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Short Communication |




From the Department of Pathology*
and Division of
Human Cancer Genetics,
Ohio State University,
Columbus, Ohio; and the Department of
Surgery,
University College Hospital,
Ibadan, Nigeria
| Abstract |
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| Introduction |
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Tumor MSI is often determined by comparing the sizes of repeated sequences at multiple microsatellite loci between tumor and normal DNA from the same person. Using this system, the definition of MSI can sometimes be vague, due largely to varying opinions on the number of microsatellite loci to examine and the percentage of loci exhibiting instability needed for the classification of MSI. In 1998, The International Workshop on Microsatellite Instability and RER Phenotypes in Cancer Detection and Familial Predisposition published their endorsement of a five-microsatellite panel for the determination of MSI in tumors.6 According to their recommendations, tumors may be characterized as having high-frequency MSI (MSI-H) if two or more of the five markers exhibit variations in microsatellite sequences or low-frequency MSI (MSI-L) if only one marker shows instability.6 Furthermore, microsatellite-stable (MSS) tissue is defined by microsatellite sequences of normal length at all five loci. The recommended five-marker panel consists of three dinucleotide microsatellites and two mononucleotide microsatellites, BAT-25 and BAT-26.
The BAT-26 locus contains a 26-repeat adenine tract and is located within the fifth intron of the MSH2 gene, whereas the BAT-25 locus contains a 25-repeat thymine tract located within intron 16 of the c-kit oncogene. Both loci have been shown to be sensitive markers of MSI, which manifests as a shortening in the size of the respective mononucleotide repeat in tumor DNA.7-11 However, in MSS tumor or normal tissue, BAT-25 and BAT-26 have been described as quasimonomorphic loci exhibiting little polymorphic variation in the size of the poly(T) or poly(A) tracts, respectively.8,9,11 Because of the quasimonomorphic profile of both loci, BAT-25 and BAT-26 have proven very useful for the identification of MSI, because shortened, unstable alleles can easily be differentiated from alleles of normal size.
While conducting a retrospective genetic analysis of the involvement of MSI in endometrial adenocarcinomas, one of the most common extracolonic tumors associated with HNPCC,12 we observed a sample that exhibited the same shift in repeat size at the BAT-26 allele in both tumor and matched normal tissue DNA. Because DNA from both tissues exhibited an allele of identical size, our findings seemed consistent with a polymorphic variant at the BAT-26 locus. Investigation into the ethnic background of this sample revealed its African-American origin, which led us to screen a population of 103 healthy African-Americans to determine the frequency of the BAT-26 polymorphism within this population. To investigate if mononucleotide polymorphisms were restricted to poly(A) tracts in the African-American population, the poly(T) tract at BAT-25 was also analyzed for size variation. Finally, a small population of Nigerians was analyzed at both microsatellite loci to confirm the ethnic origin of the polymorphisms.
| Materials and Methods |
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All procedures for the handling of subjects were performed in compliance with the ethical standards of Ohio State University. The African-American population DNA was obtained from healthy blood donors and the DNA was isolated from peripheral blood using standard extraction techniques.13 The Nigerian population lymphocyte DNA was obtained from eight patients with sporadic colorectal cancer.
Microsatellite Analysis
Microsatellite sequences at BAT-25 and BAT-26 were amplified using
previously reported primers.11
DNA derived from leukocytes
or tumor cells was amplified by polymerase chain reaction (PCR) in a
26-ul reaction mixture containing 200 µmol/L dNTPs, 1 U
Taq polymerase (Perkin Elmer Biosystems, Foster City, CA),
40 ng of each primer, 2.0 mmol/L MgCl2, 2.5 µl
10x PCR buffer (670 mmol/L Tris, 100 mmol/L ß-Mercaptoethanol, 166
mmol/L
(NH4)2SO4,
67 µmol/L EDTA, 0.5 µg/ml bovine serum albumin, final pH adjusted
to 8.8), and 0.15 µl [
-32P] ATP (10
µci/µl; Amersham, Life Sciences, Arlington Heights, IL). The
PCR reaction itself involved an initial denaturation period of 3
minutes at 95°C, 30 cycles of 1 minute at 95°C, 2 minutes at
55°C, and 3 minutes at 72°C, and a final extension step of 8
minutes at 72°C. A volume of loading dye (95% formamide, 10 mmol/L
EDTA, 0.1% bromophenol blue, and 0.1% xylene cyanol) equaling
one-half the total reaction volume was then added to each reaction.
