The effect of mismatch repair deficiency on tumourigenesis; microsatellite instability affecting genes containing short repeated sequences

We have investigated microsatellite instability (MSI) in colorectal, gastric, endometrial and ovarian cancer as a result of mismatch repair (MMR) deficiency. We detected frameshift mutations in several genes that carry short repeated sequences and are important in cell fidelity and growth control; hMSH3, hMSH6, BAX, IGFIIR, TGFbetaIIR, E2F4 and BRCA2. Accumulation of mutations was heterogeneous and mainly restricted to tumours showing MSI at several loci (MSI-H). Both insertions and deletions were evident and occasional intratumour heterogeneity was evident with more than one different additional allele in the tumour. Most MSI-H tumours had acquired mutations in more than one gene and longer repeated sequences were more frequently targets for mutations. The TGFbetaIIR gene was mutated in 62%, the hMSH3 gene in 43%, the E2F4 gene in 35%, the hMSH6 in 32%, the BAX gene in 32%, the IGFIIR gene in 26%, and the BRCA2 gene in 2% of the MSI-H tumours. Homozygous mutations or mutation of both alleles were evident in all genes except BRCA2, in total 23/105 mutated cases, varying from 7% for BAX to 50% for E2F4. E2F4 mutations were exclusively found in colon tumours and E2F4 polymorphisms was found in 8% of cases. No difference in mutation prevalence was noted between cancer types apart from TGFbetaIIR mutations, which were frequently found in colon and gastric tumours but not in endometrial tumours, suggesting that endometrial tumours progress by a different route where TGFbetaIIR mutations are less favourable.

Chromosome alterations and E-cadherin gene mutations in human lobular breast cancer

We have studied a set of 40 human lobular breast cancers for loss of heterozygosity (LOH) at various chromosome locations and for mutations in the coding region plus flanking intron sequences of the E-cadherin gene. We found a high frequency of LOH (100%, 31/31) at 16q21-q22.1. A significantly higher level of LOH was detected in ductal breast tumours at chromosome arms 1p, 3p, 9p, 11q, 13q and 18q compared to lobular breast tumours. Furthermore, we found a significant association between LOH at 16q containing the E-cadherin locus and lobular histological type. Six different somatic mutations were detected in the E-cadherin gene, of which three were insertions, two deletions and one splice site mutation. Mutations were found in combination with LOH of the wild type E-cadherin locus and loss of or reduced E-cadherin expression detected by immunohistochemistry. The mutations described here have not previously been reported. We compared LOH at different chromosome regions with E-cadherin gene mutations and found a significant association between LOH at 13q and E-cadherin gene mutations. A significant association was also detected between LOH at 13q and LOH at 7q and 11q. Moreover, we found a significant association between LOH at 3p and high S phase, LOH at 9p and low ER and PgR content, LOH at 17p and aneuploidy. We conclude that LOH at 16q is the most frequent chromosome alteration and E-cadherin is a typical tumour suppressor gene in lobular breast cancer.

Replication error in colorectal carcinoma: association with loss of heterozygosity at mismatch repair loci and clinicopathological variables

Instability of microsatellite DNA or replication error (RER) is characteristic of tumours caused by mismatch repair (MMR) deficiency. Germline mutations in MMR genes are associated with Hereditary non-polyposis colorectal carcinoma (HNPCC) and somatic mutations in these genes are also found in a substantial fraction of colorectal cancers (CRC). In this study we concurrently screened colorectal tumours for the RER phenotype and loss of heterozygosity (LOH) at MMR gene loci. The RER phenotype was evident in 47/197 (24%) tumours. RER was more commonly detected in young patients (< 50 years) and in tumours located in the proximal colon. RER was positively associated with LOH at the hMSH2/hMSH6 loci on chromosome 2p, where LOH was observed in 46% of the RER+ tumours. LOH at hMLH1 and hPMS1 loci was more frequent in the younger patients (< 50 years). RER was not associated with clinicopathological parameters, such as Duke's stage and tumour differentiation (grade). The RER phenotype was associated with better overall survival, but there was a trend towards significance when multivariate analysis was used. This indicates that loss of MMR genes generate a less aggressive phenotype, and raises the question about RER being a useful indicator of prognosis for CRC patients.

Loss of heterozygosity at chromosome 1p in different solid human tumours: association with survival

The distal half of chromosome 1p was analysed with 15 polymorphic microsatellite markers in 683 human solid tumours at different locations. Loss of heterozygosity (LOH) was observed at least at one site in 369 cases or 54% of the tumours. LOHs detected ranged from 30-64%, depending on tumour location. The major results regarding LOH at different tumour locations were as follows: stomach, 20/38 (53%); colon and rectum, 60/109 (55%); lung, 38/63 (60%); breast, 145/238 (61%); endometrium, 18/25 (72%); ovary, 17/31 (55%); testis, 11/30 (37%); kidney, 22/73 (30%); thyroid, 4/14 (29%); and sarcomas, 9/14 (64%). High percentages of LOH were seen in the 1p36.3, 1p36.1, 1p35-p34.3, 1p32 and 1p31 regions, suggesting the presence of tumour-suppressor genes. All these regions on chromosome 1p show high LOH in more than one tumour type. However, distinct patterns of LOH were detected at different tumour locations. There was a significant separation of survival curves, with and without LOH at chromosome 1p, in the breast cancer patients. Multivariate analysis showed that LOH at 1p in breast tumours is a better indicator for prognosis than the other variables tested in our model, including nodal metastasis.

Replication error in human breast cancer: comparison with clinical variables and family history of cancer

Replication errors (RER) at microsatellite repeats indicate genomic instability in hereditary nonpolyposis colorectal cancer (HNPCC) and in some sporadic cancers. We have studied genomic instability in 313 sporadic breast tumors and in 106 tumors from BRCA2, 999del5 carriers at 43 genomic loci on 13 chromosomes. RER was observed in 8/419 (1.9%) of the cases at one or more chromosomal loci. The frequencies of type I and type II RER were similar. The majority of RER+ tumors showed ER+, PgR+, high S-phase fraction, tumor size >2 cm and LOH at 2p, 2q and 3p. All 8 RER+ tumors were of the ductal histotype. The breast cancer cases with RER are not part of an HNPCC syndrome and a family history of colorectal cancer growth is not detected in relatives, with the exception of one case. However, four of the RER+ cases are from individuals carrying the BRCA2, 999del5 mutation. We conclude that RER is a rare somatic event during human breast carcinogenesis and may be associated with progression of breast carcinomas.