Epigenetics in Turner syndrome

Abstract

Background: Monosomy of the X chromosome is the most frequent genetic abnormality in human as it is present in approximately 2% of all conceptions, although 99% of these embryos are spontaneously miscarried. In postnatal life, clinical features of Turner syndrome may include typical dysmorphic stigmata, short stature, sexual infantilism, and renal, cardiac, skeletal, endocrine and metabolic abnormalities.

Main text: Turner syndrome is due to a partial or total loss of the second sexual chromosome, resulting in the development of highly variable clinical features. This phenotype may not merely be due to genomic imbalance from deleted genes but may also result from additive influences on associated genes within a given gene network, with an altered regulation of gene expression triggered by the absence of the second sex chromosome. Current studies in human and mouse models have demonstrated that this chromosomal abnormality leads to epigenetic changes, including differential DNA methylation in specific groups of downstream target genes in pathways associated with several clinical and metabolic features, mostly on autosomal chromosomes. In this article, we begin exploring the potential involvement of both genetic and epigenetic factors in the origin of X chromosome monosomy. We review the dispute between the meiotic and post-zygotic origins of 45,X monosomy, by mainly analyzing the findings from several studies that compare gene expression of the 45,X monosomy to their euploid and/or 47,XXX trisomic cell counterparts on peripheral blood mononuclear cells, amniotic fluid, human fibroblast cells, and induced pluripotent human cell lines. From these studies, a profile of epigenetic changes seems to emerge in response to chromosomal imbalance. An interesting finding of all these studies is that methylation-based and expression-based pathway analyses are complementary, rather than overlapping, and are correlated with the clinical picture displayed by TS subjects.

Conclusions: The clarification of these possible causal pathways may have future implications in increasing the life expectancy of these patients and may provide informative targets for early pharmaceutical intervention.

Keywords: Aneuploidy; Chromatin; DNA methylation; Embryonic stem cells; Epigenetics; Gene expression; Mouse models; Turner syndrome.

Fig. 1

Effects of X chromosome monosomy on gene transcription. X chromosome monosomy and epigenetic modifications. DNA is packed (compacted) into chromatin (DNA associated with histone and non-histone proteins) whose packaging unit is the nucleosome: 147 base pairs of DNA wound around an eight-histone protein (octamer) core. Gene expression rests on the chromatin state: (a) an open, accessible, and active chromatin (euchromatin) is associated with gene expression due to the combination of an histone code (H4Kac, H3K4, H3K36) and un(hypo)methylated CpG islands, which allow access of the transcriptional machinery (RNA polymerase and associated factors), and (b) a closed, inaccessible, and inactive chromatin (facultative heterochromatin) is associated with gene silencing due to the combination of an histone code (histone deacetylation, H3K9me3, H3K27me3) and (hyper)methylated CpG islands, which prevent access of the transcriptional machinery. H4Kac: lysine acetylation at histone 4; H3K4: methylation of lysine 4 residues at histone 3; H3K36: methylation of lysine 36 residues at histone 3; H3K9me3: trimethylation of lysine 9 residues at histone 3; H3K27me3: trimethylation of lysine 27 residues at histone 3; CpG: un-, hypo-, hyper-methylated: addition or removal of methyl groups to the 5′-carbon of cytosine, especially on CpG dinucleotides enriched in small regions of DNA (< 500 bp); MI or MII: meiosis I or meiosis II; ncRNA: noncoding RNA; miRNA: micro-noncoding RNA; IUGR: intrauterine growth retardation; HTA: arterial hypertension; T2DM type 2 diabetes mellitus