Not really identical: Epigenetic differences in monozygotic twins and implications for twin studies in psychiatry

@article{Haque2009NotRI,
  title={Not really identical: Epigenetic differences in monozygotic twins and implications for twin studies in psychiatry},
  author={Fahmida Nipa Haque and Irving I Gottesman and Albert Hung Choy Wong},
  journal={American Journal of Medical Genetics Part C: Seminars in Medical Genetics},
  year={2009},
  volume={151C}
}
Classical twin studies in the field of psychiatry generally fall into one of two categories: (1) those designed to identify environmental risk factors causing discordance in monozygotic (MZ) twins and (2) those geared towards identifying genetic risk factors. However, neither environment nor differences in DNA sequence can fully account for phenotypic discordance among MZ twins. The field of epigenetics – DNA modifications that can affect gene expression – offers new models to understand… 
Genetic, environmental and stochastic factors in monozygotic twin discordance with a focus on epigenetic differences
TLDR
The subject of epigenetics is explored as another component in human phenotypic variability and its links to disease focusing particularly on evidence from MZ twin studies.
Monozygotic Twins and Epigenetics
TLDR
Monozygotic twins have near-identical DNA sequences, and offer unique opportunities to study epigenetic effects on phenotype, and the way in which epigenetic information is modified and maintained.
Epigenetics as an Explanation for Phenotypic Variation in Monozygotic Twins
TLDR
The classical method is comparing traits in identical, or monozygotic, twins to those of dizygotic twins, which has had a significant impact on the current understanding of etiologic factors in many diseases which do not follow simple Mendelian law.
Methylation as an epigenetic source of random genetic effects in the classical twin design
TLDR
It is conceptualized epigenetic processes as giving rise to randomness in the effects of polygenetic influences, which means that the regression coefficient in the regression of the phenotype on the polygenic factor, as specified in the twin model, varies over individuals.
Genome-wide methylation study on depression: differential methylation and variable methylation in monozygotic twins
TLDR
It is indicated that both differential methylation and differential variability have a role in the etiology and clinical manifestation of depression, and clues on specific genomic loci of potential interest in the epigenetics of depression are provided.
Why are monozygotic twins different?
TLDR
The genotypic and phenotypic differences between MZ twins are reviewed and their main causes are discussed to have long-term implications on complex diseases and their predisposition, organ transplantation and interpretation of twin-based studies.
Epigenetic Epidemiology of Psychiatric Disorders
TLDR
A clear role for epigenetic mechanisms in development and aging of the brain is described, with experiences and environmental exposures particularly during early life having considerable impact on the development of functional abilities of thebrain.
The Relationship of DNA Methylation with Age, Gender and Genotype in Twins and Healthy Controls
TLDR
The identification of CpG loci of which DNA methylation levels are under genetic control or are related to age or gender will facilitate further studies into the role ofDNA methylation and disease.
Epigenetics, Gender, and Sex in the Diagnosis of Depression
TLDR
Depression is redefined as a response to being in a seemingly hopeless situation accompanied by despair, either externalizing (more often diagnosed as substance use disorders, impulse control disorders, antisocial personality disorder, or bipolar disorder) or internalizing (the more standard diagnosis of depression).
...
...

References

SHOWING 1-10 OF 67 REFERENCES
Genetic or epigenetic difference causing discordance between monozygotic twins as a clue to molecular basis of mental disorders
TLDR
If the genetic or epigenetic difference responsible for the discordant phenotype is found, it will have impact on the biology of mental disorder, in which few conclusive molecular genetic evidences have been obtained.
Monozygotic twins exhibit numerous epigenetic differences: clues to twin discordance?
TLDR
Although the epigenetic analysis was conducted for only several hundred base pairs of DRD2, the fact that numerous studies identified nonuniform methylation patterns across the clones of bisulfite-modified DNA from the same individual, as well as non uniform patterns across different individuals, argues for the universality of intra- and interindividual epigenetic variation.
Differential methylation of the X‐chromosome is a possible source of discordance for bipolar disorder female monozygotic twins
  • A. Rosa, M. Picchioni, I. Craig
  • Biology, Psychology
    American journal of medical genetics. Part B, Neuropsychiatric genetics : the official publication of the International Society of Psychiatric Genetics
  • 2008
TLDR
Examination of X‐chromosome inactivation patterns in DNA samples from blood and/or buccal swabs in a series of 63 female MZ twin pairs concordant or discordant for BD or SZ and healthy MZ controls suggests a potential contribution from X‐linked loci to discordance within twin pairs for BD but is inconclusive for SZ.
Epigenetics and twins: three variations on the theme.
Mechanisms for differences in monozygous twins.
DNA methylation profiles in monozygotic and dizygotic twins
TLDR
The first annotation of epigenetic metastability of ∼6,000 unique genomic regions in MZ twins is provided, suggesting that molecular mechanisms of heritability may not be limited to DNA sequence differences.
Does Higher Concordance in Monozygotic Twins Than in Dizygotic Twins Suggest a Genetic Component?
TLDR
It is demonstrated that, even in the complete absence of any genetic factor and of any biases, the greater environmental similarity alone in MZ twins can result in higher concordance rate in Mz twins than in DZ twins.
Genomic discordance between monozygotic twins discordant for schizophrenia.
TLDR
Genomic DNA of monozygotic twins discordant for schizophrenia was analyzed to determine whether their genomes were truly identical, and at least two spots with autoradiographic intensities that obviously differed in the two twins were detected.
Phenotypic differences in genetically identical organisms: the epigenetic perspective.
TLDR
Epigenetic mechanisms may explain paradoxical findings in twin and inbred animal studies when Phenotypic differences occur in the absence of observable environmental differences and also when environmental differences do not significantly increase the degree of phenotypic variation.
Twin studies of schizophrenia: from bow-and-arrow concordances to star wars Mx and functional genomics.
TLDR
The five newest studies since 1995 from Europe and Japan have confirmed earlier findings on the genetic basis of clinical heterogeneity within schizophrenia and provided further insights into non-inherited factors that contribute to the multifactorial etiology of this disorder.
...
...