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Advances in Brief |
Division of Human Cancer Genetics, Comprehensive Cancer Center, The Ohio State University, Columbus, Ohio 43210
| ABSTRACT |
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| Introduction |
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CCND1, located on 11q13 and encoding cyclin D1, is a good candidate for a modifier locus in cancer for several reasons. First, it encodes a protein that plays an important role in cell cycle control in both normal cells and neoplasia (11) . Cyclin D1 is the major cyclin involved in transition from G1 to S phase, which is regulated by cyclin-dependent kinases (12 , 13) . Second, cyclin D1 mRNA exhibits alternate splicing, and translation of the different transcripts (transcripts a and b) results in protein products with nonidentical COOH-terminal domains (14) and with possible functional differences (15) . Transcript a is normally spliced, whereas transcript b is a variant transcript reading into intron 4 and skipping exon 5. Third, a G to A polymorphism at the splice donor site of exon 4 has been postulated to increase alternate splicing (transcript b; Ref. 14 ). These observations together lay a theoretical background for a possible role of CCND1 as a cancer modifier gene. Indeed, Betticher et al. (14) reported that patients with resected non-small cell lung cancer and AG or AA genotype showed a shorter relapse-free survival as compared with those with the GG genotype. Most interestingly from the present point of view, Kong et al. (16) recently reported an earlier age of onset by 11 years in HNPCC patients who were homozygous or heterozygous for the mutant allele (A) as compared with patients who were homozygous for the normal allele (G).
The present report addresses the significance of cyclin D1 A/G polymorphism and mRNA expression as modifiers of clinical characteristics of HNPCC. The study was based on a large series (n = 146) of affected carriers from clinically well-characterized families segregating mutations in the DNA MMR genes MSH2 or (mostly) MLH1. Mutation-negative individuals (n = 186) from the same families served as controls.
| Materials and Methods |
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Detection of A/G Polymorphism.
The A/G polymorphism in codon 242 of exon 4 of CCND1 was ascertained by single-strand conformational polymorphism analysis using primers 1 and 2 reported previously (16)
. The genotype of the three polymorphic types (AA, AG, and GG) was further confirmed by sequencing.
Allele-specific Expression Analysis.
This analysis was carried out on the available 31 RNA samples from lymphoblastoid cells and/or normal mucosa from patients with colon cancer and from 12 RNAs of colon tumor tissues. Total RNA (2.5 µg) was reverse transcribed using Moloney murine leukemia virus reverse transcriptase (Promega), followed by two rounds of RT-PCR as reported previously (14)
. The final PCR products were digested with ScrF1 restriction enzyme and visualized on 3.5% NuSieve agarose gel (BioWhittaker Molecular Applications), and the results were scored for allelic imbalance.
Multiplex RT-PCR.
To study the relative expression of transcripts a and b simultaneously, multiplex RT-PCR was performed using primer sets Cy10/Cy6 (for transcript a) in combination with Cy10/Cy32 (for transcript b; Ref. 14
). We used 55°C annealing temperature for 35 cycles, followed by a second-round PCR using 1:50 dilution and primer sets Cy1F/Cy4R (for transcript a) and Cy1F/Cy27 (for transcript b) at 60°C annealing temperature with 32 cycles. Cy27 was as reported previously (14)
, whereas the other primers were designed by us [Cy1F, 5'-GGAGCTGCTCCTGGTGAACAA-3' (nucleotides 566587 in exon 3); Cy4R, 5'-CCTTGGGGTCCATGTTCTGC-3' (nucleotides 936955 in exon 5)]. The PCR products were resolved on a 3% NuSieve agarose gel and subjected to overnight blotting on Zeta-Probe (Biorad). Nested primer Cy1F was end-labeled with [
-32P]-dATP (Pharmacia) and used as a hybridization probe at 60°C. The blot was processed and exposed to Kodak X-OMAT film for 45 min. Autoradiographic signals corresponding to 389 and 324 bp indicated the expression of transcripts a and b, respectively.
LOH Analysis.
Tumor DNA was studied in comparison with the matched normal mucosa by amplifying the polymorphic exon 4 region with primers Cy26/Cy27 (14)
, followed by ScrF1 (New England Biolabs) restriction digestion.
Statistical Analysis.
The comparison between the observed and expected allele frequencies in the mutation-positive and -negative groups was studied by
2 test. The difference between the mean ages at onset in patients with transcripts a versus b was evaluated using two-tailed t test.
| Results |
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2 = 0.03; P = 0.98) or mutation-negative group (
2 = 0.39, P = 0.82; see "Total" columns in Table 1
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Multiplex RT-PCR Analysis of Transcripts a and b and Correlation with Genotypes.
