Frequent Loss of Heterozygosity at the MCC Locus on Chromosome 5q21-22 in Sporadic Colorectal Carcinomas

Recent studies have identified a gene on chromosome 5q, designated MCC (mutated in colorectal cancers), as a candidate for the putative colorectal tumor suppressor gene that is located at 5q21. We examined loss of heterozygosity (LOH) at the MCC locus and its vicinity in sporadic colorectal carcinomas, using 12 RFLP (restriction fragment length polymorphism) markers. One clone, L5.71, had been used to identify the MCC gene; all 12 markers also had tight linkage to the gene responsible for adenomatous polyposis coli. All 40 cases studied were informative with at least one marker, and 22 of them (55%) showed LOH at one or more loci. LOH in the tumors was more frequent in the immediate vicinity of L5.71 than in distant parts of the chromosome, and a common region of deletion was detected between markers L5.62 and 15A6. In one case, alleles were retained at L5.71 and at loci proximal to L5.71, but alleles were lost at loci distal to L5.71. In another case, both alleles were retained at L5.71 but alleles were lost at loci proximal and distal to L5.71. These results support the conclusion that a tumor suppressor gene for colorectal carcinoma is located within or around locus L5.71.     

Frequent loss of heterozygosity at the MCC locus on chromosome 5q21-22 in sporadic colorectal carcinomas.

Recent studies have identified a gene on chromosome 5q, designated MCC (mutated in colorectal cancers), as a candidate for the putative colorectal tumor suppressor gene that is located at 5q21. We examined loss of heterozygosity (LOH) at the MCC locus and its vicinity in sporadic colorectal carcinomas, using 12 RFLP (restriction fragment length polymorphism) markers. One clone, L5.71, had been used to identify the MCC gene; all 12 markers also had tight linkage to the gene responsible for adenomatous polyposis coli. All 40 cases studied were informative with at least one marker, and 22 of them (55%) showed LOH at one or more loci. LOH in the tumors was more frequent in the immediate vicinity of L5.71 than in distant parts of the chromosome, and a common region of deletion was detected between markers L5.62 and 15A6. In one case, alleles were retained at L5.71 and at loci proximal to L5.71, but alleles were lost at loci distal to L5.71. In another case, both alleles were retained at L5.71 but alleles were lost at loci proximal and distal to L5.71. These results support the conclusion that a tumor suppressor gene for colorectal carcinoma is located within or around locus L5.71.     

Molecular genetic studies of colon cancer

Molecular genetic studies of tumor-specific allele loss, originally associated primarily with research regarding childhood hereditary cancers such as retinoblastoma and Wilms' tumors, only lately have been recognized as a relatively fast and fruitful way of locating cancer genes on human chromosomes. To date, over 25 different cancers have been tied to a gene (or genes) on a specific chromosome when this method has been used. During the past year alone, this approach has permitted detection of three genes involved in either hereditary or sporadic colorectal cancers. These three genes, located on chromosomes 5q, 17p, and 18q, are believed to belong to the newly described tumor suppressor (or growth suppressor) gene class, whose effects are opposite those of activated cellular oncogenes, which promote uncontrolled cell growth. Present studies, however, have not shown losses of any of these tumor suppressor genes to be correlated with the presence of activated ras genes in colorectal adenomas or carcinomas. During progression from adenoma to carcinoma, ras gene mutations and 5q allelic deletions are likely to be earlier events, whereas allelic losses from chromosomes 18q and 17p seem to occur more often in advanced tumors. Involvement of the genes on 5q (FAP) and 18q (Lynch syndrome II) in hereditary colon cancer syndromes is supported by linkage studies, but their respective roles (as well as that of the gene on 17p) in familial and sporadic colorectal cancer remain to be precisely defined. Probable isolation of these three genes by molecular cloning within the next few years will help elucidate their specific biologic functions. It will also permit early detection, and thus prevention, of some familial colon cancers (such as FAP), and possibly allow DNA marker-based separation of different colon cancer subtypes of similar histologic appearance.     

