Background Spinal bulbar muscular atrophy is a slowly progressive neurodegenerative disease which affects lower motor neurons (MNs). It is caused by a trinucleotide polyglutamine (CAG) repeat expansion in the androgen receptor (AR) gene but the precise mechanisms of degeneration of the MNs in SBMA are complex and remain poorly understood. There are currently no available treatments for patients.

Objectives The aim of this study was to develop a human iPSC model of SBMA patient MNs in order to characterise early transcriptomic events in SBMA.

Methods Human iPSCs were differentiated into spinal MNs using an established, developmentally-rational- ised protocol. Samples were collected at key stages in motor neurogenesis for RNA-sequencing followed by transcriptomic and splicing analysis. Live-cell imaging assays for mitochondrial membrane potential and axonal transport of mitochondria and signal endosomes were performed to validate transcriptomic findings.

Results Over-representation analysis of differentially expressed genes revealed enrichment of DNA damage response and cell cycle pathways. There were reduced transcripts of genes involved in mito- chondrial pathways with reduced mitochondrial membrane potential and anterograde mitochondrial transport in SBMA on live-cell imaging. Alternative splicing events were common and involved adherens junction and extracellular matrix pathways.

Conclusion Early transcriptional dysregulation, alternative splicing and mitochondrial dysfunction were seen in SBMA MNs as well as genes linked to defective p53/DNA damage response and the cell cycle.

These cellular phenotypes offer potential targets for future clinical treatments.