The insulin-like growth factor-II (IGF2) and H19 genes are imprinted in mouse and human, with expression of the paternal IGF2 and maternal H19 alleles. IGF2 undergoes loss of imprinting (LOI) in most Wilms' tumours (WT). We now show that: (i) LOI of IGF2 is associated with a 80-fold down regulation of H19 expression; (ii) these changes are associated with alterations in parental-origin-specific, tissue-independent sites of DNA methylation in the H19 promoter; and (iii) loss of heterozygosity is also associated with loss of H19 expression. Thus, imprinting of a large domain of the maternal chromosome results in a reversal to a paternal epigenotype. These data also suggest an epigenetic mechanism for inactivation of H19 as a tumour suppressor gene.

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Loss of imprinting (LOI) is an epigenetic alteration of some cancers involving loss of parental origin-specific expression of imprinted genes. We observed LOI of the insulin-like growth factor-II gene in twelve of twenty-seven informative colorectal cancer patients (44%), as well as in the matched normal colonic mucosa of the patients with LOI in their cancers, and in peripheral blood samples of four patients. Ten of eleven cancers (91%) with microsatellite instability showed LOI, compared with only two of sixteen tumors (12%) without microsatellite instability (P < 0.001). Control patients without cancer showed LOI in colonic mucosa of only two of sixteen cases (12%, P < 0.001) and two of fifteen blood samples (13%, P < 0.001). These data suggest that LOI in tumor and normal tissue identifies most colorectal cancer patients with microsatellite instability in their tumors, and that LO! may identify an important subset of the population with cancer or at risk of developing cancer.

Epigenetic alterations in the genome of tumor cells have attracted considerable attention since the discovery of widespread alterations in DNA methylation of colorectal cancers over 10 years ago. However, the mechanism of these changes has remained obscure. el-Deiry and coworkers [el-Deiry, W. S., Nelkin, B. D., Celano, P., Yen, R. C., Falco, J. P., Hamilton, S. R. & Baylin, S. B. (1991) Proc. Nati. Acad. Sci. USA 88, 3470-3474], using a quantitative reverse transcription-PCR assay, reported 15-fold increased expression of DNA methyltransferase (MTase) in colon cancer, compared with matched normal colon mucosa, and a 200-fold increase in MTase mRNA levels compared with mucosa of unaffected patients. These authors suggested that increases in MTase mRNA levels play a direct pathogenetic role in colon carcinogenesis. To test this hypothesis, we developed a sensitive quantitative RNase protection assay of MTase, linear over three orders of magnitude. Using this assay on 12 colorectal carcinomas and matched normal mucosal specimens, we observed a 1.8-to 2.5-fold increase in MTase mRNA levels in colon carcinoma compared with levels in normal mucosa from the same patients. There was no significant difference between the normal mucosa of affected and unaffected patients. Furthermore, when the assay was normalized to histone H4 expression, a measure of S-phase-specific expression, the moderate increase in tumor MTase mRNA levels was no longer observed. These data are in contrast to the previously reported results, and they indicate that changes in MTase mRNA levels in colon cancer are nonspecific and compatible with other markers of cell proliferation.

The number of maternal deaths is substantially underestimated when death certificates alone are used to identify deaths, and it is unlikely that the Healthy People 2010 objective of reducing the maternal mortality rate to no more than 3.3 deaths per 100000 live births by 2010 can be achieved. Increasing numbers of births to older women and multiple-gestation pregnancies are likely to complicate efforts to reduce maternal mortality.