**7. Hypomethylation and chromosome decondensation may involve genome rearrangement**

Thus, the trophoblast cell population are prone to hypomethylation that may be a prerequisite of some other cytogenetic effects. Thus, prolonged culture of normal chorionic villus cells involves chromatin decondensation and rearrangements that mimics the ICF syndrome, i.e. immunodeficiency, centromeric region instability, and facial anomalies [69]. Thus, untreated cultures from normal chorionic villus or amniotic fluid-derived samples displayed dramatic cell passage-dependent increases in aberrations in the juxtacentromeric heterochromatin of chromosomes 1 or 16

**67**

*Genome Modifications Involved in Developmental Programs of the Placental Trophoblast*

(1qh or 16qh). By passage 8 or 9, 82 ± 7% of the chorionic villi metaphases from all eight studied samples exhibited 1qh or 16qh decondensation and 25 ± 16% had rearrangements in these regions. At early and late passages, chorionic villi DNA was hypomethylated, and amniotic fluid DNA was hypermethylated both globally and at Sat2. *DNMT1*, *DNMT3A*, or *DNMT3B* RNA levels did not differ significantly between chorionic and amniotic cultures, or late and early passages. Sat2 hypomethylation may favor 1qh and 16qh anomalies because the chorionic villi cultures, with their Sat2 hypomethylation, displayed 1qh and 16qh decondensation Therefore, hypomethylation characteristic of the embryonic and trophoblast cells may affect the structure heterochromatin regions leading to chromosome rearrangement that may be a source of genome variability at the stage of differentiation of trophoblast

The data of the present paper indicate that the multifold genome multiplication

The author is grateful to the Institute of Cytology for the support of the work as

well as Prof. N. Nikolsky and Dr. Grigori Stein for their helpful advices.

is not the only peculiarity of the placental trophoblast cell lineages. There are a range of other peculiarities that, most probably, result from cell cycle modification or play a role connected with genome duplication. Thus, underreplication and gene amplification may result in fine orchestration of gene copies necessary at a definite trophoblast cell line in order to accomplish its function. "Loose nucleosomes", most probably, represent one of peculiarities of endoreduplication; it cannot be ruled out that lack of mitoses do not require chromosome to proceed all levels of condensation and packaging of chromosomes that makes possible fastening of replication cycles and high level of transcription of different factors regulating formation of the provisory organ – placenta. Unmethylation of chromosomes including centromere regions may contribute to chromosome rise of aberrations that may be a source of genetic variability and selection of the optimal genetic structure that may

*DOI: http://dx.doi.org/10.5772/intechopen.97247*

have an adaptive effect during pregnancy.

The author declares no conflict of interests.

**Acknowledgements**

**Conflict of interest**

cell population.

**8. Conclusion**

*Genome Modifications Involved in Developmental Programs of the Placental Trophoblast DOI: http://dx.doi.org/10.5772/intechopen.97247*

(1qh or 16qh). By passage 8 or 9, 82 ± 7% of the chorionic villi metaphases from all eight studied samples exhibited 1qh or 16qh decondensation and 25 ± 16% had rearrangements in these regions. At early and late passages, chorionic villi DNA was hypomethylated, and amniotic fluid DNA was hypermethylated both globally and at Sat2. *DNMT1*, *DNMT3A*, or *DNMT3B* RNA levels did not differ significantly between chorionic and amniotic cultures, or late and early passages. Sat2 hypomethylation may favor 1qh and 16qh anomalies because the chorionic villi cultures, with their Sat2 hypomethylation, displayed 1qh and 16qh decondensation Therefore, hypomethylation characteristic of the embryonic and trophoblast cells may affect the structure heterochromatin regions leading to chromosome rearrangement that may be a source of genome variability at the stage of differentiation of trophoblast cell population.

## **8. Conclusion**

*Cytogenetics - Classical and Molecular Strategies for Analysing Heredity Material*

depletion of both *DNMT-3a* and *DNMT-3b* [65].

course of placenta formation.

proper placental development in mice [61].

