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Short Communication |




From the Department of Medical Genetics,*Programme of Neurosciences, and the Department of Neurology,
Biomedicum Helsinki, University of Helsinki, Helsinki, Finland; the Molecular and Population Genetics Laboratory,
Cancer Research UK, London, United Kingdom; the Departments of Pathology
and Surgery,¶Kuopio University Central Hospital, Kuopio, Finland; the Department of Surgery,||Jyväskylä Central Hospital, Jyväskylä, Finland; the Second Department of Surgery,**Helsinki University Central Hospital, Helsinki, Finland
| Abstract |
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Recently susceptibility to multiple adenomatous polyps and carcinoma of the colon has been found as an autosomal-recessive trait. The base excision repair gene human homolog of mutY (MYH) has been proposed as a new CRC predisposing gene.17 In base excision repair an adenine-specific DNA glycosylase MYH removes adenines mispaired with guanines or damaged DNA base 8-oxo-dG.18,19 Human homolog of mutM (OGG1) and human homolog of mutT (MTH1) remove the oxidized base from 8-oxo-G:C bp and prevent the incorporation of 8-oxo-dGMP into DNA.20,21
Al-Tassan and colleagues17 linked a somatic APC mutation pattern biased toward G to A transversions in adenomas derived from three siblings with multiple lesions to a base excision repair defect caused by compound germline heterozygosity for Y165C and G382D MYH variants. This elegant work, including functional analysis of the variants in a bacterial homologue, showed that biallelic inactivation of MYH predisposes to colorectal neoplasia. Al-Tassan and colleagues17 found both Y165C and G382D once in 100 British controls demonstrating that heterozygosity on population level is not rare. Jones and colleagues22 and Sieber and colleagues23 studied patients with multiple colorectal adenomas and reported several additional patients with MYH germline mutations. Importantly, some patients had displayed more than 100 colorectal adenomas fulfilling the classical clinical criteria of FAP. Sieber and colleagues23 showed that 9% of 157 multiple adenoma patients had germline MYH mutations. Five percent had double mutations. Twenty-nine percent of 28 patients with 15 to 100 adenomas were double-mutation carriers and the CRC frequency differed significantly from the general population.
Earlier studies assessing contribution of MYH to colorectal neoplasia susceptibility have been performed in series of patients with multiple colorectal adenomas. MYH mutation frequencies have not, to our knowledge, yet been determined in unselected and population-based series of CRC patients. We examined the contribution of MYH in such a series of 1042 patients, collected from Finland May 1994 to July 1998.24,25 We screened germline missense variants Y165C and G382D of the MYH gene by solid-phase minisequencing,26,27 to detect cases associated with biallelic loss of MYH function. DNA samples with only one mutant allele were subjected to sequencing of the whole gene, to detect possible Finnish founder mutations. DNAs from 424 anonymous blood donors were used as controls. The occurrence of MYH-associated CRC was compared with the occurrence of FAP-associated CRC in the studied population in the study period, using data from the Finnish Polyposis Registry.
| Materials and Methods |
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Normal and tumor DNA samples were collected at nine Finnish regional central hospitals between May 1994 and July 1998. Samples were derived from 1042 patients diagnosed with CRC. This is approximately two-thirds of the total number of patients treated in the units within the time frame. One third was missed but no bias toward any particular phenotype such as young age could be observed, and this series of 1042 cases is expected to represent well the examined population.24,25 BAT26 mononucleotide marker was used to determine the MSI status of the tumors. MSI cases were sequenced for MLH1 and MSH2 germline mutations, and 29 of 130 MSI patients had an MLH1 or MSH2 defect (unpublished data).24,25
Eighteen DNA samples from the series of 1042, representing a sampling of patients with three or more polyps, were earlier screened for MYH mutations by Sieber and colleagues.23 One MYH double-mutation carrier, C585, was detected in this selected group of samples.23 For the purpose of this study, all samples with sufficient DNA amounts, 1003 samples, entered the analysis. In addition to the CRC patient DNAs, DNAs from 424 anonymous cancer-free blood donors were used to examine occurrence of Y165C and G382D in the Finnish population.
