Low bone mass and high material bone density in two patients with Loeys-Dietz syndrome caused by transforming growth factor beta receptor 2 mutations

Loeys-Dietz syndrome (LDS) is a rare autosomal-dominant connective tissue disorder caused by heterozygous mutations in the genes encoding transforming growth factor beta receptor 1 or 2 (TGFBR1 or TGFBR2). Although an association between LDS and osteoporosis has been reported, the skeletal phenotype regarding bone mass is not well characterized. Here, we report on two LDS patients with mutations in TGFBR2. Patient 1 was a 24-year-old man who had a total of three fractures involving the left radius, the left metacarpal, and the right femur. At the age of 14 years, lumbar spine areal bone mineral density Z-score was -4.0 and iliac bone histomorphometry showed elevated bone turnover (bone formation rate per bone surface: 91 μm3/μm2/year; age-matched control values 37 [10], mean [SD]) and mildly low trabecular bone volume per tissue volume (17.2%; age-matched control values 25.7 [5.3]). Bone mineralization density distribution (BMDD) in trabecular bone was increased (Ca(Peak) 22.70 wt% Ca; age-matched control values 21.66 [0.52]). Patient 2, a 17-year-old girl, suffered from diffuse bone pain but had not sustained fractures. At 14 years of age, her lumbar spine areal bone mineral density Z-score was -3.4. Iliac bone histomorphometry at that age confirmed low bone mass (bone volume to tissue volume 10.1%, same control values as above) and high bone turnover (bone formation rate per bone surface 70 μm3/μm2/year). BMDD in trabecular bone was significantly shifted toward increased mineralization (Ca(Peak) 22.36 wt% Ca). Thus, it appears that LDS can be associated with low bone mass and high bone turnover but increased matrix mineralization of trabecular bone.