Frequency of replication/transcription errors in (A)/(T) runs of human genes

To estimate the error rate of the gene expression machinery and its possibl e age-related increase, we compared the occurrence of polymerase errors dur ing replication and transcription in (A)/(T) runs, in DNA and RNA of young and old individuals and of early- and late-passage cultured fibroblasts. We analyzed three human genes: TPRD, TGFBR2, and ATRX containing stretches of (A) 8, (A) 10, and (T)13, respectively. The error rate was determined by s equencing 100 cloned PCR or RT-PCR fragments from each DNA and RNA sample. The error rates in replication and transcription increased with the stretch length. The pooled error rates for genomic DNA were: TPRD (A)8, TGFBR2 (A) 10, and ATRX (T)13: 1%+/-0.41, 15.8%+/-1.3, and 31.3%+/-2.9, while those fo r RNA were: 3.8%+/-0.5, 19.3%+/-2.1, and 54.3%+/-1.8, respectively. The del etions of one nucleotide were the most frequent errors. In the replication analysis, a significant difference was found in old versus young individual s for the ATRX (T)13. In the transcription analysis, significantly higher e rror rates were obtained in old versus young individuals for the TPRD (A)8 and TGFBR2 (A) 10. For these genes, the error rate in RNA isolated from fib roblasts was significantly higher than that in blood. The data show a trend of age-related increase in replication/transcription errors; however furth er studies are necessary to confirm this hypothesis, since the sample size is small. This imperfect fidelity of the gene expression process may explai n the evolutionary disadvantage of nucleotide repeats within coding sequenc es, and that these repeats are targets for mutations in human diseases.