The allele-specific analysis described above was suitable for efficient individual amplification of each transcript, but not for their relative quantification. We therefore designed multiplex RT-PCR analysis using a common forward primer (from exon 4) and different reverse primers (corresponding to the exon 5 cDNA sequence for transcript a and intron 4 DNA sequence for transcript b). The primers and reactions were carefully designed and optimized not to interfere with each other (see Fig. 1
), thereby allowing the simultaneous analysis of the two transcripts. As shown in Table 3
, the normally spliced transcript a was predominant over the variant transcript b in blood/normal mucosa from HNPCC patients with colon cancer, irrespective of the CCND1 genotype. Transcript a alone was visible in 65% of the cases, and transcript b was visible in 10% of the cases, whereas coexpression (a + b) was seen in 26% of the cases.
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Correlation of Splice Variants with Age at Onset.
The expression of the two transcripts a and b in blood/normal mucosa was analyzed with respect to the age at onset of colon cancer (Table 4)
. When transcript a was exclusively expressed, the average age at onset was 46 years (range, 2969 years), whereas it was as low as 24 years (range, 1930 years) when only transcript b was expressed (by t statistics, P = 0.003; df = 21). In general, the presence of transcript b, alone or in combination with transcript a, showed a significant association with early average age at onset compared with patients with transcript a only (35 versus 46 years, P = 0.02; df = 29).
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| Discussion |
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There are several possible reasons for the discordance between the findings of the present study and that of Kong et al. (16) . First, the predisposing mutations may mask the modifying effects of CCND1 by being strong independent determinants of clinical outcome or, alternatively, because the MMR genes, also participate in cell cycle control (17) , by possibly interacting with CCND1. Thus, it may be important that the study of Kong et al. (16) was based predominantly on MSH2 mutation carriers (57 of 86 patients, 66%), whereas our investigation was based on MLH1 mutation carriers (141 of 146 patients, 97%). Furthermore, a significant fraction of patients studied by Kong et al. (16) had missense mutations (40 of 86 patients, 47%) as compared to only 8 of 146 (5%) in our study; the remaining mutations were truncating mutations. The pathogenicity of missense mutations is sometimes questionable, whereas truncating mutations are likely to be pathogenic. Second, our patient series was larger (146 affected mutation carriers versus 49 affected + 37 unaffected carriers), and, unlike the affected carriers studied by Kong et al. (16) , our population was in Hardy-Weinberg equilibrium and therefore less likely to be biased. Third, there may be an ethnic or environmental variation that may confound the results. Importantly, in our investigation, carriers of mutation 1 formed as homogeneous a population as possible, having common ancestry and shared geographic origin (18) , and the results were the same as in the pooled series, thus emphasizing the reliability of our data.
In our investigation, the presence of the variant transcript b in blood/normal mucosa was associated with a significantly lower age at onset of colon cancer as compared with individuals with the normally spliced transcript a only. Transcript b that lacks the PEST destruction box may have a longer half-life than transcript a (14)
and may thereby promote the proliferative effects of CCND1. Importantly, our multiplex RT-PCR analysis showed that transcript a was predominant over b in all three genotypes (AA, AG, and GG) and especially in individuals with the AG genotype (Table 4)
, thus being compatible with our finding of a later age of onset in the latter group. In accordance with our results, a recent study on 100 sporadic colon cancers failed to demonstrate any association between CCND1 genotype and overall survival; additionally, CCND1 genotype was unrelated to the level of expression of cyclin D1 protein, which unlike genotype, was a prognostic indicator (19)
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In summary, whereas the data available to date suggest that cyclin D1, with its polymorphic nature and the alternatively spliced forms, may contribute to the clinical characteristics of HNPCC and other cancers, the relationships are complex, and more studies are needed to explore the biological basis of the proposed interactions.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by the Sigrid Juselius Foundation, the Academy of Finland, and NIH Grants CA67941, CA82282, and P30 CA16058. ![]()
2 To whom requests for reprints should be addressed, at the Division of Human Cancer Genetics, Comprehensive Cancer Center, Ohio State University, 690 Medical Research Facility, 420 West 12th Avenue, Columbus, OH 43210. Phone: (614) 688-4493; Fax: (614) 688-4245; E-mail: peltomaki-1{at}medctr.osu.edu ![]()
3 The abbreviations used are: HNPCC, hereditary nonpolyposis colorectal cancer; RT-PCR, reverse transcription-PCR; LOH, loss of heterozygosity; MMR, mismatch repair. ![]()
Received 4/12/01. Accepted 6/29/01.
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