High-mobility group AT-hook 2, also known as HMGA2, is a protein that, in humans, is encoded by the HMGA2 gene. [3] [4] [5]
High-mobility group AT-hook 2, also known as HMGA2, is a protein that, in humans, is encoded by the HMGA2 gene. [3] [4] [5]
This gene encodes a protein that belongs to the non-histone chromosomal high-mobility group (HMG) protein family. HMG proteins function as architectural factors and are essential components of the enhanceosome. This protein contains structural DNA-binding domains and may act as a transcriptional regulating factor. Identification of the deletion, amplification, and rearrangement of this gene that are associated with lipomas suggests a role in adipogenesis and mesenchymal differentiation. A gene knock-out study of the mouse counterpart demonstrated that this gene is involved in diet-induced obesity. Alternate transcriptional splice variants, encoding different isoforms, have been characterized. [5]
The expression of HMGA2 in adult tissues is commonly associated with both malignant and benign tumor formation, as well as certain characteristic cancer-promoting mutations. Homologous proteins with highly conserved sequences are found in other mammalian species, including lab mice ( Mus musculus ).
HMGA2 contains three basic DNA-binding domains (AT-hooks) that cause the protein to bind to adenine-thymine (AT)-rich regions of nuclear DNA. HMGA2 does not directly promote or inhibit the transcription of any genes, but alters the structure of DNA and promotes the assembly of protein complexes that do regulate the transcription of genes. With few exceptions, HMGA2 is expressed in humans only during early development, and is reduced to undetectable or nearly undetectable levels of transcription in adult tissues. [6] The microRNA let-7 is largely responsible for this time-dependent regulation of HMGA2. [7] The apparent function of HMGA2 in proliferation and differentiation of cells during development is supported by the observation that mice with mutant HMGA2 genes are unusually small (the pygmy or mini-mouse phenotype), [8] and genome-wide association studies linking HMGA2-associated SNPs to variation in human height. [9]
Let-7 inhibits production of specific proteins by complementary binding to their mRNA transcripts. The HMGA2 mature mRNA transcript contains seven regions complementary or nearly complementary to let-7 in its 3' untranslated region (UTR). [10] Let-7 expression is very low during early human development, which coincides with the greatest transcription of HMGA2. The time-dependent drop in HMGA2 expression is caused by a rise in let-7 expression. [7]
Heightened expression of HMGA2 is found in a variety of human cancers, but the precise mechanism by which HMGA2 contributes to the formation of cancer is unknown. [11] [12] The same mutations that lead to pituitary adenomas in mice can be found in similar cancers in humans. [11] Its presence is associated with poor prognosis for the patient, but also with sensitization of the cancer cells to certain forms of cancer therapy. [13] To be specific, HMGA2-high cancers display an abnormally strong response to double strand breaks in DNA caused by radiation therapy and some forms of chemotherapy. Artificial addition of HMGA2 to some forms of cancer unresponsive to DNA damage cause them to respond to the treatment instead, although the mechanism by which this phenomenon occurs is also not understood. [13] However, the expression of HMGA2 is also associated with increased rates of metastasis in breast cancer, and both metastasis and recurrence of squamous cell carcinoma. These properties are responsible for patients' poor prognoses. As with HMGA2's effects on the response to radiation and chemotherapy, the mechanism by which HMGA2 exerts these effects is unknown. [13]
A very common finding in HMGA2-high cancers is the under-expression of let-7. [14] This is not unexpected, given let-7's natural role in the regulation of HMGA2. However, many cancers are found with normal levels of let-7 that are also HMGA2 high. Many of these cancers express the normal HMGA2 protein, but the mature mRNA transcript is truncated, missing a portion of the 3'UTR that contains the critical let-7 complementary regions. Without these, let-7 is unable to bind to HMGA2 mRNA, and, thus, is unable to repress it. The truncated mRNAs may arise from a chromosomal translocation that results in loss of a portion of the HMGA2 gene. [10]
Overexpressed HMGA2 may play a role in the frequent repression of ERCC1 in cancers. The let-7a miRNA normally represses the HMGA2 gene, and in normal adult tissues, almost no HMGA2 protein is present. [15] (See also Let-7 microRNA precursor.) Reduction or absence of let-7a miRNA allows high expression of the HMGA2 protein. As shown by Borrmann et al., [16] HMGA2 targets and modifies the chromatin architecture at the ERCC1 gene, reducing its expression. These authors noted that repression of ERCC1 (by HGMA2) can reduce DNA repair, leading to increased genome instability.
ERCC1 protein expression is reduced or absent in 84% to 100% of human colorectal cancers. [17] [18] ERCC1 protein expression was also reduced in a diet-related mouse model of colon cancer. [19] As indicated in the ERCC1 article, however, two other epigenetic mechanisms of repression of ERCC1 also may have a role in reducing expression of ERCC1 (promoter DNA methylation and microRNA repression).