DNA Pooling Strategy, Polymerase Chain Reaction (PCR), and Solid-Phase Minisequencing
DNA samples (n = 16 to 22) were mixed in one pool. Fifty-three master pools representing 1003 DNA samples were amplified by PCR for screening known mutations in exons 7 and 13. PCR reactions were performed in 50-µl reaction volume containing 125 ng of pooled genomic DNA, 10x PCR buffer (Applied Biosystems, Branchburg, NJ), 500 µmol/L of each dNTP (Finnzymes, Espoo, Finland), 10 pmol of biotin-labeled forward primer, and 50 pmol of nonbiotinylated reverse primer (Table 1)
, MgCl2 in 5 mmol/L (for exon 7 PCR) or in 3.5 mmol/L (for exon 13) concentrations, 5 U of AmpliTaqGOLD polymerase (AB). The following PCR cycles were used for amplification: 95°C for 10 minutes, 35 cycles of 95°C for 45 seconds denaturation, annealing temperature of 60°/58°C (exons 7 and 13) for 45 seconds, and 72°C for 30 seconds extension. Final extension was 72°C for 10 minutes.
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Patients with only one mutated allele in the germline were sequenced for all 16 exons from both normal and tumor DNA samples, to search for additional MYH defects. Primer sequences were obtained from Al-Tassan and colleagues17 Big Dye 3 Terminator chemistry (AB) was used for direct sequencing. Cycle-sequencing products were run on ABI 3100 capillary sequencer (AB) according to the manufacturers instructions.
Somatic APC Mutation Analysis in MYH-Associated CRCs
Tumors from MYH mutation carriers were examined for mutations in the APC gene by fluorescent single-stranded nucleotide polymorphism analysis and by denaturing high performance liquid chromatography analysis. Exons 15A to 15L of APC were screened, which encompasses the mutation cluster region, codons 1286 to 1513. The majority of somatic APC mutations in MYH carriers have been found within this region.22 For fluorescent single-stranded nucleotide polymorphism analysis, tumor DNA was isolated from fresh-frozen samples using standard methods. Primer pairs to amplify overlapping fragments of APC from exon 15A to 15L inclusive were used for the coding regions including exon-intron boundaries (sequences from the authors available on request).
Each 25-µl PCR reaction contained 1x PCR reaction buffer without MgCl2 (Promega, Madison, WI), 1.5mmol/L MgCl2, 200 µmol/L dNTPs, 200 nmol/L of each primers, 50 ng of genomic DNA, and 1 U of TaqDNA polymerase (Qiagen), and the PCR conditions consisted of 95°C for 5 minutes, followed by 35 cycles of 95°C for 1 minute, 55°C or 60°C for 1 minute, 72°C for 1 minute, and a final extension step at 72°C for 10 minutes. The resulting PCR products were screened for variants by being run at 18°C and 24°C on the ABI 3100 and analyzed using Genotyper 2.5 software (Perkin-Elmer Applied Biosystems). Fragments showing aberrant migration were reamplified, purified using Qiaquick columns (Qiagen), and then sequenced in both forward and reverse orientations using the ABI Big Dye Terminator kit (PE AB) in parallel with control samples.
Denaturing high performance liquid chromatography was done using the 3500HT WAVE nucleic acid fragment analysis system (Transgenomic, Crewe, UK). The same primer set was used as for fluorescent single stranded-nucleotide polymorphism. To enhance the formation of heteroduplexes before analysis, the PCR products were denatured at 94°C and reannealed by cooling to 50°C at a rate of 1°C per minute. Denaturing high performance liquid chromatography was performed at the melting temperature predicted by Wavemaker (version 4.0) software (Transgenomic) with a 12% acetonitrile gradient throughout 2.5 minutes. Samples displaying aberrant denaturing high performance liquid chromatography elution profiles were sequenced directly.