散发性结直肠癌患者5号染色体杂合缺失分析

目的：探讨散发性结直肠癌患者5号染色体上与抑癌基因相关的杂合缺失（LOH）情况,并探讨新的抑癌基因位点,方法：对83例散发性结直肠癌患者基因组DNA,以15个不同荧光标记的高度多态性微卫星引物（平均遗传距离12．67cm)扩增相应的微卫星位点,用ABI PRISM 377测序仪进行基因扫描,统计各位点杂合缺失率。结果：在15个微卫星位点中,平均杂合缺失率为25．80％,5p中最高为D5S416,占48．15％,5q中最高为D5S471,占38．71％,D5S471周围的3个位点（D5S428,D5S2027 和 D5S2115）也存在高频的杂合缺失（＞30．00％）,结论：5号染色体上存在着高频的杂合缺失,其中5q13.3-31.1区域中,有与结直肠癌发生密切相关的APC,MCC,CTNNA1及IL家族等基因,而在5p15.1上的D5S416的杂合缺失率高达48．15％,此区域至今尚未发现与结直肠癌相关的基因位点,估计可能有未知的抑癌基因存在。     

LOH of PTPRJ occurs early in colorectal cancer and is associated with chromosomal loss of 18q12-21

Recently, the gene PTPRJ (protein tyrosine phosphatase receptor type J) was identified as the candidate gene for the mouse colon cancer susceptibility locus Scc1. Its human homologue PTPRJ is frequently deleted in several cancer types, including colorectal cancer. To elucidate the role of PTPRJ loss in different stages of colorectal cancer and in its pathways of progression, we expanded the previously published comparative genomic hybridization results with novel data on loss of heterozygosity (LOH) at the PTPRJ locus. We identified a strong association between the LOH of PTPRJ and the loss of chromosomal region 18q12-21 (P=0.009). This observation is specific for progressed colorectal adenomas, suggesting that an interaction between LOH of PTPRJ and loss of 18q12-21 may be involved in the development of a more progressed form of adenomas.     

Allele loss from 5q21 (APC/MCC) and 18q21 (DCC) and DCC mRNA expression in breast cancer.

Thirty-four primary, untreated sporadic breast cancers were examined for loss of heterozygosity (LOH) at tumour suppressor loci involved in colorectal cancer: APC/MCC at 5q21 and DCC at 18q21. LOH was identified in 28% informative patients at 5q21 and 31% at 18q21. LOH at 5q21 and 18q21 was compared with allele loss at 17p13 and concurrent LOH at two or more of the loci was noted in 24% of tumours. Expression of a 12 kb DCC mRNA was demonstrated in 14/34 (42%) of the cancers and in all five tumours with LOH at the DCC locus there was an additional 11 kb DCC mRNA. Abnormalities of three loci involved in colorectal cancer (5q21, 17p13 and 18q21) therefore also occur in sporadic breast cancer. The accumulation of such genetic abnormalities may confer a growth advantage important in the development of breast cancer.     

High-frequency of loss of expression and allelic deletion of the apc and mcc genes in human prostate-cancer

Loss of heterozygosity (LOH) or allelic deletion at various loci has been reported in the majority of human tumors. The frequently deleted targets are believed to be tumor suppressor genes. Recent studies have identified the APC and MCC genes at 5q21, as putative tumor suppressor genes. The APC and MCC genes have been implicated in the development of familial adenomatous polyposis coli and cancers of the gastrointestinal tract, ovary, breast and lung. In the present study, we investigated a possible role of the APC and MCC genes in prostate cancer development. mRNA expression of the APC and MCC genes and LOH at the APC and MCC loci were determined in prostate cancer tissues from 28 patients and 5 human prostatic adenocarcinoma cell lines. Of the informative cases, the frequency of LOH at the APC and MCC loci was 63% (10/16) and 54% (7/13), respectively. Overall, 65% (15/23) of the informative cases showed LOH at the APC and/or MCC gene. All prostate cancer cell lines showed homozygosity at all APC and MCC polymorphic sites studied. Approximately half (57%) of the tumor tissues examined showed a decreased expression of APC and MCC mRNA. Our data suggest that the APC and MCC genes may be involved in the formation of human prostate cancer (HPC).     

