Generalized metabolic bone disease in Neurofibromatosis type I

Abstract

Skeletal abnormalities are a recognized component of Neurofibromatosis type I (NF1) but a generalized metabolic bone defect in NF1 has not been fully characterized thus far. The purpose of this study was to characterize at the densitometric, biochemical and pathological level the bone involvement in NF1 patients. Using dual energy X-ray absorptiometry (DXA) we analyzed bone status in 73 unselected NF1 subjects, 26 males and 47 females, mainly children and adolescents (mean age: 16.6 years). In a subgroup of subjects with low bone mass, we measured indices of calcium-phosphate metabolism, bone turnover, and bone density before and after vitamin D and calcium treatment. We found statistically significant and generalized reduction in bone mass with the mean lumbar bone mineral density (BMD) z-score being -1.38+/-1.05 (CI 95% -1.62 to -1.13), and whole body bone mineral content (BMC) z-score -0.61+/-1.19 (CI 95% -0.94 to -0.29), both significantly reduced compared to normal controls (p<.001). PTH was moderately elevated and after 4 months of supplemental therapy with calcium and vitamin D, it decreased to the normal range. However, BMD z-scores did not significantly improve after 2 years of follow-up. Histological analysis of bone samples from NF1 patients revealed substantial alteration of bone microarchitecture due mainly to reduced trabecular bone. Our observations are consistent with a generalized bone metabolic defect due to loss of the function of neurofibromin. Early identification of patients with osteoporosis may permit more timely and aggressive treatments to prevent the likely substantial morbidity associated with increased fracture risk later in life.

Figure 1

A. Lumbar spine BMD z-score. Distribution of BMD z-score in NF1 compared to a normal distribution. B. Whole body BMC z-score. Distribution of BMC z-score in NF1 compared to a normal distribution. The y axis represents the percentage of patients and z-scores are shown on the x axis. Comparison of z-score mean with the zero mean reference value was made with the one sample t-test using SPSS 11.5 (SPSS Inc., Chicago, Illinois, USA). 95^th centile confidence intervals were calculated: results are shown as mean ± SD.

Figure 2

Serum level of PTH before (pre) and after (post) 4 months of vitamin D and calcium therapy.

Figure 3

Representative section of the bone biopsies from controls (A, B, C) and from three subjects with NF1 (D, E, F) (H&E, decalcified section). The bony trabeculae are reduced in number and thickness. The control bone samples from healthy age-, and gender-matched child demonstrate the typical trabecular structure of a vertebral body and a marrow space occupied by typical bone marrow elements (H&E, decalcified section).

Figure 4

Electron microscopy of the bone from two NF1 patients. Examples of the numerous round cells adjacent to the bone surface observed in two independent bone samples from two distinct NF1 patients are shown in A and B. These cells were not detected in the bone sample obtained from a patient with osteogenesis imperfecta used for comparison in C.