Long-Term Follow-Up Outcomes of 19 Patients with Osteogenesis Imperfecta Type XI and Bruck Syndrome Type I Caused by FKBP10 Variants.

Osteogenesis imperfecta type XI (OI-XI) and Bruck syndrome type I (BS1) are two rare disorders caused by biallelic variants in the FKBP10, characterized by early-onset bone fractures and progressive skeletal deformities. The patients with OI-XI, also co-segregated with autosomal-recessive epidermolysis bullosa simplex caused by KRT14 variant, have been reported. In this study, the follow-up clinical features of the patients with OI-XI and BS1 phenotypes due to biallelic FKBP10 variants are compared. The aim of this study is to investigate the follow-up findings of OI-XI and BS1 phenotypes in patients with the FKBP10 variants. A total of 19 children, ten males and nine females, from 16 unrelated families were included in the study. FKBP10 variants were investigated by next-generation sequencing (NGS) based panel gene test or Sanger sequencing. Seventeen patients were followed between 1.5 and 16.8 years, and the last follow-up age was between 2 and 24.6 years (median 10.7 years). They received intravenous bisphosphonate infusions once every 3 months in follow-up period. We identified four different biallelic FKBP10 variants, two of which are novel (c.890_897dup TGATGGAC, p.Gly300Ter and c.1256 + 1G > A) in 16 families. Five of these patients also had findings of epidermolysis bullosa simplex, and the same biallelic c.612T > A (p.Tyr204Ter) variant in KRT14, as well as FKBP10, were identified. Twelve patients were diagnosed with OI-XI; whereas, seven were diagnosed with BS1. The BS1 phenotype was late-onset and the annual fracture number was lower. After bisphosphonate treatment, bone mineral densitometry Z score at L1-L4 increased (p = 0.005) and the number of annual fractures decreased (p = 0.036) in patients with OI-XI. However, no significant effect of bisphosphonate treatment was found on these values in BS1 patients. Despite the treatment, the rate of scoliosis and long bone deformity had increased in both groups at the last examination; and, only two patients could take a few steps with the aid of a walker, while others were not ambulatory, and they used wheelchairs for mobility. We identified two novel variants in FKBP10. Families originating from the same geographic region and having the same variant suggest founder effects. Although the number of fractures decreased with bisphosphonate treatment, none of our patients were able to walk during the follow-up. This study is valuable in terms of showing the follow-up findings of patients with FKBP10 variants for the first time.