Patient Records
Patient records of MYH mutation carriers were studied for polyp number, sex, age at the time of carcinoma diagnosis, the degree of tumor differentiation, histology of tumor, Dukes stage, and tumor site (Table 2)
. Family history of cancer in first-degree relatives had been examined through population and Cancer Registry24,25
for all of the 1042 cases.
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Chi-square test statistic was used for examining MYH mutation distribution between colorectal carcinoma patients and cancer-free controls. P < 0.05 was considered statistically significant.
| Results and Discussion |
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Little is known about cancer in families segregating MYH defects. Table 2
lists all first-degree relatives of the nine CRC probands with biallelic or heterozygous MYH mutation, as well as possible other cancers diagnosed in the CRC proband. Although the data are compatible with the notion that no obvious excess of a particular type of malignancy is associated with heterozygous MYH defects, this aspect should be studied in a larger material because eight of nine probands had one to five first-degree relatives with cancer. Some of the cancers have occurred at relatively early age. Unfortunately, determination of the MYH genotype was possible only in the CRC probands.
In the series of CRC patient samples analyzed the allele frequency of Y165C was 0.002 (4 of 2006) and allele frequency of G382D was 0.004 (9 of 2006). Allele frequency of any of these two MYH mutations was 0.006 (13 of 2006). In a British control sample series of 100 DNAs both MYH mutant alleles Y165C and G382D were found once.17 In the Finnish population examining a set of 424 blood donor samples did not reveal any mutations, and the difference between CRC patients and controls reached significance (P < 0.025). Furthermore, none of the five heterozygous Y165C and G382D carriers displayed another MYH defect elsewhere in the gene. Thus it appears that, similar to APC defects,28 MYH mutations are not particularly enriched in the Finnish population. The low frequency of MYH mutations in blood donors supports the view that at present large-scale screening for biallelic MYH mutations is not justified.29
The CRCs from the four patients with biallelic MYH germline mutations and five heterozygous carriers were analyzed for mutations in the APC mutation cluster region. One mutation, E1378X (G>T), was found in the MYH heterozygote C544 (Table 2)
. The analysis was initially performed by single-stranded nucleotide polymorphism, and after the primarily negative results confirmed by denaturing high performance liquid chromatography. Because of the small number of cases analyzed little can be concluded from this effort.
Table 3
depicts the spectrum of predisposing gene defects observed in our population-based series of 1042 CRC cases in this and previous studies (unpublished).24,25,28,30
At present altogether 35 patients (3.4%) have a molecular diagnosis of a hereditary CRC syndrome and this number is likely to increase in the future. According to the Finnish Polyposis Registry between May 1994 and July 1998 four FAP-associated CRCs were diagnosed in the target population. Table 3
depicts only one patient with molecular diagnosis of APC mutation.28
One case was missed, in one case the tumor sample contained adenoma tissue only and the case was excluded from the series, and in one case a predisposing APC mutation has not been searched for. The comparison between occurrence of FAP- and MYH-associated disease is of interest. Four CRCs patients with biallelic MYH mutations in this population-based series represent an absolute minimum. That at the same time the national Polyposis registry recorded four cases of FAP-associated CRCs suggests that the contributions of APC and MYH germline variants are relatively equal, in a population with active clinical screening of FAP families. Thus the role of MYH as a cancer predisposition gene is not limited to extremely rare cases.
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| Acknowledgements |
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| Footnotes |
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Supported by the European Commission (QLG2-CT-2001-01861), the Helsinki University Central Hospital, Biocentrum Helsinki, the Sigrid Juselius Foundation, the Paulo Foundation, the Finnish Cancer Society, and the Academy of Finland (44870, Finnish Center of Excellence Program 20002005).
Accepted for publication June 11, 2003.
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