Creatine kinase brain isoenzyme (BB‐CK) presence in serum distinguishes osteopetroses among the sclerosing bone disorders

Abstract Creatine kinase (CK) isoenzyme BB‐CK is predominantly found in brain and is not normally detected in the blood. A few recent reports, however, have described BB‐CK in serum from several patients with osteopetrosis (OP). To evaluate the presence and specificity of BB‐CK in serum in the osteopetroses among disorders that increase skeletal mass, we quantitated total CK activity and CK isoenzymes in 15 patients representing the five major clinical forms of OP (2 infantile, 3 intermediate, 7 adult [2 type I, 5 type II], and 3 carbonic anhydrase II [CA II] deficiency cases) and in 22 patients representing 14 other types of sclerosing bone disease. All OP patients (except the two adult type I subjects) had BB‐CK readily detected in their serum. Conversely, only 1 of the 22 patients with other sclerosing bone disorders had detectable BB‐CK in serum (1 of 3 patients with fibrodysplasia [myositis] ossificans progressiva who had barely measurable activity). In three OP patients (one of two with the infantile form and two of five with adult, type II disease), BB‐CK values were sufficiently high that serum total CK activity was elevated. In a newborn with malignant OP, both cord blood plasma and peripheral blood serum had substantial amounts of BB‐CK. In three subjects (with adult type II OP), who were restudied 2‐6 years later, BB‐CK was still elevated in their blood. BB‐CK in serum appears to distinguish the osteopetroses among the sclerosing bone disorders. Absence of serum BB‐CK in adult type I disease suggests that this condition may not be a genuine form of OP. Assay of BB‐CK in fetal blood could be studied as a means for prenatal diagnosis of malignant OP. Why the osteoclast failure that characterizes all true forms of OP is associated with BB‐CK in the circulation is a new question for skeletal biologists.