X-linked adult form of spinal muscular atrophy

Pathogenesis and molecular targeted therapy of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) or Kennedy's disease is a motor neurone disease characterized by muscle atrophy, weakness, contraction fasciculations and bulbar involvement. SBMA mainly affects males, while females are usually asymptomatic. SBMA is caused by expansion of a polyglutamine (polyQ)-encoding CAG trinucleotide repeat in the androgen receptor (AR) gene. AR belongs to the heat shock protein 90 (Hsp90) client protein family. The histopathologic hallmarks of SBMA are diffuse nuclear accumulation and nuclear inclusions of the mutant AR with expanded polyQ in residual motor neurones in the brainstem and spinal cord as well as in some other visceral organs. There is increasing evidence that the ligand of AR and molecular chaperones play a crucial role in the pathogenesis of SBMA. The success of androgen deprivation therapy in SBMA mouse models has been translated into clinical trials. In addition, elucidation of its pathophysiology using animal models has led to the development of disease-modifying drugs, that is, Hsp90 inhibitor and Hsp inducer, which inhibit the pathogenic process of neuronal degeneration. SBMA is a slowly progressive disease by nature. The degree of nuclear accumulation of mutant AR in scrotal skin epithelial cells was correlated with that in spinal motor neurones in autopsy specimens; therefore, the results of scrotal skin biopsy may be used to assess the efficacy of therapeutic trials. Clinical and pathological parameters that reflect the pathogenic process of SBMA should be extensively investigated.

CAG repeat length in an infertile male population of Irish origin

The androgen receptor (AR) gene, located on the X chromosome, is an important regulator of human spermatogenesis. In the past decade, the link between the CAG polyglutamine tract, situated on exon one of the AR gene, and reduced spermatogenesis has become a controversial one. Alterations in the length of the CAG polyglutamine tract have been associated with prostate cancer at a reduced intrinsic length and neuromuscular diseases at a CAG repeat length of > or = 40. Minimal intermediate increases have been linked with depressed spermatogenesis in infertile males. Asian and Australian groups have published an association between increased CAG repeat length and reduced spermatogenesis while many European studies have found no such association. The aim of this study was to document the association between increased CAG repeat length and reduced spermatogenesis in a group of Irish infertile males and controls known to have fathered at least one child. The study employed the ABI 377 DNA sequencer to size the CAG repeat region of exon one of the AR gene in each group. Statistical analysis revealed no actual link between the length of the CAG tract and a reduction of spermatogenesis in a cohort of infertile patients (n = 66) of Irish ethnic origin when compared to a fertile control group (n = 77) (p = 0.599).

Clinical aspects of CAG repeat diseases

Seven neurodegenerative disorders are known to be caused by unstable expansions of the trinucleotide CAG within human genes, and more will be discovered in the coming years. These disorders share some clinical similarities, as well as some differences, which are summarized here. These diseases have unusual clinical genetic properties related to the dynamic nature of CAG repeat expansions, including instability of the repeat expansion in meiosis, particularly male meiosis; a strong correlation between onset age and size of the repeat expansion; anticipation (earlier disease onset in succeeding generations); new mutations arising from unstable, mutable alleles with a high-normal CAG repeat number; and reduced penetrance for alleles in the low-affected range. Much more remains to be learned about the molecular biology and clinical pathophysiology of this new class of genetic diseases.

Prenatal diagnosis of X-linked spinal and bulbar muscular atrophy in a Greek family

X-linked spinal and bulbar muscular atrophy (SBMA) is a late-onset motor neuron disorder which is caused by an expansion of the trinucleotide repeat (CAG)n in the first exon of the androgen receptor gene. Two cases of prenatal testing for the disease in a Greek family are reported. An affected male died in his late 50s of this disorder and his 30-year-old daughter (an obligate carrier) asked for prenatal testing for SBMA. DNA analysis revealed that she indeed carried an expanded allele of 40 repeats, as well as a normal size allele of 24 repeats. Prenatal diagnosis of SBMA was performed when, on two successive pregnancies, two male fetuses with the expanded (CAG)n allele were found.

X-linked recessive bulbospinal neuronopathy (Kennedy's syndrome): a neurophysiological study

We examined 8 men with X-linked recessive bulbospinal neuronopathy. Quantitative electromyography showed large amplitude motor unit action potentials of prolonged duration and increased polyphasia. There was a pronounced loss of motor units in all but one muscle at maximal volition even in muscles with normal or only mildly decreased force. Denervation activity was present in 53% sampled limb muscles, and fasciculation was recorded in 33% of limb muscles. Nerve conduction studies showed small amplitude sensory action potentials in 6 out of 8 patients. The motor and sensory conduction velocity was normal or borderline slow. The electrophysiologic findings were consistent with chronic partial denervation (motor axonopathy) combined with large fibre sensory axonopathy.

X-linked bulbospinal muscular atrophy (Kennedy's syndrome): a report of three cases

Two cases of X-linked bulbo-spinal muscular atrophy and one sporadic case with the same clinical features are presented. All three cases were extensively studied by electrophysiological methods. One of the patients showed some transient weakness, which was partly improved by pyridostigmin. In this patient the Decrement test and jitter revealed a mild degree of motor-end plate dysfunction. Clinical and electrophysiological findings obtained from all three patients suggest that in Kennedy syndrome cell bodies of group IA muscle afferents are also affected selectively, with other peripheral afferents.

[Sensory nerve involvement in X-linked bulbospinal amyotrophy (Kennedy syndrome). Contributions of electrophysiologic and histologic data]

Two new cases of X-chromosome linked bulbospinal muscular atrophy associated with gynecomastia (described by Kennedy in 1968) are reported. In one patient, an electrophysiological study and a muscle nerve biopsy were performed. Motor nerve conduction studies were normal but sensory action potentials were small or unrecordable in the absence of clinical sensory loss. Superficial peroneal nerve biopsy showed axonal lesions. These findings suggest that this disease is not purely due to degeneration of spinal motor neurons and is also associated with an axonopathy.