High bone turnover and accumulation of osteoid in patients with neurofibromatosis 1

Although it is known that neurofibromatosis 1 (NF1) patients suffer from vitamin D deficiency and display decreased bone mineral density (BMD), a systematic clinical and histomorphometrical analysis is absent. Our data demonstrate that NF1 patients display high bone turnover and accumulation of osteoid and that supplementation of vitamin D has a beneficial effect on their BMD. Neurofibromatosis 1 results in a wide range of clinical manifestations, including decreased BMD. Although it has been reported that NF1 patients have decreased vitamin D serum levels, the manifestation of the disease at the bone tissue level has rarely been analyzed. Thus, we performed a clinical evaluation of 14 NF1 patients in comparison to age- and sex-matched control individuals. The analysis included dual X-ray absorptiometry osteodensitometry, laboratory parameters, histomorphometric and quantitative backscattered electron imaging (qBEI) analyses of undecalcified bone biopsies. NF1 patients display significantly lower 25-(OH)-cholecalciferol serum levels and decreased BMD compared to control individuals. Histomorphometric analysis did not only reveal a reduced trabecular bone volume in biopsies from NF1 patients, but also a significantly increased osteoid volume and increased numbers of osteoblasts and osteoclasts. Moreover, qBEI analysis revealed a significant decrease of the calcium content in biopsies from NF1 patients. To address the question whether a normalization of calcium homeostasis improves BMD in NF1 patients, we treated four patients with cholecalciferol for 1 year, which resulted in a significant increase of BMD. Taken together, our data provide the first complete histomorphometric analysis from NF1 patients. Moreover, they suggest that low vitamin D levels significantly contribute to the skeletal defects associated with the disease.