Samples were denatured for 3 minutes at 95°C and kept on ice until
loaded onto a 5% polyacrylamide +7M urea gel. Gels were run for
2000 volts for 2 hours at 55°C, transferred to blotting paper (VWR
Scientific, Chicago, IL), dried, and exposed to Hyperfilm MP
(USB-Amersham Life Science, Cleveland, OH) for 1 hour at room
temperature. The mononucleotide repeat sizes of unusual alleles were
measured in comparison to that of the most frequently occurring
allele, which was scored as 0.
| Results |
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| Discussion |
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The literature does contain some suggestive evidence of greater allelic variation at the BAT-26 locus. The analysis of 78 gliomas resulted in two samples, each of which exhibited one shortened BAT-26 allele of either 7 or 12 bases in both tumor and matched normal DNA.17 Similarly, the examination of MSI in 31 thyroid cancers revealed one sample that exhibited a shortening of one BAT-26 allele by 7 bp but no size variation at two other mononucleotide repeat microsatellites.17 Although the authors referred to these three samples as false positives, the failure to detect instability at other microsatellite loci and their presence in both tumor and normal DNA suggest the presence of a polymorphism at BAT-26. Additionally, Weber and Rodriguez-Bigas have reported to the MSH2 database (www.nfdht.nl/) a 10-bp deletion at BAT-26. Although no attempts were made to determine the frequencies or to identify the ethnic background of any of the above-mentioned alleles, these results do support the polymorphic nature of this locus. Finally, Perucho has recently described unpublished observations suggesting that 2 to 4% of the population, especially African-Americans, could carry shortened BAT-26 alleles.18
This study has clearly demonstrated the presence of BAT-26 alleles in the size range characteristic of MSI from healthy African-American DNA. Of the African-Americans screened, 12.6% demonstrated alleles within this range and 11 of 13 individuals shared a common allele of 16 adenosine repeats. Additionally, 18.4% of African-Americans were polymorphic at BAT-25 and 2.9% were found to be polymorphic at both loci. Screening a small Nigerian population further supported the high frequency and African origin of these polymorphisms. Unfortunately, we were not able to demonstrate the stable inheritance of polymorphic alleles at either the BAT-25 or BAT-26 loci, because the one extended family we examined was not informative at either loci. However, the identification of alleles of common size at both BAT-25 and BAT-26 in the African-American population screened suggests that the transmission of alleles at both loci is stable and does not result in the progressive loss of mononucleotide repeats at either site over generations.
The presence of a common BAT-26 allele of 16 repeats in both populations examined could be explained by either a founder chromosome, suggesting a common origin, or by recombinogenic sequences such as Alu repeats resulting in independent recombination events. Because BAT-26 polymorphisms appear to be population-dependent and the most common variant 16-repeat allele has been documented in both indigenous African and African-American populations, there appears to be indirect evidence of a common origin of the16-repeat allele resulting from a single event in Africa. To support the suggested origin of this common BAT-26 allele, extensive haplotype analysis would need to be conducted to establish the existence of a founder chromosome. In any case, this study has shown a higher degree of polymorphism exists at both BAT-25 and BAT-26 in ethnic African populations compared to either the European or Asian populations that have been examined thus far. Because evolutionarily older populations have been shown to demonstrate more genetic diversity, these results further support the older age of the African population compared to either the European or Japanese-Hawaiian populations.
The identification of multiple alleles at both BAT-25 and BAT-26 clearly disputes the quasimonomorphic allelic profile of both loci in all populations. This study has demonstrated the presence of normal alleles at both BAT-25 and BAT-26 that are within the range of size associated with MSI and thus could be misinterpreted. If MSI analyses using BAT-25 and BAT-26 were conducted on tumor DNA without matched normal tissue, then approximately 28% of African-Americans would be polymorphic at one of the loci and therefore could be incorrectly classified as MSI-positive. Consequently, we recommend that matched normal tissue DNA be required for the comparison of test results for all samples showing microsatellite sequences suggestive of MSI. Most importantly, when new microsatellite sequences are elucidated, thorough population studies must be conducted to identify the allelic profiles and frequencies at these loci before they can be used appropriately for diagnostic testing.
| Footnotes |
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Supported by National Institutes of Health grants CA67941 (to A. d. l. C.) and CA 16058 (to Ohio State University Comprehensive Cancer Center).
Accepted for publication April 28, 1999.
| References |
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