Neonatal Testosterone Surge Identified as Key Trigger for Spinal and Bulbar Muscular Atrophy (SBMA)
New research from Nagoya University has identified a natural testosterone surge occurring shortly after birth as a critical early trigger for Spinal and Bulbar Muscular Atrophy (SBMA), an inherited neurodegenerative disease. The study, published in Nature Communications, demonstrated that this neonatal testosterone surge activates a mutant protein in motor neurons, contributing to their eventual breakdown. Early intervention with gene-silencing drugs in newborn mice showed improved survival and reduced motor neuron degeneration, suggesting a potential therapeutic window in the disease's earliest stages.
Understanding Spinal and Bulbar Muscular Atrophy (SBMA)
Spinal and Bulbar Muscular Atrophy (SBMA) is a rare, inherited disease that exclusively affects males. It causes progressive muscle weakness and wasting, with symptoms often including hand tremors beginning in their thirties. Diagnosis typically occurs around age 40 when muscle weakness becomes more pronounced. The disease is triggered by elevated testosterone levels.
Neonatal Testosterone Surge: The Critical Early Trigger
Researchers at Nagoya University focused on the earliest phase of SBMA, specifically the initial days after birth. They discovered that a natural testosterone surge, sometimes referred to as "mini-puberty," activates a mutant protein in the motor neurons of newborn male mice carrying the SBMA mutation.
This natural testosterone surge lasts approximately 10 days in mice and about six months in humans, representing the earliest point of disease initiation.
The defective mutant androgen receptor protein, which is central to SBMA, requires testosterone to enter the nuclei of motor neurons and initiate damage. Confirmation showed that the mutant protein accumulated in the nuclei of motor neurons in male SBMA mice within their first day of life, driven by this surge. Female mice with the same mutation did not exhibit these effects, underscoring testosterone's role as a primary trigger.
Unraveling the Mechanism of Damage
The activation by testosterone leads to the overactivation of nerve cells that control muscles. Additionally, genes responsible for activating nerve cells, particularly glutamate receptors, showed abnormal overactivity in SBMA mice during their first week of life, contributing to this motor neuron overactivity.
Similar abnormal overactivity was observed in motor neurons grown in a laboratory from cells of human SBMA patients, suggesting a parallel disease process in humans. This sustained overactivity ultimately contributes to the breakdown of nerve cells in adulthood.
Promising Early Intervention: Lasting Protection
To evaluate the impact of early intervention, researchers administered two gene-silencing drugs to newborn mice with the SBMA mutation. One drug specifically targeted the mutant protein directly, while the other targeted REST4, a protein identified as driving abnormal nerve cell overactivity.
Both treatments resulted in improved survival rates, enhanced motor performance, and reduced motor neuron degeneration in mice assessed at 13 weeks of age.
A notable finding was that a single dose of the drug targeting the mutant protein, administered at birth, continued to protect motor neurons for months, even though the drug's direct effects had subsided within two weeks.
This suggests that intervention during a critical early life window may have long-lasting protective consequences. REST4 has been identified as a potential new therapeutic target.
Next Steps: From Lab to Clinic
Nagoya University has a history of contributions to SBMA treatment, including the development of leuprorelin acetate, the only drug approved for SBMA in Japan. The research team's next priority is to determine if similar abnormal nerve cell overactivity occurs in human SBMA patients. This aspect is currently challenging to study directly in living newborns. The goal is to translate these findings into patient care, alongside evaluating the safety of gene-silencing drugs and the effectiveness of repeated treatments in humans.