Testosterone Therapy Reshapes Muscle Gene Activity in Klinefelter Syndrome

Klinefelter syndrome affects roughly 1 in 600 males and is the leading chromosomal cause of male hypogonadism. Beyond infertility, it drives profound metabolic and body composition changes — most notably reduced skeletal muscle mass — that raise long-term risks for cardiometabolic disease and reduced healthspan. Testosterone replacement therapy is standard care, yet exactly how it reshapes muscle at the cellular level has remained poorly understood.

Researchers at Aarhus University Hospital applied single-nucleus RNA sequencing to vastus lateralis (thigh) muscle biopsies from four KS patients at diagnosis and again after 12 months of TRT, alongside biopsies from four age-matched healthy male controls. This approach captures the transcriptional activity of individual nuclei across all major cell types residing in muscle tissue, providing an unprecedented resolution of biological change.

The study profiled 81,786 nuclei across ten distinct cell types. At baseline, untreated KS muscle displayed pervasive transcriptional reprogramming: elevated fibrotic and adipogenic gene programs, heightened inflammatory signaling, and disrupted progression of muscle stem cells through normal developmental trajectories — a pattern suggesting the tissue was structurally compromised and poorly primed for repair. After one year of TRT, testosterone and gonadotropin levels normalized, and gene expression shifted meaningfully toward pro-regenerative pathways, improved structural integrity, and vascular remodeling. Three of four patients gained measurable muscle mass.

Despite these gains, a substantial KS-specific transcriptional signature persisted post-treatment, most notably chronic inflammatory signaling. This suggests that while TRT is beneficial, it does not fully reverse the cellular consequences of long-standing testosterone deficiency.

For clinicians and longevity-focused individuals, these findings underscore that earlier testosterone intervention in KS — before irreversible fibrotic and inflammatory remodeling sets in — may be critical to preserving muscle healthspan. The study is exploratory given its small sample size, and findings await replication in larger cohorts.