Epigenetic differences arise during the lifetime of monozygotic twins
This article examines a possible epigenetic cause for differences in the phenotypes of monozygotic twins. It has been described by one commentator as "the most comprehensive and detailed study of age-associated epigenetic changes so far reported, certainly for the case of human subjects. It is a technical tour de force with the utilisation of a battery of powerful molecular genetic methodologies coupled with competitive chromosomal hybridisations."(Martin 2005)
Phenotypical discordance between monozygotic twins is readily observed but poorly understood. Previous studies have indicated that differences do arise in the placenta, amniotic sac, and vascularisation of separate cell masses , however there has been a lack of published experimental data. As a result, the underlying causes of phenotypical discordance in traits and for common diseases between monozygotic twins remain unknown.
Eighty monozygotic twins from Spain of varying ages were used in the study. Despite sharing a common genotype, substantial epigenetic disparity was indicated between the subject twins in examinations of the global and locus-specific differences in DNA methylation and histone acetylation of the paired subjects. It was shown that older monozygotic twins exhibited substantial differences in overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation which was indicated in their respective gene expression portraits. Epigenetic differences in younger twins were almost indistinguishable.
The observable tendency of older twins to exhibit phenotypical differences as a factor of aging appeared to be caused by both external (diet, lifestyle, health) and internal (epigenetic) factors. The epigenetic differences which were amplified over the lifetime of the subjects have been coined "epigenetic drift" and may be the result of defective or degraded signalling of epigenetic information through successive cell divisions.
The writers consider that epigenetic defects may occur at a much faster rate than genetic mutations as mechanisms for their correction do not appear to be as prevalent or well developed as those corresponding to genetic mutations, nor do the results of epigenetic defects appear to be so dramatic. The test results indicated that there was a role that epigenetic differences played in the discordant frequency/onset of diseases in monozygotic twins.
The study therefore led to the postulation that there may be an epigenetic cause for phenotypical discordance between monozygotic twins and that further investigation of epigenetic mechanisms is warranted.
Matt Sulman
Phenotypical discordance between monozygotic twins is readily observed but poorly understood. Previous studies have indicated that differences do arise in the placenta, amniotic sac, and vascularisation of separate cell masses , however there has been a lack of published experimental data. As a result, the underlying causes of phenotypical discordance in traits and for common diseases between monozygotic twins remain unknown.
Eighty monozygotic twins from Spain of varying ages were used in the study. Despite sharing a common genotype, substantial epigenetic disparity was indicated between the subject twins in examinations of the global and locus-specific differences in DNA methylation and histone acetylation of the paired subjects. It was shown that older monozygotic twins exhibited substantial differences in overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation which was indicated in their respective gene expression portraits. Epigenetic differences in younger twins were almost indistinguishable.
The observable tendency of older twins to exhibit phenotypical differences as a factor of aging appeared to be caused by both external (diet, lifestyle, health) and internal (epigenetic) factors. The epigenetic differences which were amplified over the lifetime of the subjects have been coined "epigenetic drift" and may be the result of defective or degraded signalling of epigenetic information through successive cell divisions.
The writers consider that epigenetic defects may occur at a much faster rate than genetic mutations as mechanisms for their correction do not appear to be as prevalent or well developed as those corresponding to genetic mutations, nor do the results of epigenetic defects appear to be so dramatic. The test results indicated that there was a role that epigenetic differences played in the discordant frequency/onset of diseases in monozygotic twins.
The study therefore led to the postulation that there may be an epigenetic cause for phenotypical discordance between monozygotic twins and that further investigation of epigenetic mechanisms is warranted.
Matt Sulman
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