**genome rearrangement**

Such a parallel with carcinogenesis may be connected with the trophoblast invasive pathways. Inhibition of DNA methylation by 5-azacytidine treatment disrupts trophoblast invasive and migratory potential *in vitro* [65] and proper placental development *in vivo* [66]. Thus, threatment of BeWo cell with DNA methyltransferase inhibitor, 5′-aza-2′-deoxycytidine (AZA) resulted in conversion to non-migratory phenotype. AZA was found to increase mRNA level of E-cadherin and plakoglobin, components of cell junction structures - zonula adherens and desmosomes [65]. AZA treatment also resulted in decrease their gene promoter activity and protein levels. Increases in plakoglobin and E-cadherin promoter activity and inhibition of BeWo cell migration was also achieved with small interfering RNA-mediated

Meantime, beside the trophoblast invasion, some DNA methylation (locusspecific and/or repeat-based) is important for differentiation of the functionally different trophoblast lineages forming the placental barrier and performing other placental specific functions. Thus, most homeobox genes were hypomethylated in the human placenta throughout gestation**.** Nevertheless, three homeobox genes, *TLX1*, *HOX10* and *DLX5* showed progressive methylation and decrease of their expression in the course of pregnancy – from first to third trimester. Using siRNA treatment the key role of *TLX1, HOXA10* and *DLX5* in trophoblast proliferation, differentiation and apoptosis was demonstrated [67]. It cannot be ruled out that progressive methylation of some homeobox genes promotes trophoblast differentiation into highly proliferative ones that, in turn, gives rise to villous cyto- and syncytiotrophoblast and the invasive trophoblast lineages. The data suggest an important role of several homeobox gene methylation in gene expression in the

In mice, *Dnmt3L* is expressed at high levels in the chorion, containing a multipotent trophoblast stem cell population. Disruption of *Dnmt3L* disturbs placental development including spongiotrophoblast and labyrinth malformation, leads to excess of TGC and defective attachment of the chorion to the ectoplacental cone. Excessive TGC development indicates that such a phenotype is bound to the hypomethylation. This is associated with an arrest of proliferation of the extraembryonic tissue [68]. It may be suggested that *Dnmt3L*-mediated *de novo* methylation is connected with initiation of differentiation of trophoblast into multiple lineages that imply maintenance of high level of mitotic proliferation regulated by Mash2 expression as well as syncytiotrophoblast formation demonstrated by *GCM1*

expression [68]. It suggests that *Dnmt3L*-mediated *de novo* methylation is critical for

Demonstration of placenta-specific hypomethylation of the *DNMT3L* gene supports a role in human placental development [61] probably bound to the start of differentiation of trophoblast into a range of lineages with different proliferative

**7. Hypomethylation and chromosome decondensation may involve** 

Thus, the trophoblast cell population are prone to hypomethylation that may be a prerequisite of some other cytogenetic effects. Thus, prolonged culture of normal chorionic villus cells involves chromatin decondensation and rearrangements that mimics the ICF syndrome, i.e. immunodeficiency, centromeric region instability, and facial anomalies [69]. Thus, untreated cultures from normal chorionic villus or amniotic fluid-derived samples displayed dramatic cell passage-dependent increases in aberrations in the juxtacentromeric heterochromatin of chromosomes 1 or 16

**66**

capacity.

The data of the present paper indicate that the multifold genome multiplication is not the only peculiarity of the placental trophoblast cell lineages. There are a range of other peculiarities that, most probably, result from cell cycle modification or play a role connected with genome duplication. Thus, underreplication and gene amplification may result in fine orchestration of gene copies necessary at a definite trophoblast cell line in order to accomplish its function. "Loose nucleosomes", most probably, represent one of peculiarities of endoreduplication; it cannot be ruled out that lack of mitoses do not require chromosome to proceed all levels of condensation and packaging of chromosomes that makes possible fastening of replication cycles and high level of transcription of different factors regulating formation of the provisory organ – placenta. Unmethylation of chromosomes including centromere regions may contribute to chromosome rise of aberrations that may be a source of genetic variability and selection of the optimal genetic structure that may have an adaptive effect during pregnancy.

#### **Acknowledgements**

The author is grateful to the Institute of Cytology for the support of the work as well as Prof. N. Nikolsky and Dr. Grigori Stein for their helpful advices.

## **Conflict of interest**

The author declares no conflict of interests.

*Cytogenetics - Classical and Molecular Strategies for Analysing Heredity Material*
