Lethal and mild hypophosphatasia in half-sibs.

This report concerns a family showing both the lethal and mild form of hypophosphatasia in half-sibs. In addition, several other paternal family members with the mild form are documented. The lethal form is characterized by extremely low to absent alkaline phosphatase activity in serum with hypomineralization of the skeleton, whereas mildly affected individuals have enzyme levels intermediate between normal and lethal states. On the basis of this pedigree and because the mildly affected individuals have both biochemical abnormalities and the clinical phenotype of premature tooth loss, we prefer to designate hypophosphatasia as a dominant trait affecting both osteogenesis and cementogenesis which has mild clinical expression in the heterozygote but lethality in the homozygote. This situation resembles the dominantly inherited enzymopathy acute intermittent porphyria.