Effects of comorbid chronic kidney disease on late-onset hypophosphatasia mice under treatment with asfotase alfa

ABSTRACT Hypophosphatasia (HPP) is a rare bone disorder caused by loss-of-function mutations in the ALPL gene, leading to deficient tissue-nonspecific alkaline phosphatase (TNAP) activity and impaired skeletal/dental mineralization. Asfotase alfa (AA), the only FDA-approved therapy, improves survival and skeletal/dental mineralization. Because of its mineral-binding properties, AA also binds to sites of ectopic calcification, including vasculature. Overexpression of TNAP in either the vascular media or intima has been shown to induce severe calcification. Here we examined what might occur if a disease that features medial artery calcification, such as chronic kidney disease-mineral and bone disorder (CKD-MBD), were to develop in a subject with HPP undergoing AA treatment. Two-month-old AlplPrx1/- (HPP) or Alplflox/- (non-HPP control) mice were treated with subcutaneous AA injections (8.2 mg/kg, 3×/week) for four months. After three months of treatment, CKD was induced via a 0.2% adenine and 1.8% phosphorus diet. Mice were euthanized at six months of age, and skeletal, renal, cardiac, and vascular tissues were analyzed. ALP levels slightly increased with AA but significantly increased after inducing CKD in control and HPP female mice. In males, ALP activity was elevated in the AA and AA+CKD cohorts. Micro-CT analysis showed improved bone parameters in HPP under AA treatment. However, the improvements were dampened when CKD was induced, compared to non-CKD Alplflox/-. IHC of the kidney revealed increased TNAP immunolocalization in the AA+CKD cohort, in both females and males. Ectopic renal and vascular calcification was observed in the AA+CKD group. Gene expression profiling in the kidney revealed that AA treatment modulates the immune and mineral metabolism pathways, while CKD superimposition induces a shift toward pro-inflammatory and pro-fibrotic responses, compromising renal and skeletal homeostasis. These findings underscore the importance of carefully evaluating HPP patients under treatment with mineral-targeted TNAP at risk of developing comorbid conditions associated with ectopic calcification.