C-Type Natriuretic Peptide and Achondroplasia

Achondroplasia is the most common form of dwarfism and is characterized by short limbs with the thighs and upper arms being the most affected. Achondroplasia is also associated with a narrowing of the foramen magnum and spinal stenosis. Hypochondroplasia is a related, but less severe form of dwarfism that does not have the neurologic problems. Achondroplasia and hypochondroplasia are caused by mutations in the fibroblast growth factor receptor-3 (FGFR-3) gene that causes constitutive activation of the receptor. FGFR-3 signals primarily through the MAP kinase pathway, which is overactivated in growth plate chondrocytes in achondroplasia. C-type natriuretic peptide (CNP) is a hormone that is produced and acts in the growth plate as a potent positive regulator of linear growth. CNP signals through natriuretic peptide receptor-B (NPR-B), generating cGMP. Studies in mice show that activation of the MAP kinase pathway inhibits signaling through NPR-B. Hence the achondroplasia phenotype may be due in part to inhibition of CNP signaling. Conversely, CNP intracellular signaling inhibits the MAP kinase pathway and CNP analogs are being studied as a potential specific therapy for achondroplasia. The objective of this project is to define the state of the CNP system in children and adults with achondroplasia or hypochondroplasia. Our hypotheses are 1) blood levels of CNP and its aminoterminal propeptide (NTproCNP) are elevated and blood levels of cGMP are reduced in children with achondroplasia or hypochondroplasia, due to inhibition of NPR-B; 2) CNP and NTproCNP levels are normal in adults with achondroplasia and hypochondroplasia, due to their lack of growth plate cartilage; and 3) as in healthy children, NTproCNP levels predict height velocity in children with achondroplasia or hypochondroplasia. These hypotheses will be addressed with two specific aims. Specific aim 1 is to determine plasma levels of CNP, NTproCNP, and cGMP in children and adults with achondroplasia or hypochondroplasia. Specific aim 2 is to determine if NTproCNP levels correlate with height velocity in children with achondroplasia or hypochondroplasia.

The study is an observational, cross-sectional/partially longitudinal study of children and adults with achondroplasia or hypochondroplasia. Children will be seen as part of routine clinic visits. Children seen more than once during the study period will provide longitudinal data. Adult subjects with achondroplasia or hypochondroplasia will be studied a single time. Data collected will include anthropometrics, information on neurologic complications of achondroplasia, and blood levels of CNP, NTproCNP, and cGMP. We anticipate 100 subjects will be recruited, with about 20 being studied as many as three times during the course of the study.

By studying the potential role of the CNP system in achondroplasia and hypochondroplasia, not only will we provide further insight into the pathophysiology of these common syndromes, we will also provide greater insight into the regulation of normal linear growth.