Table 1 Epigenetic studies using discordant MZ twins*

Adolescent depression

Illumina 450 K

Buccal cells (no correction)

Two differentially methylated positions reproducible in brain

18 discordant pairs of MZ twins

Validation: bisulfite pyrosequencing

[[67]]

Alzheimer’s disease

Immunohistochemistry for 5-methylcytosine

Temporal neocortex (no correction)

Significantly reduced levels of DNA methylation

1 discordant MZ twin pair

[[68]]

Autism spectrum disorders

Custom array^§

Lymphoblastoid cells (no correction)

73 differentially methylated CpG islands

3 pairs of male discordant MZ twin pairs

Validation: bisulfite sequencing and methylation-specific PCR

[[69]]

Autism spectrum disorders

Illumina 27 K

Whole blood (no correction)

Differentially methylated regions in genes already mentioned in the literature

34 discordant and 5 concordant MZ twin pairs for ASD or a related trait

Validation: bisulfite pyrosequencing

[[70]]

Bipolar disorder

Illumina 27 K

Whole blood (no correction)

Hypomethylation in the upstream region of GPR24

11 discordant MZ twin pairs

[[71]]

Bipolar disorder

MS-RDA

Lymphoblastoid cells (no correction)

Hypomethylation of PPIEL

1 discordant MZ twin pair, 23 unrelated cases, and 18 unrelated controls

Validation: bisulfite pyrosequencing

[[72]]

Birth weight

Illumina 450 K

Saliva (correction for buccal epithelium and leukocyte subtypes)

45 differentially methylated CpG sites

17 discordant monochorionic MZ female twin pairs

Validation: bisulfite sequencing

[[60]]

Breast cancer

Illumina 450 K

Whole blood (no correction)

DOK7 as a candidate for blood-based cancer diagnosis

15 discordant MZ twin pairs

Validation: bisulfite pyrosequencing of 21 discordant MZ twin pairs

[[73]]

Caudal duplication anomaly

Targeted bisulfite sequencing

PBMCs and buccal cells (no correction)

Hypermethylation of the AXIN1 promoter in PBMCs, but not in buccal epithelia

1 discordant MZ twin pair

[[74]]

Childhood leukemia and secondary thyroid carcinoma

Targeted bisulfite pyrosequencing

Primary skin fibroblasts (no correction)

Increased methylation of BRCA1

1 discordant MZ twin pair

[[75]]

Congenital renal agenesis

RRBS

Whole blood (no correction)

514 DMRs in 25 genes, including 6 related to organ development

1 discordant MZ twin pair

[[76]]

Major depressive disorder

MeDIP-seq

Whole blood (correction for lymphocytes, monocytes, neutrophils, and eosinophils)

Hypermethylation of a region within the ZBTBT20 gene

50 discordant MZ twin pairs

Replication: MeDIP-seq in 356 unrelated case–control samples

[[77]]

Major psychosis

Illumina 27 K

Whole blood (no correction)

Hypomethylation in the promoter region of ST6GALNAC1

22 discordant MZ twin pairs

Replication: EpiTYPER in 45 post-mortem brain samples

[[71]]

Multiple sclerosis

RRBS

CD4⁺ lymphocytes (no correction)

No reproducible differences were detected

3 discordant MZ twin pairs

[[78]]

Pain sensitivity

MeDIP-seq

Whole blood (correction for lymphocytes, monocytes, neutrophils, and eosinophils)

Significant association signal in the promoter of the gene TRPA1

25 discordant MZ twin pairs

Replication: MeDIP-seq in 50 unrelated samples. Validation: bisulfite pyrosequencing and Illumina 450 k

[[79]]

Psoriasis

Illumina 27 K

CD4⁺ and CD8⁺ cells (no correction)

Correlation between DNA methylation differences and gene expression differences of genes involved in the immune response only in CD4⁺

CD4⁺: 17 discordant MZ twins pair; CD8⁺: 13 discordant MZ twins pairs

[[80]]

Schizophrenia

Illumina 27 K

Whole blood (no correction)

Hypermethylation in the upstream region of PUS3

11 discordant MZ twin pairs

[[71]]

Schizophrenia

Targeted bisulfite sequencing

Lymphocytes (no correction)

Patterns of methylation in the 5'-regulatory region of DRD2 gene are closer between affected individuals

1 discordant and 1 concordant MZ twin pair

[[81]]

Systemic lupus erythematosus

Illumina GoldenGate Methylation Cancer Panel I

White blood cells (no correction)

49 differentially methylated genes potentially relevant in autoimmune inflammatory diseases processes

5 discordant MZ twin pairs

Validation: bisulfite sequencing

[[82]]

Type 1 diabetes

Illumina 27 K

CD14⁺ monocytes (no correction)

132 differentially methylated CpG sites

15 discordant MZ pairs

Validation: bisulfite pyrosequencing. Replication: Illumina 27 K of 4 twin pairs

[[17]]

Type 1 diabetes

Illumina 27 K

Immortalized B cell lines (no correction)

88 differentially methylated CpG sites

3 discordant and 6 concordant MZ twin pairs

Validation: bisulfite sequencing

[[83]]

Ulcerative colitis

Illumina 27 K and MeDIP-chip

Intestinal biopsies (no correction)

61 disease-associated genes

10 discordant MZ pairs

Validation: bisulfite pyrosequencing

[[84]]