Spondyloenchondrodysplasia: An enigmatic immuno-osseus type I interferonopathy

Spondyloenchondrodysplasia (SPENCD) is a rare immuno-osseus disease due to biallelic mutations in ACP5, resulting in a loss of tartrate-resistant acid phosphatase (TRAP) activity and enhanced type I interferon signalling. While TRAP was identified in the 1950s, ACP5 was cloned in the 1990s, an Acp5 knockout mouse was reported in 1996, and >3,000 articles are retrievable on PubMed using the terms "tartrate-resistant acid phosphatase" + "TRAP", the immunopathology of SPENCD remains unclear. Here we describe the clinical phenotype and molecular architecture of SPENCD, review the biology of TRAP, and consider how TRAP deficiency leads to disturbed innate immunity.