Monozygotic twins discordant for constitutive BRCA1 promoter methylation, childhood cancer and secondary cancer

Abstract

We describe monozygotic twins discordant for childhood leukemia and secondary thyroid carcinoma. We used bisulfite pyrosequencing to compare the constitutive promoter methylation of BRCA1 and several other tumor suppressor genes in primary fibroblasts. The affected twin displayed an increased BRCA1 methylation (12%), compared with her sister (3%). Subsequent bisulfite plasmid sequencing demonstrated that 13% (6 of 47) BRCA1 alleles were fully methylated in the affected twin, whereas her sister displayed only single CpG errors without functional implications. This between-twin methylation difference was also found in irradiated fibroblasts and untreated saliva cells. The BRCA1 epimutation may have originated by an early somatic event in the affected twin: approximately 25% of her body cells derived from different embryonic cell lineages carry one epigenetically inactivated BRCA1 allele. This epimutation was associated with reduced basal protein levels and a higher induction of BRCA1 after DNA damage. In addition, we performed a genome-wide microarray analysis of both sisters and found several copy number variations, i.e., heterozygous deletion and reduced expression of the RSPO3 gene in the affected twin. This monozygotic twin pair represents an impressive example of epigenetic somatic mosaicism, suggesting a role for constitutive epimutations, maybe along with de novo genetic alterations in recurrent tumor development.

Keywords: :

• BRCA1
• childhood cancer
• DNA Methylation
• epimutation
• monozygotic twins
• secondary cancer
• somatic mosaicism

Acknowledgments

This work was supported by the Stiftung Rheinland-Pfalz für Innovation (project no. 698). First of all, we thank the patients whose participation made this study possible. We also thank Annalisa Tiozzo (Affymetrix), Tanja Zeller (Clinical Chemistry and Laboratory Medicine, University Medical Center Mainz), Niels Boehm and Franz Grus (Experimental Ophthalmology, Ocular Proteomics and Immunology Center, University Medical Center Mainz) for help with the DNA and antibody arrays, respectively.