Biological Understanding of Spondyloenchondrodysplasia (SPENCD) Caused by Damaging ACP5 Variants

Introduction Inborn errors of immunity are genetic disorders that result in the absence or dysfunction of critical components of the immune system. Spondyloenchondrodysplasia (SPENCD) is one of these diseases, causing immune dysfunction, abnormal bone development, and neurological changes. SPENCD is caused by damaging variants in ACP5, which encodes tartaric acid-resistant acid phosphatase (TRAP). TRAP has dual roles, acting as a phosphatase and generating reactive oxygen species (ROS). While previous studies have shown that some SPENCD variants impair phosphatase activity, the impact of these variants on ROS production has not, to our knowledge, been investigated. Methods Clinical assessments and panel sequencing were performed. ACP5 variant-expressing constructs were generated using site-directed mutagenesis and expressed in HEK293 cells. Immunoblotting and p-nitrophenyl phosphate (PNPP) assay were performed on cells transfected with wild-type, patient, or known SPENCD variants. Flow cytometry was performed on PBMCs and THP-1 cells treated with the TRAP inhibitor AubipyOMe and stained for ROS with H2DCF. Results We report a novel case, elucidating the functional impact of two ACP5 variants: ACP5 (NM_001611.5) c. 550C>T, p.Q184* and c. 740T>G, p.L247R. This patient presented with hallmark SPENCD characteristics, including autoimmune hemolytic anemia, autism, intracranial calcification, radiolucent lesions, and short stature. In silico variant assessment confirmed a high likelihood of each variant being damaging with deleterious scores in multiple models. TRAP protein expression was comparable to wild type for patient and known SPENCD variants, except for truncating variants, which resulted in higher levels of truncated protein. Phosphatase activity was abolished for both the patient and known SPENCD variants. ROS production was impaired in cells treated with the TRAP inhibitor. Conclusion This work describes a newly identified SPENCD case, demonstrating the damaging effects of two ACP5 variants (p.Q184* and p.L247R) on TRAP production and function, reclassifying p.L247R as pathogenic. We also show impairment of ROS production with TRAP inhibition and are assessing how the patient’s ACP5 variants influence ROS production. These findings elucidate the dual roles of TRAP in dephosphorylation and ROS production, showing TRAP’s physiological role and SPENCD’s biological mechanism.