Bone mass in subjects with Klinefelter syndrome: role of testosterone levels and androgen receptor gene CAG polymorphism.

Context: Klinefelter syndrome (KS) is a chromosomal alteration characterized by supernumerary X-chromosome(s), primary hypogonadism, decreased pubertal peak bone mineral density (BMD), and accelerated bone loss during adulthood. Decreased bone mass has been traditionally related to low testosterone levels. However, testosterone replacement therapy does not necessarily increase bone mass in these patients, and low BMD can be observed also in patients with normal testosterone levels. The androgen receptor (AR) gene CAG polymorphism seems to modulate the sensitivity to testosterone and previous studies have related it to some clinical aspects of KS, to include BMD, gynecomastia, testes and prostate volume, and hemoglobin concentration.

Objective: To analyze the relation between bone mass, testosterone, and AR CAG polymorphism in men with KS.

Design: Cross-sectional cohort study.

Setting: University department.

Patients: One hundred twelve consecutive treatment-naïve 47,XXY Klinefelter patients (mean age 33.5 ± 4.7 yr) and 51 age-matched normal male controls.

Main Outcome Measures: Dual-energy x-ray absorptiometry, CAG repeat length polymorphism, X-chromosome inactivation, and testosterone levels.

Results: Forty-nine of 112 KS subjects (42.5%) had low bone mass (osteopenia or osteoporosis). Lumbar and/or femoral T-scores were lower in KS patients compared with controls. No significant relationship was observed between testosterone levels and bone parameters, and the prevalence of osteopenia/osteoporosis was similar in subjects with normal and low testosterone levels (43.7% and 40.5%, respectively). The mean CAG repeat length calculated after X-chromosome inactivation analysis showed no differences between patients with normal and low bone mass.

Conclusions: Testosterone levels and AR CAG polymorphism are not associated with bone mass phenotype in KS.