Genetic changes and histopathological types in colorectal tumors from patients with familial adenomatous polyposis

Loss of heterozygosity (LOH) and K-ras mutation were analyzed in 111 colorectal polyps and 26 invasive carcinomas from 40 patients with familial adenomatous polyposis of distinct histopathological types. LOH, being less than 2% in moderate adenomas, was detected on chromosome 5q (20%) in severe adenomas, on 5q (26%) and 17p (38%) in intramucosal carcinomas, and on 5q (52%), 17p (56%), 18 (46%), and 22q (33%) in invasive carcinomas. LOH on chromosome 5q occurred most frequently in the region close to the APC gene both in adenomas and carcinomas, and a loss of the normal allele of the APC gene was demonstrated in 3 cases. K-ras mutation markedly increased in the step of development from moderate (11%) to severe (36%) adenomas. These results suggest the following mechanisms for the development of colon tumors in patients with familial adenomatous polyposis: (a) the heterozygous mutant/wild-type condition at the APC gene causes formation of mild or moderate adenoma; (b) the loss of the normal allele in the APC gene leads to a change from moderate to severe adenoma; (c) LOH on chromosome 17p contributes to the conversion of adenoma to intramucosal carcinoma; (d) LOH on other chromosomes, such as 18 and 22q, are involved in the progression of intramucosal carcinoma to invasive carcinoma; and (e) K-ras mutation may also affect the development of moderate to severe adenoma.     

Somatic mutations of the PPP2R1B candidate tumor suppressor gene at chromosome 11q23 are infrequent in ovarian carcinomas

Previous studies have demonstrated frequent allelic losses of distal chromosome 11q in ovarian carcinomas. The tumor suppressor gene(s) presumably targeted by these losses have not yet been identified. PPP2R1B is a candidate tumor suppressor gene at 11q23 that has recently been shown to be mutated in a subset of colorectal and lung cancers. We evaluated 5 ovarian carcinoma cell lines and 27 primary ovarian carcinomas for allelic losses of 11q23 and for mutations in the open reading frame of PPP2R1B. We also evaluated the primary tumors for allelic losses at 17p13, another chromosomal region frequently affected by losses of heterozygosity (LOH) in ovarian cancers. 11q23 and 17p13 allelic losses were identified in 25% and 74% of the carcinomas, respectively. No mutations within PPP2R1B coding sequences were found. These findings indicate that mutations of the PPP2R1B gene are infrequent in ovarian cancer and that deletions affecting the distal portion of chromosome 11q in ovarian cancer likely target inactivation of other genes.     

散发性结直肠癌染色体5p15区杂合性缺失精细定位

目的 研究散发性结直肠癌5号染色体的杂合性缺失,寻找新的结直肠癌抑癌基因.方法 取83例结直肠癌患者的肿瘤和正常组织,先后应用分布于5号染色体上的16对微卫星DNA标记和5p15区D5S416附近的6对微卫星DNA标记进行聚合酶链反应(PCR),PCR产物在自动荧光测序仪进行电泳3h,以GeneScan3.1和Genotyper2.1软件进行基因分型.结果 经5号染色体上的16对微卫星DNA标记的杂合性缺失分析,发现散发性结直肠癌5号染色体上有2个明显的高频杂合性缺失区域,即染色体短臂上的D5S416位点和染色体长臂上的D5S428～D5S410区.进一步对5p15区的精细定位,界定了1个遗传距离为1 cM大小、跨越D5S416位点的杂合性缺失区,该区的位点大致顺序是pter-D5S630-D5S1987-D5S1991-D5S1954-D5S1963-D5S416-D5S2114-D5S486.本组54例有效标本的杂合性缺失率达48.2%.结论 散发性结直肠癌在5号染色体短臂存在1个杂合性缺失区域,即5p15.2-15.3,该区很可能存在1个或多个与结直肠癌相关的新的抑癌基因.     

Frequent loss of heterozygosity at the DCC locus in gastric cancer.

