The mechanisms underlying how monozygotic (or identical) twins arise are yet to be determined. Here, the authors investigate this in an epigenome-wide association study, showing that monozygotic... Show moreThe mechanisms underlying how monozygotic (or identical) twins arise are yet to be determined. Here, the authors investigate this in an epigenome-wide association study, showing that monozygotic twinning has a characteristic DNA methylation signature in adult somatic tissues.Monozygotic (MZ) twins and higher-order multiples arise when a zygote splits during pre-implantation stages of development. The mechanisms underpinning this event have remained a mystery. Because MZ twinning rarely runs in families, the leading hypothesis is that it occurs at random. Here, we show that MZ twinning is strongly associated with a stable DNA methylation signature in adult somatic tissues. This signature spans regions near telomeres and centromeres, Polycomb-repressed regions and heterochromatin, genes involved in cell-adhesion, WNT signaling, cell fate, and putative human metastable epialleles. Our study also demonstrates a never-anticipated corollary: because identical twins keep a lifelong molecular signature, we can retrospectively diagnose if a person was conceived as monozygotic twin. Show less
DNA methylation quantitative trait locus (mQTL) analyses on 32,851 participants identify genetic variants associated with DNA methylation at 420,509 sites in blood, resulting in a database of >... Show moreDNA methylation quantitative trait locus (mQTL) analyses on 32,851 participants identify genetic variants associated with DNA methylation at 420,509 sites in blood, resulting in a database of >270,000 independent mQTLs.Characterizing genetic influences on DNA methylation (DNAm) provides an opportunity to understand mechanisms underpinning gene regulation and disease. In the present study, we describe results of DNAm quantitative trait locus (mQTL) analyses on 32,851 participants, identifying genetic variants associated with DNAm at 420,509 DNAm sites in blood. We present a database of >270,000 independent mQTLs, of which 8.5% comprise long-range (trans) associations. Identified mQTL associations explain 15-17% of the additive genetic variance of DNAm. We show that the genetic architecture of DNAm levels is highly polygenic. Using shared genetic control between distal DNAm sites, we constructed networks, identifying 405 discrete genomic communities enriched for genomic annotations and complex traits. Shared genetic variants are associated with both DNAm levels and complex diseases, but only in a minority of cases do these associations reflect causal relationships from DNAm to trait or vice versa, indicating a more complex genotype-phenotype map than previously anticipated. Show less
Osteoarthritis (OA) is a prevalent and complex disorder with a high hereditary probability. Previous genetic research into OA has yielded several common gene variants contributing to joint... Show moreOsteoarthritis (OA) is a prevalent and complex disorder with a high hereditary probability. Previous genetic research into OA has yielded several common gene variants contributing to joint specific OA phenotypes. Only a few studies allowed an examination of multiple joint sites in the patients included. In this thesis, we have focused, on the identification and investigation of OA susceptibility of rare and common generalised OA (GOA) in family based studies and common OA in the population. We investigated previously reported relationships between two candidate genes (FRZB and MATN3) with OA in a random sample from the population-based Rotterdam study and in siblings from the Genetics Osteoarthritis and Progression (GARP) study. A functional variant in FRZB indeed associates to OA but this seems not confined to hip only. Associations of MATN3 variants suggest that genetic variation in this gene determines susceptibility to spinal disc degeneration and OA of the first carpometacarpal joint. Different linkage areas were identified for early and late onset GOA. We mapped a major locus for OA at multiple joint sites on 14q32.11 in middle aged siblings from the GARP study and on 2q33.3 in seven early onset families. From our studies so far, DIO2, IDH1 and NRP2 may be new OA loci. Show less
