle is a primary target of polyQ-AR toxicity [62].…”

“…Therefore its inhibition, for example using the non-steroid anti-androgen flutamide, may be an ideal target for therapy in SBMA [79] [80]. Although the phenotype does not improve, some benefits are detectable in transgenic mice overexpressing wild type AR, when flutamide is administered at a prenatal stage [62] [81] [66].…”

Androgen receptor and Kennedy disease/spinal bulbar muscular atrophy

“…We detected no transgene expression in other tissues. Importantly, we could not detect any transgene expression in nervous tissue including spinal motoneurons (Monks et al, 2007).…”

“…The HSA promoter was cloned from the human ACTA1 gene, which is an actin isoform specific for muscle lineage cells. In our hands, this promoter does not appear to be expressed in other muscle types, including cardiac or smooth muscle (Monks et al, 2007). We verified HSA expression in two ways: we generated HSA-LacZ reporter mice, which use the same promoter to express β-galactosidase, an enzyme that is easily detected histochemically, and we measured AR protein in several tissues in HSA-AR Tg mice.…”