We examined 28 cases of surgically resected gastric cancer, excluding the diffuse type, for loss of heterozygosity (LOH) on 12 chromosomal arms using polymorphic DNA markers. LOH on chromosome 18q was detected in 61% (14 of 23) of the cases by the probes OLVIIA8, OLVIIE10, p15-65, SAM 1.1, and OS-4, and a putative common region showing LOH included the locus of the DCC tumor suppressor gene. LOH on chromosome 17p was also frequently found (8 of 19 or 42% of the cases) by the probes p10-3 and pHF12-1, and in 5 of these 6 cases the LOH on chromosome 17p was accompanied by LOH on chromosome 18q. On the other hand, the incidence of LOH was 30% or less using probes pHRnES, pHF12-65, p-c-mybE2.6, NJ3 3.2, pHF12-8, pHINS6.0, p9D11, hp2-alpha, pCMM6, and P1A5 on chromosomes 1q, 5, 6q, 7q, 9, 11p, 13q, 16q, 20, and 22q, respectively. LOH on chromosome 18q was frequent irrespective of the depth of tumor invasion, whereas the incidence of LOH on chromosome 17p was higher in the cases in which the tumor invaded beyond the muscularis propria than in those in which tumor invasion was limited to the submucosa and muscularis propria. These results suggest that LOH on chromosome 18q occurs at an earlier stage than LOH on chromosome 17p and that the inactivation of tumor suppressor genes located on chromosome 17p and 18q (e.g., the p53 and DCC genes) is critically involved in the development of the majority of gastric cancers. While alteration of the p53 gene is observed in various human cancers, that of the DCC gene is considered to occur more selectively in gastrointestinal cancers.     

Chromosome 7q31 allelic imbalance and somatic mutations of RAY1/ST7 gene in colorectal cancer

ST7 is a putative tumor suppressor gene on chromosome 7q31. However, the role of ST7 as a tumor suppressor is uncertain as somatic mutations have been difficult to demonstrate. In order to investigate the genetic role of RAY1/ST7 in tumorigenesis, we have screened 135 colorectal cancers for loss of heterozygosity (LOH) at chromosome 7q31. The entire RAY1/ST7 gene, including intron/exon boundaries and alternate 5' and 3' sequences of 15/124 (12%) informative cancers with LOH were characterized. No somatic mutations of the RAY1/ST7 gene were observed. Our results do not support a role for RAY1/ST7 as a colorectal cancer tumor suppressor gene.     

Molecular karyotyping of human hepatocellular carcinoma using single-nucleotide polymorphism arrays

Genomic amplification of oncogenes and inactivation of suppressor genes are critical in the pathogenesis of human cancer. To identify chromosomal alterations associated with hepatocarcinogenesis, we performed allelic gene dosage analysis on 36 hepatocellular carcinomas (HCCs). Data from high-density single-nucleotide polymorphism arrays were analysed using the Genome Imbalance Map (GIM) algorithm, which simultaneously detects DNA copy number alterations and loss of heterozygosity (LOH) events. Genome Imbalance Map analysis identified allelic imbalance regions, including uniparental disomy, and predicted the coexistence of a heterozygous population of cancer cells. We observed that gains of 1q, 5p, 5q, 6p, 7q, 8q, 17q and 20q, and LOH of 1p, 4q, 6q, 8p, 10q, 13q, 16p, 16q and 17p were significantly associated with HCC. On 6q24-25, which contains imprinting gene clusters, we observed reduced levels of PLAGL1 expression owing to loss of the unmethylated allele. Finally, we integrated the copy number data with gene expression intensity, and found that genome dosage is correlated with alteration in gene expression. These observations indicated that high-resolution GIM analysis can accurately determine the localizations of genomic regions with allelic imbalance, and when integrated with epigenetic information, a mechanistic basis for inactivation of a tumor suppressor gene in HCC was elucidated.     

Genetic pathways in the evolution of morphologically distinct colorectal neoplasms

Colorectal adenomas can be morphologically classified as exophytic or flat. Polypoid cancers and cancers arising de novo (ie., without any adenomatous component) might be the results of genetic progression from exophytic and flat adenomas, respectively. In this study, we examined 94 morphologically distinct neoplastic specimens for mutations in K-RAS and analyzed 10 microsatellite loci tightly linked to the tumor suppressor genes APC, p53, DCC/SMAD4, hMSH2, and hMLH1. K-RAS mutations were significantly associated with exophytic adenomas [11 of 21 (52%)] compared to flat adenomas [2 of 13(15%), P < 0.03] and polypoid cancers [17 of 25 (68%)] compared to cancers arising de novo [7 of 25 (28%), P < 0.01]. Two polypoid cancer cases demonstrated three and four different K-RAS mutations, respectively, suggesting multiple areas of clonal expansion. Cancers arising de novo were significantly associated with loss of heterozygosity (LOH) at chromosome 3p compared to pol ypoid cancers [6 of 18(33%) versus 1 of 20(5%), P < 0.03], whereas the prevalence of LOH at chromosomes 2p, 5q, 17p, and 18q and microsatellite instability were not different between the groups. For all cancers, LOH at chromosomes 17p and 18q occurred in 47 and 51%, respectively. However, LOH at 17p and 18q occurred in 0 and 16% of benign lesions, respectively, suggesting their role in malignant transformation. There was no difference in LOH at chromosomes 17p and 18q between exophytic and flat lesions. These findings suggest that (a) mutant K-RAS is associated with the exophytic growth of colonic neoplasms, and that (b) some colorectal cancers arising de novo lose chromosome 3p during their evolution, which is not seen in polypoid cancers. Half of all cancers lose chromosomes 17p and 18q at or near the malignant transition of benign lesions as reported previously, irrespective of morphology. There may be more than one genetic avenue for colorectal cancer formation, and this correlates with the morphological characteristics.     

