Molecular Characterization Of The SLC22A18AS Gene From The Imprinted Human Chromosome Segment 11p15.5
The imprinting status of the SLC22A18AS gene, located in the human chromosome segment 11p15.5, was studied using PCR-SSCP analysis and fetal tissues from a battery of 17 abortuses. This gene showed monoallelic expression (genomic imprinting) in different tissues from two abortuses which were heterozygous for an SNP (c.473G>A) in its coding region. This gene was found to be paternally imprinted (maternally expressed) in five tissues namely lung, liver, brain, kidney and placenta from an abortus. The parental origin of the expressed allele could not be determined in the second abortus as both the mother and the abortus were heterozygous for the SNP. Since paternal blood samples from none of the 17 abortuses could be collected for DNA isolation, the mother's genotype was used to find the origin of the expressed allele. In order to understand the mechanism underlying imprinting of this gene, it was important to understand the nature of the epigenetic marks (imprints) on the two alleles of this gene. Since these epigenetic marks are generally observed in promoters or CpG islands associated with the imprinted genes, the promoters of the SLC22A18AS gene was characterized using transient transfection of putative SLC22A18AS promoter fragments cloned in the pGL3-Basic vector in human cells followed by luciferase reporter assay. Since the promoter of a gene lies upstream to the transcription start site (TSS), TSS of this gene was mapped. In silico approach revealed an EST (CB129046) which had an additional 39 bases upstream to the known mRNA sequence. TSS was then identified by the 5’ primer extension analysis. TSS was found to be 166 bases upstream to the 5’ end of this EST. In order to select cell lines for transient transfection of putative promoter constructs for promoter charaterization, RT-PCR analysis was used to see the expression of this gene in the following available cell lines in the lab: HuH7, HepG2, A549, HeLa, LNCaP and PC3. This gene was found to be maximally expressed in HepG2 cells. Expression of this gene was also observed in A549, HeLa, LNCaP and PC3 cells. HuH7, on the other hand, did not show any detectable expression of this gene. Based on the above data, HepG2 and A549 cells were selected for promoter characterization. Seven putative promoter constructs were transiently transfected in these cells and the promoter activity of different constructs was measured by luciferase assay. The assay identified two promoters for the gene: P1 promoter in a region from -855 to -254 bp and P2 promoter in a region from -1441 to -855. In order to see the presence of putative transcription factor binding sites in the upstream region of the gene, the MatInspector Professional program was used. The gene was found to be devoid of TATA and CCAAT boxes. Most of the putative transcription factor binding sites were present in a region from -855 to -254 bp which spans the P1 promoter, including a binding site for the Sp1 transcription factor. In order to see if Sp1 binds to the promoter of this gene, ChIP assay was performed. Sp1 was shown to bind the region harboring the P1 promoter. In order to see if Sp1 has a role in the regulation of this gene, Sp1 constructs were co-transfected with the SLC22A18AS P1 promoter construct in HepG2 and Sp1-null Drosophila SL2 cells. The results showed that the Sp1 has a positive regulation on the SLC22A18AS promoter activity. As stated earlier, epigenetic marks such as differential methylation of CpG dinucleotides in two alleles are associated with promoters of the genes. Since the promoters for SLC22A18AS were characterized, the presence of allele-specific differentially methylated regions (DMRs) associated with the promoters was investigated. In order to differentiate the two alleles in the promoter regions by SNPs, DNA sequence analysis of the promoter regions was performed in a battery of 17 abortuses to search for SNPs. Abortus no. 3 showed heterozygosity for a C to A change at nucleotide position -445 in the P1 promoter region, while abortus no. 2 showed heterozygosities for G to A and A to G changes at nucleotide positions -919 and -1321 respectively in the P2 promoter region. The alleles in the abortus no. 3 were designated as allele C and allele A. The alleles in abortus no. 2 were designated as allele GG and allele AA. Once the two alleles were differentiated by these SNPs, identification of DMRs was performed using sodium bisulfite genomic DNA sequencing. Genomic DNA from the abortus no. 3 