Nam Hwa Kyung, Emmanouil Emmanouil, Hatch Nan E
Department of Orthodontics and Pediatric Dentistry, School of Dentistry, University of Michigan, Ann Arbor, MI, United States.
Front Dent Med. 2022;3. doi: 10.3389/fdmed.2022.846962. Epub 2022 Apr 28.
Hypophosphatasia is a rare heritable metabolic disorder caused by deficient Tissue Non-specific Alkaline Phosphatase (TNAP) enzyme activity. A principal function of TNAP is to hydrolyze the tissue mineralization inhibitor pyrophosphate. ENPP1 (Ectonucleotide Pyrophosphatase/Phosphodiesterase 1) is a primary enzymatic generator of pyrophosphate and prior results showed that elimination of ENPP1 rescued bone hypomineralization of skull, vertebral and long bones to different extents in TNAP null mice. Current TNAP enzyme replacement therapy alleviates skeletal, motor and cognitive defects but does not eliminate craniosynostosis in pediatric hypophosphatasia patients. To further understand mechanisms underlying craniosynostosis development in hypophosphatasia, here we sought to determine if craniofacial abnormalities including craniosynostosis and skull shape defects would be alleviated in TNAP null mice by genetic ablation of ENPP1. Results show that homozygous deletion of ENPP1 significantly diminishes the incidence of craniosynostosis and that skull shape abnormalities are rescued by hemi- or homozygous deletion of ENPP1 in TNAP null mice. Skull and long bone hypomineralization were also alleviated in TNAP/ENPP1 compared to TNAP/ENPP1 mice, though loss of ENPP1 in combination with TNAP had different effects than loss of only TNAP on long bone trabeculae. Investigation of a relatively large cohort of mice revealed that the skeletal phenotypes of TNAP null mice were markedly variable. Because FGF23 circulating levels are known to be increased in ENPP1 null mice and because FGF23 influences bone, we measured serum intact FGF23 levels in the TNAP null mice and found that a subset of TNAP/ENPP1 mice exhibited markedly high serum FGF23. Serum FGF23 levels also correlated to mouse body measurements, the incidence of craniosynostosis, skull shape abnormalities and skull bone density and volume fraction. Together, our results demonstrate that balanced expression of TNAP and ENPP1 enzymes are essential for microstructure and mineralization of both skull and long bones, and for preventing craniosynostosis. The results also show that FGF23 rises in the TNAP model of murine lethal hypophosphatasia. Future studies are required to determine if the rise in FGF23 is a cause, consequence, or marker of disease phenotype severity.
低磷酸酯酶症是一种罕见的遗传性代谢紊乱疾病，由组织非特异性碱性磷酸酶（TNAP）酶活性缺乏引起。TNAP的主要功能是水解组织矿化抑制剂焦磷酸。ENPP1（胞外核苷酸焦磷酸酶/磷酸二酯酶1）是焦磷酸的主要酶促生成者，先前的研究结果表明，在TNAP基因敲除小鼠中，消除ENPP1可在不同程度上挽救颅骨、椎骨和长骨的骨矿化不足。目前的TNAP酶替代疗法可缓解小儿低磷酸酯酶症患者的骨骼、运动和认知缺陷，但不能消除颅缝早闭。为了进一步了解低磷酸酯酶症中颅缝早闭发展的潜在机制。在此，我们试图确定通过基因敲除ENPP1，TNAP基因敲除小鼠的颅面异常（包括颅缝早闭和颅骨形状缺陷）是否会得到缓解。结果表明，ENPP1的纯合缺失显著降低了颅缝早闭的发生率，并且在TNAP基因敲除小鼠中，ENPP1的半合子或纯合子缺失可挽救颅骨形状异常。与TNAP/ENPP1小鼠相比，TNAP/ENPP1小鼠的颅骨和长骨矿化不足也得到了缓解，尽管ENPP1与TNAP同时缺失对长骨小梁的影响与仅TNAP缺失不同。对一大群小鼠的研究表明，TNAP基因敲除小鼠的骨骼表型存在显著差异。由于已知ENPP1基因敲除小鼠的循环FGF23水平会升高，并且FGF23会影响骨骼，我们测量了TNAP基因敲除小鼠血清中完整FGF23的水平，发现一部分TNAP/ENPP1小鼠的血清FGF23水平显著升高。血清FGF23水平还与小鼠身体测量值、颅缝早闭的发生率、颅骨形状异常以及颅骨骨密度和体积分数相关。总之，我们的结果表明，TNAP和ENPP1酶的平衡表达对于颅骨和长骨的微观结构和矿化以及预防颅缝早闭至关重要。结果还表明，在小鼠致死性低磷酸酯酶症的TNAP模型中FGF23水平升高。未来需要进一步研究以确定FGF23水平升高是疾病表型严重程度的原因、结果还是标志物。
