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Loss of heterozygosity at the c-raf locus in small cell lung carcinoma
G Sithanandam1, M Dean, U Brennscheidt
1Laboratory of Viral Carcinogenesis, National Cancer Institute, Frederick, Maryland 21701.
Abstract:
The c-raf-1 oncogene is located at chromosome 3p25, near a region known to be specifically deleted in patients with renal cell carcinoma and small cell lung carcinoma (SCLC). From cytogenetic analyses of SCLC cell lines, we have estimated that one c-raf-1 allele was deleted in approximately 80% of the cases. However, c-raf-1 was generally thought to be distal to the most common deletion in SCLC, 3p14-23. Using restriction site polymorphisms (RFLPs) located within the c-raf-1 locus, we have examined DNA from 84 human lung carcinomas. In an analysis of 11 paired (normal versus tumor) SCLC DNA samples, all five informative cases showed loss of heterozygosity at this locus in the corresponding tumor sample. Analysis of 73 unpaired lung carcinoma DNAs showed that out of 31 non-SCLC samples, 19% were heterozygous for the BglI polymorphism and 25% showed heterozygosity with TaqI. However, all of the 42 SCLC samples were homozygous for both of these RFLPs. This striking loss of heterozygosity at the c-raf-1 locus in SCLC indicates that one allele of c-raf-1 is deleted in SCLC. The kinase activity of the c-raf protein appears to be constitutively activated in these cells. Whether this apparent activation results from genetic or epigenetic events is under investigation.
Insights
The c-raf-1 oncogene is frequently deleted in small cell lung carcinoma (SCLC). Loss of heterozygosity at this locus in SCLC suggests c-raf-1 deletion contributes to the disease.
Area of Science:
- Oncology
- Molecular Biology
- Genetics
Background:
- The c-raf-1 oncogene is located at chromosome 3p25.
- This region is known to be deleted in renal cell carcinoma and small cell lung carcinoma (SCLC).
- Previous cytogenetic analyses suggested c-raf-1 deletion in approximately 80% of SCLC cases.
Purpose of the Study:
- To investigate the role of c-raf-1 gene deletion in lung carcinoma, particularly SCLC.
- To determine if c-raf-1 loss of heterozygosity (LOH) occurs in SCLC.
- To analyze c-raf-1 allele status in both SCLC and non-SCLC tumors.
Main Methods:
- Restriction site polymorphisms (RFLPs) within the c-raf-1 locus were used.
- DNA from 84 human lung carcinomas (paired and unpaired) was analyzed.
- Analysis included paired normal and tumor SCLC DNA samples and unpaired SCLC and non-SCLC samples.
Main Results:
- Loss of heterozygosity at the c-raf-1 locus was observed in all informative paired SCLC samples.
- All 42 SCLC samples analyzed were homozygous for BglI and TaqI RFLPs, indicating deletion.
- Non-SCLC samples showed varying heterozygosity rates for the analyzed polymorphisms.
Conclusions:
- The study demonstrates a striking loss of heterozygosity at the c-raf-1 locus in SCLC.
- This indicates that one allele of c-raf-1 is deleted in SCLC.
- Constitutive activation of c-raf protein kinase activity in SCLC is suggested, with underlying genetic or epigenetic causes under investigation.