To study the effect of nucleotide excision repair on the spectrum of mutations induced in diploid human fibroblasts by UV light (wavelength, 254 nm), we synchronized repair-proficient cells and irradiated them when the HPRT gene was about to be replicated (early S phase) so that there would be no time for repair in that gene before replication, or in G, phase 6 h prior to S, and determined the kinds and location of mutations in that gene. As a control, we also compared the spectra of mutations induced in synchronized populations of xeroderma pigmentosum cells (XP12BE cells, which [HPRT]) is significantly higher than in cells irradiated in early G1 phase (9). This difference in frequency cannot be attributed to differences in the physical state of the DNA, since no such difference in mutation frequency is found when diploid xeroderma pigmentosum cells (XP12BE, complementation group A), which are virtually devoid of nucleotide excision repair (18), are irradiated in early S phase or in early G1 phase (9). These data suggest that S-phase replication is centrally involved in the conversion of potentially mutagenic DNA damage into mutations (fixation) and that excision repair prior to the onset of S phase decreases the frequency of mutants by eliminating such lesions.Of the two major classes of potentially mutagenic UV photoproducts, i.e., cyclobutane pyrimidine dimers (Py-Py dimers) and 6-4 pyrimidine-pyrimidone (6-4 Py-Py) lesions, the latter have been shown to be removed from human genomic DNA more rapidly than the former (15). It has also been shown that Py-Py dimers are excised more rapidly from an actively transcribed gene of human cells, the dihydrofolate reductase gene (DHFR), than from bulk DNA (1, 12). Moreover, Mellon et al. (14) demonstrated that there is a strand bias in the rate of removal of Py-Py dimers from the * Corresponding author. DHFR gene in human cells. The majority of such lesions are removed within 24 h, but dimers in the transcribed strand are excised much faster than those in the nontranscribed strand. Information on strand-specific excision repair of UV-induced damage from the HPRT gene of human cells is not yet available, but if such repair occurs, HPRT mutations induced by UV in excision repair-proficient human cells irradiated at the onset of S phase should arise from 6-4 Py-Py lesions and Py-Py dimers located in either strand of DNA. In contrast, the lesions responsible for the mutations induced in such cells allowed 6 h for repair before the onset of S phase should arise mainly from Py-Py dimers, and these should be located primarily in the nontranscribed strand. This switch in strands should not occur in XP12BE cells.There are no published data on the kinds and locations (i.e., the spectrum) of UV-induced mutations in an endogenous gene of human cells, nor are there data showing whether excision repair in such cells alters the spectrum. This study was designed to examine the spectrum of mutations induced by UV in the HPRT gene of diploid human cells and to determine, at the DNA ...

Cell adhesion, mediated by N-cadherin, is critical for embryogenesis since N-cadherin-null embryos die during mid-gestation with multiple developmental defects. To investigate the role of N-cadherin in heart muscle development, N-cadherin was specifically deleted from myocardial cells in mice. The structural integrity of the myocardial cell wall was compromised in the N-cadherin mutant embryos, leading to a malformed heart and a delay in embryonic development. In contrast, cardiac-specific deletion of αE-catenin, found in adherens junctions, or β-catenin, did not cause any morphological defects in the embryonic heart, presumably due to compensation by αT-catenin that is normally found in intercalated disks and γ-catenin (plakoglobin), respectively. Embryos lacking β-catenin in the heart also exhibited a cardiac defect, but only later in development resulting in partial lethality. These genetic studies underscore the importance of the N-cadherin/catenin complex in cardiogenesis.

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