A new gene of unknown function, called FAM135B, is expressed at lower levels in a cellular model of spinal and bulbar muscular atrophy (SBMA) and is thought to play a role in supporting spinal motor neurons’ growth and survival, a study found.
SBMA, also known as Kennedy’s disease, is a rare, late-onset neurodegenerative disorder that mainly affects men. It disrupts the nerve cells that control muscle movement (motor neurons) and particularly affects facial and swallowing muscles, as well as arm and leg muscles, especially proximal ones, i.e. those nearest to the center of the body.
SBMA is triggered by a mutation in the androgen receptor (AR) gene, which provides instructions for the androgen receptor protein, found at high levels in motor neurons. This results in a section of the gene, called a CAG trinucleotide repeat, to become abnormally expanded, leading to a much longer than normal and structurally altered protein.
Several models have been used to study SBMA, including cellular models, but also animal ones (e.g. fly, mouse). However, the mechanisms underlying SBMA have yet to be made clear.
Induced pluripotent stem cells (iPSCs) are adult human cells that have been reprogrammed back into an embryonic-like state and can therefore give rise to several types of cells.
Researchers from the Genome Institute of Singapore previously generated iPSCs from two SBMA patients and their healthy siblings.
Now, the same team has transformed four healthy and five SBMA patient-derived iPSC lines into spinal motor neurons to study molecular changes and gene expression patterns in Kennedy’s disease. Gene expression is the process by which information in a gene is synthesized to create a working product, such as a protein.
SBMA patient-derived iPSCs had a very high differentiation efficiency (approximately 81.2%), which meant they were reliably transformed into spinal motor neurons. This rate was higher than those reported in previous SBMA studies.
iPSC lines derived from healthy controls were also efficiently generated.
By comparing wild-type (healthy) and SBMA iPSC lines, the team found disease-specific changes in SBMA spinal motor neurons, including lower survival and shortened and swollen neuronal projections. These changes in nerve cell anatomy can indicate poor neuron health and lead to reduced neuronal transmission (communication).
“In our study, the neurite blebs [reduced-size neuronal projections] occurred as early as day 21 and could be the first sign of degeneration before [death of spinal motor neurons],” researchers said.
Contrary to other studies, “the [spinal motor neurons] from our SBMA model recapitulated the loss of [spinal motor neurons] observed in SBMA patients,” they added.
Molecular analysis showed that SBMA spinal motor neurons had lower androgen receptor (AR) expression, compared to those derived from healthy cells, which is consistent with low AR levels observed in SBMA patients.
A process called protein translation, i.e. when messenger RNA is translated into a protein, was disrupted in SBMA spinal motor neurons.
The severity of “cellular disease” appeared not to correlate with CAG repeat length, suggesting other factors can modify SBMA outcome, “including that of differences in genetic makeup of individuals which lend a unique twist to disease progression rate,” researchers said.
When a molecule known as a ligand binds to a cellular receptor, it triggers a molecular cascade of events. To find out whether ligand binding could alter disease outcome, researchers treated cells with dihydrotestosterone (DHT), a hormone that binds to the androgen receptor and selectively activates it.
DHT did not increase SBMA cellular/genetic changes. However, the team found that several genes involved in the regulation of important neurological processes were dysregulated in SBMA spinal motor neurons upon treatment with DHT.
Among these, the FAM135B gene was reduced by more than 10,000-fold in SBMA spinal motor neurons when compared to healthy cells.
Interestingly, this gene was present at low levels in iPSCs derived from either SBMA or healthy individuals. Its differential expression was only observed once these cells became spinal motor neurons, leading researchers to hypothesize that “this gene plays a functional role in supporting sMN [spinal motor neurons] growth and survival.”
To investigate the functional role of this gene in SBMA, the team genetically engineered healthy spinal motor neurons to express lower levels of FAM135B (similar to what was observed in SBMA spinal motor neurons).
The results revealed a decrease in neuronal projections’ length and spinal motor neurons’ survival, suggesting a specific role for this gene in SBMA.
Interestingly, low levels of FAM135B were not observed in amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) iPSC-derived spinal motor neurons, indicating that this differential gene expression pattern is unique to SBMA.
“To date there have been no detailed studies that define a role of FAM135B in the nervous system,” researchers said.
“Future work could be focused on detailed investigation of the role of FAM135B in SBMA. With FAM135B proposed to play roles in neuronal survival, growth and maintenance of structural integrity, it could be speculated that an upregulation of which in [spinal motor neurons] could potentially [reverse the disease-related neuronal changes],” they concluded.