Heterozygous mutations in cyclic AMP phosphodiesterase-4D (PDE4D) and protein kinase A (PKA) provide new insights into the molecular pathology of acrodysostosis.

Abstract

Acrodysostosis without hormone resistance is a rare skeletal disorder

characterized by brachydactyly, nasal hypoplasia, mental retardation and

occasionally developmental delay. Recently, loss-of-function mutations in the

gene encoding cAMP-hydrolyzing phosphodiesterase-4D (PDE4D) have been reported to

cause this rare condition but the pathomechanism has not been fully elucidated.

To understand the pathogenetic mechanism of PDE4D mutations, we conducted 3D

modeling studies to predict changes in the binding efficacy of cAMP to the

catalytic pocket in PDE4D mutants. Our results indicated diminished enzyme

activity in the two mutants we analyzed (Gly673Asp and Ile678Thr; based on PDE4D4

residue numbering). Ectopic expression of PDE4D mutants in HEK293 cells

demonstrated this reduction in activity, which was identified by increased cAMP

levels. However, the cells from an acrodysostosis patient showed low cAMP

accumulation, which resulted in a decrease in the phosphorylated cAMP Response

Element-Binding Protein (pCREB)/CREB ratio. The reason for this discrepancy was

due to a compensatory increase in expression levels of PDE4A and PDE4B isoforms,

which accounted for the paradoxical decrease in cAMP levels in the patient cells

expressing mutant isoforms with a lowered PDE4D activity. Skeletal radiographs of

10-week-old knockout (KO) rats showed that the distal part of the forelimb was

shorter than in wild-type (WT) rats and that all the metacarpals and phalanges

were also shorter in KO, as the name acrodysostosis implies. Like the G-protein

alpha-stimulatory subunit and PRKAR1A, PDE4D critically regulates the cAMP signal

transduction pathway and influences bone formation in a way that

activity-compromising PDE4D mutations can result in skeletal dysplasia. We

propose that specific inhibitory PDE4D mutations can lead to the molecular

pathology of acrodysostosis without hormone resistance but that the pathological

phenotype may well be dependent on an over-compensatory induction of other PDE4

isoforms that can be expected to be targeted to different signaling complexes and

exert distinct effects on compartmentalized cAMP signaling.