Deletion mapping on the short arm of chromosome 1 in Merkel cell carcinoma

Twenty-two Merkel cell carcinoma (MCC) biopsies and six cell lines from 24 patients were examined for loss of heterozygosity (LOH) at 11 loci on 1p and one on 1q, to determine LOH regions on chromosome 1p. Sixteen (73%) tumors had LOH for at least one locus; 14 demonstrated LOH at more than one locus, and 7 (29%) samples had more than one region of loss, with 4 of these having loss at all informative loci on 1p. Three common regions of loss (SRO) were defined by LOH in multiple tumors. Eight samples demonstrated LOH between D1S214 and D1S160 (1p36), seven between D1S234 and D1S186 (1p35), and 11 for the region centromeric of D1S211 and D1S220 (1p32-1p33). Seven samples (29%) demonstrated more than one region of loss. LOH on 1p occurs frequently in MCC and more than one tumor suppressor gene on 1p is likely to play a role in the development of this tumor type.     

Frequent allelic loss at 10q23 but low incidence of PTEN mutations in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare, highly metastatic skin tumor of neuroectodermal origin. The disease shares clinical and histopathological features with small cell lung carcinoma (SCLC). The genetic mechanisms underlying the development and tumor progression of MCC are poorly understood. We recently showed by comparative genomic hybridization (CGH) that the pattern of chromosomal abnormalities in MCC resembles that of SCLC. One of the most frequently observed losses involved the entire chromosome 10 or partial loss of the chromosome 10 long arm (33% of examined MCC cases). The PTEN tumor-suppressor gene has been mapped to 10q23.3 and was shown to be mutated in a variety of human cancers including SCLC. Germline PTEN mutations have been observed in familial predisposing cancer syndromes including Cowden disease. Interestingly, an association between Cowden syndrome and Merkel cell carcinoma has been reported. To study the possible role of PTEN in MCC oncogenesis, loss of heterozygosity (LOH) analysis for the 10q23 region was performed on 26 MCC tumor samples from 23 MCC patients. The PTEN locus was deleted in 9 of 21 (43%) informative MCC tumor samples [7 of 18 (39%) MCC patients]. Despite this high frequency of LOH at 10q23, mutation and homozygous deletion screening of the PTEN gene revealed only one tumor with a nonsense mutation and a second with a homozygous deletion of exon 9. These data suggest that either alternative mechanisms lead to inactivation of the PTEN gene or that other tumor-suppressor genes at chromosome 10 are implicated in the development of MCC.     

Loss of heterozygosity of the Mutated in Colorectal Cancer gene is not associated with promoter methylation in non-small cell lung cancer

