Elevated rate of DNA synthesis and its correlation to cAMP-phosphodiesterase activity during induction of polydactyly in mouse embryos heterozygous forHemimelia-extra toe (Hmx)

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Abstract
The induction of polydactyly in mouse embryos heterozygous for Hemimelia‐extra toe (Hmx) is associated with aberrant outgrowth of the developing autopod on day 12 of gestation. We have quantitated the rate of DNA synthesis and the activity of cAMP‐phosphodiesterase (PDE) that is characteristic of the prospective polydactylous region. Mid‐stage 18 hind‐limb buds were labeled with [3H]dThd either in situ using whole embryo culture, or as isolated preaxial autopod fragments cultured on a membrane substratum. The mean specific activities of incorporation were compared for normal (+/+) and mutant (Hmx/+) genotypes. A significant (P ≤ 0.01) 19% increase, peculiar to the prospective polydactylous region, was measured after 4 hours in embryo culture. The same increment was detected after 4 hours in organ culture, but was amplified linearly to 55% when incubation was extended to 20 hours. During this period, continuous exposure to 1.0 mM IBMX (3‐isobutyl‐1‐methyl xanthine), an inhibitor of cAMP‐PDE activity, “slowed down” the rate of DNA synthesis to untreated +/+ proportions. When cAMP‐PDE activity was assayed in uncultured autopods, a significant (P ≤ 0.01) 18% increase was detected within the prospective polydactylous region specifically on stage 18 of gestation. This is the developmental phase during which polydactylous outgrowth is induced in situ. Thus, uncontrolled cAMP‐PDE activity may, in part, provoke the enhanced rate of cell proliferation.