Very high bone mineral density in a monogenic form of obesity-associated insulin resistance

Alström syndrome (ALMS) is an ultra-rare metabolic disorder caused by biallelic loss-of-function in the Alms1 gene which encodes a ubiquitously expressed centrosomal protein of the primary cilium. Although ALMS is characterised by several metabolic and hormonal dysfunctions that can lead to an increased risk of developing osteoporosis and bone fracture, an increased BMD have been observed. The aim of this study was to characterise the anthropometric, clinical, genetic and densitometric features of bone health in a large adult UK cohort of subjects with ALMS.

Twenty-three patients with ALMS and 23 age-matched male control subjects were recruited. Lumbar spine (LS) and total hip (TH) bone mineral density (BMD) were evaluated by DXA in all subjects. A CT scan to assess the spinal bone architecture was performed in ALMS patients with raised lumbar density. Blood analysis for biochemical parameters and thyroid and sex hormones was performed in all ALMS patients.

LS Z-score levels were higher than +2 SD in 35% of all ALMS study participants, of whom 75% were men and 25% were women. TH Z-scores were higher than +1 SD 13% of patients and all of them have higher than expected lumbar Z-score. An extremely high BMD was found in two of the oldest patients (LS Z-score +10.8 and +15.3 SD).

ALMS patients tend to have high levels of BMD that increase with age, in particular of the trabecular bones. Although obesity and lifelong IR can be responsible for the increase in BMD, at least in part, of a possible signalling role of Alms1 protein as a bone-forming factor is plausible.