Treatment response to long term antiresorptive therapy in osteogenesis imperfecta type VI: does genotype matter?

OBJECTIVES

Osteogenesis imperfecta type VI (OI VI) follows a progressive and severe course, yet unlike other forms of severe OI it has a later onset of fractures, and extra-skeletal findings are not part of the clinical picture. Another difference is that there is an increase in unmineralized osteoid tissue in OI VI, which hinders the effect of bisphosphonates-the current standard of treatment for OI. Therefore, the response to standard treatments in OI VI is not satisfactory. Herein, we report long-term follow-up of two cases with novel SERPINF1 mutations, who show great variation in their treatment response to bisphosphonates.

CASE PRESENTATION

The first case was given pamidronate at the age of 15 months when he could sit independently, followed a fluctuating course under treatment, fracture rate did not decrease, however he was able to mobilize with walker at the age of 10 years. On the other hand, the second case developed severe deformities and became wheelchair-bound under pamidronate, thus the treatment was switched to denosumab. Unfortunately, there was no improvement under denosumab after 15 months too, and since bone pain increased, denosumab treatment was stopped. He was put on zoledronic acid instead.

CONCLUSION

SERPINF1 transcript amount may be an important factor to explain the variation in response to pamidronate therapy. In OI VI patients, the factors affecting the clinical course should be identified and new or combined treatment options should be established.

Novel Mutations in SERPINF1 Result in Rare Osteogenesis Imperfecta Type VI

Osteogenesis imperfecta (OI) is a group of inherited disorders characterized by recurrent fragile fractures. Serpin peptidase inhibitor, clade F, member 1 (SERPINF1) is known to cause a distinct, extremely rare autosomal recessive form of type VI OI. Here we report, for the first time, the detection of SERPINF1 mutations in Chinese OI patients. We designed a novel targeted next-generation sequencing panel of OI-related genes to identify pathogenic mutations, which were confirmed with Sanger sequencing and by co-segregation analysis. We also investigated the phenotypes of OI patients by evaluating bone mineral density, radiological fractures, serum bone turnover markers, and pigment epithelium-derived factor (PEDF) concentration. Six patients with moderate-to-severe bone fragility, significantly low bone mineral density, and severe deformities of the extremities were recruited from five unrelated families for this study. Six pathogenic mutations in SERPINF1 gene were identified, five of which were novel: (1) a homozygous in-frame insertion in exon 3 (c.271_279dup, p.Ala91_Ser93dup); (2) compound heterozygous mutations in intron 3 (c.283 + 1G > T, splicing site) and exon 5 (c.498_499delCA, p.Arg167SerfsX35, frameshift); (3) a homozygous frameshift mutation in exon 8 (c.1202_1203delCA, p.Thr401ArgfsX); (4) compound heterozygous missense mutation (c.184G > A, p.Gly62Ser) and in-frame insertion (c.271_279dup, p.Ala91_Ser93dup) in exon 3; and (5) a heterozygous nonsense mutation in exon 4 (c.397C>T + ?, p.Gln133X + ?). Serum PEDF levels were barely detectable in almost all subjects. We identified five novel mutations in SERPINF1 and confirmed the diagnostic value of serum PEDF level for the first time in Chinese patients with the extremely rare OI type VI.