Developmental and tissue-specific regulation of the Q10 class I gene by DNA methylation

De novo methylation of an MHC class I transgene following transformation with human adenoviruses is not correlated with its altered expression

The biological importance of class I histocompatibility antigens in a large variety of immune mechanisms is widely recognized, and their role in tumor rejection has been proven in several experimental tumor systems. Reduced expression of class I antigens, which is correlated with enhanced tumorigenicity, was shown in these systems to be mainly the result of transcriptional down-regulation. Mouse embryonal fibroblasts expressing H-2 antigens and the product of a miniature swine class I transgene, transformed by adenovirus 12, exhibit low levels of all class I antigens on the cell surface. Half of the cell lines demonstrate a suppressed level of class I mRNAs. Cell lines derived from transformation with the early region of adenovirus 5 express a high level of class I antigens. DNAs from adenovirus-transformed cells are extensively hypermethylated both in the 5' and the coding regions of the transgene compared to DNAs from immortalized cell lines and primary embryonal fibroblasts. Nevertheless, hypermethylation of these sequences is not correlated with mRNA level or cell-surface expression of the transgene product. Treatment of the transformed cells with high concentration of 5-azacytidine (5 Aza-C) induced merely a minor enhancement in the expression of class I mRNAs and class I antigens. Thus, this system is a perfect example of where viral transformation is associated with induced methylation of a class I gene, but hypermethylation does not affect its expression. The role of de novo methylation of genes in this system might be associated with transformation, or generation of mutations in CpG-rich sequences.

Tumorigenicity of mouse T lymphoma cells is controlled by the level of major histocompatibility complex class I H-2Kk antigens

We have previously found that an increased tumorigenicity and spontaneous metastatic potential of BW5147-derived T lymphoma cells was associated with a decrease in major histocompatibility complex (MHC) class I H-2Kk antigen expression. This suggested that H-2Kk antigens may control the tumorigenic potential of BW T lymphoma cells. Our current experiments aimed to prove this association by specifically altering H-2Kk expression by gene transfection. Transfected cells expressing a high level of H-2Kk antigens were significantly less tumorigenic and metastatic after subcutaneous inoculation. However, there was selection in vivo for cells expressing a reduced level of H-2Kk antigens, which concomitantly led to an increased tumorigenicity. These data further confirmed the strong association between H-2Kk expression and tumorigenicity. We subsequently tested whether the immune system is implicated in this phenomenon by inoculating the H-2Kk transfectants into irradiated, immunocompromised recipients. Our results indicate that the reduced tumorigenicity of the BW H-2Kk transfectants is due to an immune rejection mechanism, mediated by CD8+ immune effector cells, as revealed by in vivo depletion experiments with anti-CD8 antibodies. Hence, we hereby demonstrated that H-2Kk antigens increased the immunogenicity of BW cells, via a CD8-dependent mechanism, which consequently reduced their tumorigenicity.

Parent-specific expression of a human keratin 18/beta-galactosidase fusion gene in transgenic mice

Insertion of a human keratin 18 (K18)-bacterial beta-galactosidase (LacZ) fusion gene into mice has led to a unique transgenic line in which expression of the transgene is subject to unusual germ line-specific, genomic imprinting effects. Fetal expression of the LacZ reporter gene depends on the gender of the transmitting parent, with appropriate expression in liver after maternal inheritance, and ectopic expression in retina and mesodermal tissues after paternal inheritance. This tissue-specific imprinting pattern is superimposed upon a basic expression pattern which is unaffected by parental inheritance. Insertion of the transgene has led to a recessive-lethal phenotype, with no parent-of-origin effects on viability, suggesting that the transgene has not inserted into an imprinted region of the genome. HpaII and HhaI methylation sensitive restriction sites within the bacterial LacZ reporter gene are completely methylated when activity of the maternally inherited transgene is detected in the fetal liver, and not methylated when the paternally inherited transgene is silent. Thus DNA methylation of LacZ is correlated with maternal inheritance and may be implicated in the genomic imprinting mechanism as others have suggested. However, in contrast to the commonly found correlation of expression and low DNA methylation, the LacZ gene was expressed in fetal liver when fully methylated. This result may imply the existence of negative regulatory activities that recognize the unmethylated LacZ gene.

Perinatal expression of the atrial natriuretic factor gene in rat cardiac tissue

The expression of the atrial natriuretic factor (ANF) gene was examined in the perinatal rat ventricle. The late fetal ventricle (day -3 relative to parturition) demonstrated a modest level of ANF gene expression. This expression increased at the time of birth and peaked on day +1 after birth. Atrial ANF immunoreactivity (irANF) increased gradually during the postpartum period (day +1 to 2-3 wk of age), while ventricular irANF as well as ventricular ANF mRNA levels fell over the same interval. This decrease in ANF gene expression resulted from a decrease in activity of the same major transcription start site employed in the adult atria and ventricle. Plasma irANF levels were very high in the neonatal period and decreased over the ensuing 2-3 wk, a profile similar to that seen for ventricular ANF gene expression. In situ hybridization histochemistry revealed that, while adult rat ventricular ANF mRNA was predominantly confined to the subendocardial layer, neonatal ventricles expressed the ANF gene throughout approximately 50% of the inner wall thickness. These findings suggest that the ANF gene is responding to regulatory stimuli that appear at or near the time of parturition and dissipate in the early neonatal period.