Sommer Ss

Novel pattern of p53 gene mutations in an American black cohort with high mortality from breast cancer

The pattern of acquired mutations in the p53 gene can be used to study differences in factors contributing to carcinogenesis. We investigated mutations in exons 5-9 and adjacent intronic regions in 47 breast cancers of black women from Michigan, a population with the highest breast-cancer mortality in the US. The 16 mutations detected differed from those of other populations. In particular, the black women had an excess of A:T-->G:C transitions compared with rural white US midwest women. While the causes of the different pattern of acquired mutation remain to be determined, this molecular epidemiological approach detects the consequences of mutagenic processes in specific populations. Mutation patterns will constrain hypotheses to mechanisms consistent with the observed biochemical alterations.

High frequency of p53 gene mutations in primary breast cancers in Japanese women, a low-incidence population

The pattern of acquired mutations in the p53 tumour-suppressor gene is potentially useful for determining factors contributing to carcinogenesis in diverse populations differing in incidence and/or mortality from the disease. We previously reported differences in mutational patterns of the p53 gene in primary breast cancers from Midwest US Caucasian, African-American and Austrian women. Herein, we report 16 mutations in 27 primary breast cancers from Japanese women from Hirosaki, a population with a low incidence of breast cancer. The frequency of 59.3% of p53 mutations is the highest reported in breast cancers from a particular ethnic group thus far. A relatively high number of mutations (7/16) were heterozygous in at least some tumour cell clusters. Intergroup comparisons of the mutational pattern between this population and several other US, European and Japanese populations do not show any statistically significant differences. There were recurrent mutations at two sites, codon 273 (R --> H; three mutations), a common hotspot of mutations in breast and other cancers, and codon 183 (S --> Stop; two mutations), a very rare location for p53 mutations. These mutations were shown to be independent and presumably not in the germ line. The highest frequency of p53 mutations raises the possibility that p53 mutagenesis is a predominant factor for breast cancer development in this low-risk Japanese group, whereas in other cohorts different mechanisms are likely to account for the higher proportion of breast cancer. Further studies are needed to confirm the present observations.

DIRECT CARRIER TESTING IN 14 FAMILIES WITH HAEMOPHILIA B

Direct carrier testing was done in 54 at-risk female relatives of haemophilic patients by initially analysing 2.46 kb of the factor IX gene in 1 haemophiliac per family by genomic amplification with transcript sequencing. A presumptive mutation was found in all 14 haemophiliacs examined. Analyses were then done either by sequencing the appropriate region in at-risk female relatives or by detection of an altered restriction site. A simulation indicated that the mutation will be associated with an altered restriction site in about half the families. The technique has clinical application.

A polymorphism but no mutations in the GADD45 gene in breast cancers

The p53 gene product is part of a pathway regulating growth arrest at the G1 checkpoint of the cell cycle. Mutation of other components of this pathway, including the products of the ataxia telangiectasia (AT), GADD45, mdm2, and p21WAF1/CIP1 genes may have effects comparable to mutations in the p53 gene. The GADD45 gene is induced by ionizing radiation and several DNA-damaging xenobiotics. Induction requires the binding of wild-type p53 to an evoulutionarily highly conserved putative intronic p53 binding site in intron 3 of GADD45. We recently analyzed the entire coding region of the p53 gene in primary breast cancers of Midwestern white women and found 21 mutations among 53 tumors (39,6%). We now have shown by direct sequencing that there are no mutations in the intronic p53 binding site of the GADD45 gene in any of the 53 primary breast cancers and no mutations in the entire coding region of the GADD45 gene in a subset of 26 consecutive tumors (12 with p53 mutation and 14 without p53 mutation). The only sequence variation detected was a common polymorphism in intron 3. The absence of mutations in the GADD45 gene, including the putative p53-binding intronic site, suggests that this gene is not a frequent target of mutations in breast cancer. Although mutations of the p53 gene have been studied in a wide spectrum of human cancers, GADD45 has not been examined in any tumor or cell line to the best of our knowledge. Our results raise the possibility that mutation of the GADD45 gene alone is not functionally equivalent to loss of wild-type p53 activity.