Reduced Bone Mass in Collagen Prolyl 4‐Hydroxylase <i>P4ha1</i> ^+/− ; <i>P4ha2</i> ^−/− Compound Mutant Mice

Abstract

Proper deposition of the extracellular matrix and its major components, the collagens, is essential for endochondral ossification and bone mass accrual. Collagen prolyl 4-hydroxylases (C-P4Hs) hydroxylate proline residues in the -X-Pro-Gly- repeats of all known collagen types. Their product, 4-hydroxyproline, is essential for correct folding and thermal stability of the triple-helical collagen molecules in physiological body temperatures. We have previously shown that inactivation of the mouse <i>P4ha1</i> gene, which codes for the catalytic α subunit of the major C-P4H isoform, is embryonic lethal, whereas inactivation of the <i>P4ha2</i> gene produced only a minor phenotype. Instead, mice with a haploinsufficiency of the <i>P4ha1</i> gene combined with a homozygous deletion of the <i>P4ha2</i> gene present with a moderate chondrodysplasia due to transient cell death of the growth plate chondrocytes. Here, to further characterize the bone phenotype of the <i>P4ha1</i> ^+/-; <i>P4ha2</i> ^-/- mice, we have carried out gene expression analyses at whole-tissue and single-cell levels, biochemical analyses, microcomputed tomography, histomorphometric analyses, and second harmonic generation microscopy to show that C-P4H α subunit expression peaks early and that the C-P4H deficiency leads to reduced collagen amount, a reduced rate of bone formation, and a loss of trabecular and cortical bone volume in the long bones. The total osteoblast number in the proximal <i>P4ha1</i> ^+/-; <i>P4ha2</i> ^-/- tibia and the C-P4H activity in primary <i>P4ha1</i> ^+/-; <i>P4ha2</i> ^-/- osteoblasts were reduced, whereas the population of osteoprogenitor colony-forming unit fibroblasts was increased in the <i>P4ha1</i> ^+/-; <i>P4ha2</i> ^-/- marrow. Thus, the <i>P4ha1</i> ^+/-; <i>P4ha2</i> ^-/- mouse model recapitulates key aspects of a recently recognized congenital connective tissue disorder with short stature and bone dysplasia caused by biallelic variants of the human <i>P4HA1</i> gene. Altogether, the data demonstrate the allele dose-dependent importance of the C-P4Hs to the developing organism and a threshold effect of C-P4H activity in the proper production of bone matrix. © 2022 The Authors. <i>JBMR Plus</i> published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.