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Allele-specific hypermethylation of the retinoblastoma tumor-suppressor gene
T Sakai1, J Toguchida, N Ohtani
1Howe Laboratory of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston 02114.
Abstract:
Inactivation of the retinoblastoma gene appears to have a fundamental role in the genesis of retinoblastoma, osteosarcoma, and other malignant tumors. The gene is generally inactivated because of loss-of-function mutations, although epigenetic phenomena, such as hypermethylation of the promoter region, could possibly have the same effect. We investigated the methylation pattern at the 5' end of the retinoblastoma gene, including its promoter region and exon 1, in DNA purified from 56 primary retinoblastomas. We found five tumors with evidence for hypermethylation, all from unilateral, simplex patients. No methylation abnormalities were detected in DNA purified from the leukocytes from these patients. It is interesting that in one of these tumors the hypermethylation was confined to one allele. There were no mutations in a 1,306-bp sequence including the hypermethylated region that might account for the allele-specific hypermethylation. We believe that the hypermethylation of the retinoblastoma gene that we found in these tumors corresponds to the allelic inactivation of the gene, and we speculate that erroneous hypermethylation without alteration of nucleotide sequence occasionally plays a role in the genesis of this cancer. If this is true, then retinoblastomas with hypermethylation might be treatable with chemotherapeutic agents that interfere with methylation of DNA.
Insights
Retinoblastoma gene inactivation is key in certain cancers. Our study found hypermethylation, a potential epigenetic cause, in some retinoblastomas, suggesting new treatment avenues.
Area of Science:
- Oncology
- Epigenetics
- Molecular Biology
Background:
- The retinoblastoma gene (RB1) is crucial in preventing cancer, with its inactivation linked to retinoblastoma and osteosarcoma.
- Gene inactivation typically occurs via mutations, but epigenetic changes like promoter hypermethylation can also silence genes.
Purpose of the Study:
- To investigate the methylation status of the retinoblastoma gene's 5' end (promoter and exon 1) in primary retinoblastomas.
- To explore the role of epigenetic alterations, specifically hypermethylation, in retinoblastoma development.
Main Methods:
- DNA was purified from 56 primary retinoblastomas and analyzed for methylation patterns.
- Leukocyte DNA from patients with hypermethylation was examined to rule out germline methylation abnormalities.
- Sequencing was performed to identify mutations in the hypermethylated region.
Main Results:
- Five of 56 retinoblastomas exhibited hypermethylation in the retinoblastoma gene's promoter and exon 1.
- Hypermethylation was observed only in tumors from unilateral, simplex patients, not in their leukocyte DNA.
- One tumor showed allele-specific hypermethylation without detectable mutations in the affected region.
Conclusions:
- Epigenetic hypermethylation of the retinoblastoma gene can occur in retinoblastomas, acting as an inactivation mechanism.
- This hypermethylation, independent of sequence mutations, may play a role in sporadic retinoblastoma genesis.
- Retinoblastomas with hypermethylation could potentially be targeted by methylation-inhibiting chemotherapy.