Mouse chromocenters DNA content: sequencing and in silico analysis

Abstract

BackgroundChromocenters are defined as a punctate condensed blocks of chromatin in the interphase cell nuclei of certain cell types with unknown biological significance. In recent years a progress in revealing of chromocenters protein content has been made although the details of DNA content within constitutive heterochromatin still remain unclear. It is known that these regions are enriched in tandem repeats (TR) and transposable elements. Quick improvement of genome sequencing does not help to assemble the heterochromatic regions due to lack of appropriate bioinformatics techniques.ResultsChromocenters DNA have been isolated by a biochemical approach from mouse liver cells nuclei and sequenced on the Illumina MiSeq resulting in ChrmC dataset. Analysis of ChrmC dataset by the bioinformatics tools available revealed that the major component of chromocenter DNA are TRs: ~ 66% MaSat and ~ 4% MiSat. Other previously classified TR families constitute ~ 1% of ChrmC dataset. About 6% of chromocenters DNA are mostly unannotated sequences. In the contigs assembled with IDBA_UD there are many fragments of heterochromatic Y-chromosome, rDNA and other pseudo-genes and non-coding DNA. A protein coding sfi1 homolog gene fragment was also found in contigs. The Sfi1 homolog gene is located on the chromosome 11 in the reference genome very close to the Golden Pass Gap (a ~ 3 Mb empty region reserved to the pericentromeric region) and proves the purity of chromocenters isolation. The second major fraction are non-LTR retroposons (SINE and LINE) with overwhelming majority of LINE - ~ 11% of ChrmC. Most of the LINE fragments are from the ~ 2 kb region at the end of the 2nd ORF and its’ flanking region. The precise LINEs’ segment of ~ 2 kb is the necessary mouse constitutive heterohromatin component together with TR. The third most abundant fraction are ERVs. The ERV distribution in chromocenters differs from the whole genome: IAP (ERV2 class) is the most numerous in ChrmC while MaLR (ERV3 class) prevails in the reference genome. IAP and its LTR also prevail in TR containing contigs extracted from the WGS dataset. In silico prediction of IAP and LINE fragments in chromocenters was confirmed by direct fluorescent in situ hybridization (FISH).ConclusionOur data of chromocenters’ DNA (ChrmC) sequencing demonstrate that IAP with LTR and a precise ~ 2 kb fragment of LINE represent a substantial fraction of mouse chromocenters (constitutive heteroсhromatin) along with TRs.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-4534-z) contains supplementary material, which is available to authorized users.

Heterochromatin Morphodynamics in Late Oogenesis and Early Embryogenesis of Mammals

“…The formation of the SN chromatin configuration in murine GV oocytes is accompanied by a redistribution of centromeric and pericentromeric heterochromatin [ 64 , 65 , 66 , 67 , 68 ], including the chromocenters—discrete chromatin structures highly enriched in tandem repeats and transposable elements [ 69 ]. In oocytes present in primordial and primary follicles and arrested in diplotene, centromeres and chromocenters are preferentially located at the periphery of the nucleus.…”

The epigenetic regulation of centromeres and telomeres in plants and animals

“…The majority of the assembled mammalian genomes contain a 3 Mb Golden Path Gap ( GPG ) empty region around each centromere. However, gradually, more and more data on the composition of the sequence of constitutive heterochromatin regions are becoming available ( Ostromyshenskii et al 2018 ). Constitutive heterochromatin turns out to be surprisingly heterogeneous, characterized by plasticity, and its epigenetic regulators depend on the genomic context in which it is present.…”

Early Critical Phase Transitions of Pericentromere-Associated Domains in MCF-7 Breast Cancer Cells Committed to Differentiation by Heregulin

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