Positive linear growth and bone responses to growth hormone treatment in children with types III and IV osteogenesis imperfecta: high predictive value of the carboxyterminal propeptide of type I procollagen.

Extreme short stature is a cardinal feature of severe osteogenesis imperfecta (OI), types III and IV. We conducted a treatment trial of growth hormone in children with OI and followed linear growth velocity, bone metabolism markers, histomorphometrics, and vertebral bone density. Twenty-six children with types III and IV OI, ages 4.5-12 years, were treated with recombinant growth hormone (rGH), 0.1-0.2 IU/kg per day for 6 days/week, for at least 1 year. Length, insulin-like growth factor (IGF-I), insulin-like growth factor binding protein (IGFBP-3), bone metabolic markers, and vertebral bone density by DXA were evaluated at 6-month intervals. An iliac crest biopsy was obtained at baseline and 12 months. Approximately one-half of the treated OI children sustained a 50% or more increase in linear growth over their baseline growth rate. Most responders (10 of 14) had moderate type IV OI. All participants had positive IGF-I, IGFBP-3, osteocalcin, and bone-specific alkaline phosphatase responses. Only the linear growth responders had a significant increase in vertebral DXA z-score and a significant decrease in long bone fractures. After 1 year of treatment, responders' iliac crest biopsy showed significant increases in cancellous bone volume, trabecular number, and bone formation rate. Responders were distinguished from nonresponders by higher baseline carboxyterminal propeptide (PICP) values (p < 0.05), suggesting they have an intrinsically higher capacity for collagen production. The results show that growth hormone can cause a sustained increase in the linear growth rate of children with OI, despite the abnormal collagen in their bone matrix. In the first year of treatment, growth responders achieve increased bone formation rate and density, and decreased fracture rates. The baseline plasma concentration of PICP was an excellent predictor of positive response.