'Mutated in Colorectal Cancer' (MCC) is emerging as a multifunctional protein that affects several cellular processes and pathways. Although the MCC gene is rarely mutated in colorectal cancer, it is frequently silenced through promoter methylation. Previous studies have reported loss of heterozygosity (LOH) of the closely linked MCC and APC loci in both colorectal and lung cancers. APC promoter methylation is a marker of poor survival in non-small cell lung cancer (NSCLC). However, MCC methylation has not been previously studied in lung cancer. Therefore, we wanted to determine if MCC is silenced through promoter methylation in lung cancer and whether this methylation is associated with LOH of the MCC locus or methylation of the APC gene. Three polymorphic markers for the APC/MCC locus were analysed for LOH in 64 NSCLC specimens and matching normal tissues. Promoter methylation of both genes was determined using methylation specific PCR in primary tumours. LOH of the three markers was found in 41-49% of the specimens. LOH within the MCC locus was less common in adenocarcinoma (ADC) (29%) than in squamous cell carcinoma (SCC) (72%; P=0.006) or large cell carcinoma (LCC) (75%; P=0.014). However, this LOH was not accompanied by MCC promoter methylation, which was found in only two cancers (3%). In contrast, 39% of the specimens showed APC methylation, which was more common in ADC (58%) than in SCC (13%). Western blotting revealed that MCC was expressed in a subset of lung tissue specimens but there was marked variation between patients rather than between cancer and matching non-cancer tissue specimens. In conclusion, we have shown that promoter methylation of the APC gene does not extend to the neighbouring MCC gene in lung cancer, but LOH is found at both loci. The variable levels of MCC expression were not associated with promoter methylation and may be regulated through other cellular mechanisms.     

AOM-induced mouse colon tumors do not express full-length APC protein

While evidence in both sporadic and inherited human colorectal cancer and MIN mice implicate the tumor suppressor gene, APC, in the causation of colorectal carcinogenesis, this gene has not been confirmed to be involved in rodent chemically-induced colon cancer models (RCCM). These experimental models are widely used to elucidate mechanisms involved in colon carcinogenesis (initiation, promotion and progression) as well as studies on chemoprevention (dietary and other) and intervention. To validate the RCCM as relevant models for sporadic human colorectal cancer, and to facilitate research on the role of the APC gene in colon carcinogenesis, we investigated the role of APC in azoxymethane (AOM)- induced colorectal tumors in mice. Using an antibody that recognizes the carboxy terminus of APC, we have characterized the pattern of staining observed in normal mouse intestinal tissue, in MIN mouse intestinal adenomas and in AOM-induced mouse colon tumors. The APC protein was localized in the cytoplasm of normal colonic epithelial cells. In the small intestine there was APC immunoreactivity along the villous and staining of the Paneth cells at the base of the glands. In the proximal and distal colonic crypts there appeared to be a gradient of staining which increased towards the luminal surface. This gradient was not as apparent in the small intestinal villi. Nuclei and mucus in the goblet cells showed no immunoreactivity. MIN mouse small bowel and colonic adenomas, known to have lost APC, stained negatively for APC. AOM-induced adenomas and carcinomas also consistently stained negatively using this antibody. This study demonstrates for the first time the loss of wild-type APC protein in AOM-induced mouse colon tumors and suggests that alterations in expression of this tumor suppressor gene, which is so commonly mutated in human colon cancer, is also involved in this animal model of colon cancer.      

Distinct regions of frequent loss of heterozygosity of chromosome 5p and 5q in human esophageal cancer

Loss of heterozygosity (LOH) studies reported thus far suggest that tumor suppressor loci on chromosome 5q are important in esophageal cancer (EC) while little is known about the involvement of chromosome 5p. To investigate the potential existence of tumor suppressor gene(s) on chromosome 5 contributing to the development of EC, we performed LOH studies using a total of 24 polymorphic markers spanning the entire chromosome 5. Seventy primary esophageal cancers were microdissected and allelic deletions were detected by polymerase chain reaction (PCR)-single strand conformation polymorphism or by microsatellite analysis. LOH was observed in at least 1 of the loci in 47 of 70 (67%) esophageal tumors. Initially, 40 tumors [24 squamous cell carcinomas (SCC) and 16 adenocarcinomas (ADC)], each with matched histologically normal esophageal mucosa, were analyzed at 15 marker loci on 5p and 5q. A novel locus, D5S667 on 5p15.2, exhibited the highest frequency of LOH (44%) in these tumors along with another previously reported region of frequent deletion, irf-1 (5q31.1). In a series of 30 additional EC tumors (11 SCC and 19 ADC), a detailed LOH analysis of chromosome 5p15.2 region was conducted using 10 additional polymorphic markers, which mapped the frequently deleted region within 1 cM. Overall, LOH at the D5S667 locus was observed more frequently in SCC than in ADC (62% vs. 23%, p = 0.01). This significant rate of LOH of a distinct region of chromosome 5p implicates the existence of a putative tumor suppressor gene locus involved in EC. Int. J. Cancer 78:600–605, 1998. © 1998 Wiley-Liss, Inc.