was taken for the identification of DMR in the P1 promoter region, while genomic DNA from abortus no. 2 was taken for the identification of DMR in the P2 promoter region. Sodium bisulfite genomic DNA sequencing of the P1 promoter region showed heavy methylation of both the alleles. No DMR was observed in this region. Sodium bisulfite genomic DNA sequencing of the P2 promoter region using DNA from abortus no. 2 did not show any differential methylation of the two alleles. However, like the P1 promoter region, the P2 promoter region was also heavily methylated. In order to see the methylation status of both the promoter regions in human sperms, sperm DNA from an unrelated healthy volunteer was also subjected to sodium bisulfite genomic sequencing. A dense methylation was observed in both the promoter regions of the gene. Heavy methylation of CpG dinucleotides in these regions corroborates the imprinting result for this gene. Since the methylation epigenetic mark is also known to be associated with CpG islands, CpG Plot/CpG Report analysis was used to identify CpG islands in this gene. The analysis showed the presence of two CpG islands, CpG I and CpG II, in the second intron of the gene. As the CpG I island is known to lack methylated CpGs (Ali et al., unpublished result from our lab), a DMR was sought for the CpG II island region. Heterozygosity was ascertained in this region by sequencing DNA from 17 abortuses. However, none of the abortuses showed heterozygosity. It was reasoned that if there is a differential methylation of the two alleles in this region, half of the clones (alleles) should be unmethylated, and the other half should show methylation. Therefore, DNA from abortus no. 3 was randomly chosen for sodium bisulfite genomic sequence anaylsis to identify DMR. The CpG II island showed heavy methylation. However, a DMR was not identified. In order to see the methylation status of the CpG II island in human sperms, sperm DNA from an unrelated healthy volunteer was also subjected to sodium bisulfite genomic sequencing. Almost all the CpG sites showed methylation. The observation of a dense methylation of both the promoters and CpG II island suggested that methylation has a role in the expression of this gene. In order to confirm this observation, A549 and HuH7 cells were treated with a methyltransferase inhibitor, 5-aza-2’-deoxycytidine. 5-Aza-2’-deoxycytidine treatment in HuH7 cells restored the expression of this gene. Further, the expression of this gene was increased in A549 cells following the drug treatment. These results suggested that DNA methylation has a definite role in the modulation of expression of the SLC22A18AS gene. Histone acetylation is another key epigenetic player which is known to have a role in the expression of genes. In order to study the role of the histone acetylation, HuH7 and A549 cells were treated with TSA, a histone deacetylase inhibitor. Treatment of HuH7 and A549 cells with TSA didn’t have any effect on the expression of this gene. On the other hand, the expression of TPA, a gene shown to be regulated by TSA earlier, increased following the TSA treatment in both cell lines. These results suggested that histone acetylation doesn’t have any effect on the expression of this gene. Based on this observation, it was reasoned that histone acetylation is not associated with the imprinting of this gene. Therefore, we did not look for the allele-specific acetylation of histones in this gene. The SLC22A18AS gene has a weak ORF of 253 amino acids as the translation intiation site does not contain a consensus Kozak sequence for efficient translation. In order to determine if it codes for a protein, Western blot analysis was performed using lysates from A549 cells and human fetal liver tissue, and a polyclonal antibody raised in a rabbit against a bacterially expressed SLC22A18AS protein fragment from amino acids 138 to 245. The Western blot result was negative. It was reasoned that this gene might be expressed at a low level and therefore its expression could not be detected by Western blot analysis. Immunoprecipitation was then performed to enrich the SLC22A18AS protein in the lysates followed by Western blot analysis. SLC22A18AS was shown to be expressed as a 30 kDa band in the immunoprecipiates from A549 cell and human fetal liver tissue lysates. The subcellular localization of this gene was studied by immunofluorescence. The fluorescence immunolocalization was performed on A549 cells with anti-SLC22A18AS antibody. The SLC22A18AS protein was found to be localized in the cytoplasm of A549 cells.