Chu Hyun Bae, Myeongmo Kang, Clara Yongjoo Park, Bo Mi Park, Dong dong Zhang, Hee Jin Nam, Yu-Mi Yang, Dong Min Shin, Je-Yong Choi, and Sung-Kil Lim
Recently, an arginine-to-cysteine homozygous mutation at position 25 in mature parathyroid hormone was reported in a Korean patient with hypoparathyroidism.
To clarify whether the high bone mass phenotype observed in this patient was related to the hypoparathyroidism itself or to chronic elevation of mutant PTH.
A series of in vitro and in vivo experiments were performed in MC3T3E1, ROS 17/2.8, and SAOS2 cells treated with hPTH(1–34), Cys25hPTH(1–34), AL1Cys25hPTH(1–34) and Bpa1Cys25hPTH(1–34). The peptides were then subcutaneously delivered to OVX mice as daily single injections.
Compared to hPTH(1–34) and Ala1Cys25hPTH(1–34), treatment with Cys25hPTH(1–34) or Bpa1Cys25hPTH(1–34) resulted in decreases in the cAMP response and pCRE luciferase reporter activity. Although the cAMP response was sustained with hPTH(1–34) in MC3T3E1 cells, such response was not observed with the other mutated peptides. Meanwhile, all PTH analogues exhibited ERK phosphorylation and cytoplasmic Ca++ signals comparable to hPTH(1–34). On micro-CT analyses, trabecular and cortical bone parameters improved after 6 weeks of respective treatments as follows: hPTH(1–34)(80μ g/kg) = Ala1Cys25hPTH(1–34)(80μ g/kg) = Cys25hPTH(1–34)(80μ g/kg) > Bpa1Cys25hPTH(1–34)(80μ g/kg) > hPTH(1–34)(40μ g/kg). The increment of RANKL/OPG mRNA ratio in the MC3T3E1 cells after 6 hours of treatment of Cys25hPTH(1–34), AL1Cys25hPTH(1–34), and Bpa1Cys25hPTH(1–34) was less than that was obtained after hPTH(1–34) treatment. On bone histomorphometric analysis, AL1Cys25hPTH(1–34) increased the bone formation rate in both trabecular and periosteal bones compared to the control group.
The high bone mass phenotype observed in this patient with hypoparathyrodism caused by a Cys mutation at the 25th residue of hPTH(1–84) may have arisen from both direct and indirect effects exerted by the mutant PTH itself on bone.