Genome-wide analysis of HMGA2 target genes was performed by chromatin immunoprecipitation in a gastric cell line with overexpressed HMGA2, and 1,366 genes were identified as potential targets. [20] The pathways they identified as associated with malignant neoplasia progression were the adherens junction pathway, MAPK signaling pathway, Wnt signaling pathway, p53 signaling pathway, VEGF signaling pathway, Notch signaling pathway, and TGF beta signaling pathway.
Overexpression of HMGA2 delayed the release of DNA-PKcs (needed for non-homologous end joining DNA repair) from double strand break sites. Overexpression of HMGA2 alone was sufficient to induce chromosomal aberrations, a hallmark of deficiency in NHEJ-mediated DNA repair. These properties implicate HMGA2 in the promotion of genome instability and tumorigenesis. [21] showed that
HMGA2 protein can cleave DNA containing apurinic/apyrimidinic (AP) sites (is an AP lyase). In addition, this protein also possesses the related 5’-deoxyribosyl phosphate (dRP) lyase activity. An interaction between human AP endonuclease 1 and HMGA2 in cancer cells has been demonstrated indicating that HMGA2 can be incorporated into the cellular base excision repair (BER) machinery. Increased expression of HMGA2 increased BER, and allowed cells with increased HMGA2 to be resistant to hydroxyurea, a chemotherapeutic agent for solid tumors. [22]
HMGA2 has been shown to interact with PIAS3 [23] and NFKB1. [24]
The transport of HMGA2 to the nucleus is mediated by an interaction between its second AT-hook and importin-α2. [8]
In genetics, a promoter is a sequence of DNA to which proteins bind to initiate transcription of a single RNA transcript from the DNA downstream of the promoter. The RNA transcript may encode a protein (mRNA), or can have a function in and of itself, such as tRNA or rRNA. Promoters are located near the transcription start sites of genes, upstream on the DNA . Promoters can be about 100–1000 base pairs long, the sequence of which is highly dependent on the gene and product of transcription, type or class of RNA polymerase recruited to the site, and species of organism.
The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG islands.
Myc is a family of regulator genes and proto-oncogenes that code for transcription factors. The Myc family consists of three related human genes: c-myc (MYC), l-myc (MYCL), and n-myc (MYCN). c-myc was the first gene to be discovered in this family, due to homology with the viral gene v-myc.
The Let-7 microRNA precursor was identified from a study of developmental timing in C. elegans, and was later shown to be part of a much larger class of non-coding RNAs termed microRNAs. miR-98 microRNA precursor from human is a let-7 family member. Let-7 miRNAs have now been predicted or experimentally confirmed in a wide range of species (MIPF0000002). miRNAs are initially transcribed in long transcripts called primary miRNAs (pri-miRNAs), which are processed in the nucleus by Drosha and Pasha to hairpin structures of about 70 nucleotide. These precursors (pre-miRNAs) are exported to the cytoplasm by exportin5, where they are subsequently processed by the enzyme Dicer to a ~22 nucleotide mature miRNA. The involvement of Dicer in miRNA processing demonstrates a relationship with the phenomenon of RNA interference.
HMGA is a family of high mobility group proteins characterized by an AT-hook. They code for a "small, nonhistone, chromatin-associated protein that has no intrinsic transcriptional activity but can modulate transcription by altering the chromatin architecture". Mammals have two orthologs: HMGA1 and HMGA2.
DNA-dependent protein kinase, catalytic subunit, also known as DNA-PKcs, is an enzyme that in humans is encoded by the gene designated as PRKDC or XRCC7. DNA-PKcs belongs to the phosphatidylinositol 3-kinase-related kinase protein family. The DNA-Pkcs protein is a serine/threonine protein kinase consisting of a single polypeptide chain of 4,128 amino acids.
DNA excision repair protein ERCC-1 is a protein that in humans is encoded by the ERCC1 gene. Together with ERCC4, ERCC1 forms the ERCC1-XPF enzyme complex that participates in DNA repair and DNA recombination.
SRY -box 2, also known as SOX2, is a transcription factor that is essential for maintaining self-renewal, or pluripotency, of undifferentiated embryonic stem cells. Sox2 has a critical role in maintenance of embryonic and neural stem cells.
RAD52 homolog , also known as RAD52, is a protein which in humans is encoded by the RAD52 gene.
Lipoma-preferred partner is a protein that in humans is encoded by the LPP gene.
Forkhead box protein N3 is a protein that in humans is encoded by the FOXN3 gene.
The LBH gene is a highly conserved human gene that produces the LBH protein, a transcription co-factor in the Wnt/β-catenin pathway. Upon transcriptional activation of β-catenin, LBH goes on to act as a regulator of cell proliferation and differentiation through multiple transcriptional targets. The gene is located on the p arm of chromosome 2 and is roughly 28 kb long. Current ongoing studies are examining its role in developmental and oncological settings.
DNA-directed RNA polymerase I subunit RPA34 is an enzyme that in humans is encoded by the CD3EAP gene.
Insulin-like growth factor 2 mRNA-binding protein 2 is a protein that in humans is encoded by the IGF2BP2 gene.
Lipoma HMGIC fusion partner is a protein that in humans is encoded by the LHFP gene.
Lipoma HMGIC fusion partner-like 1 protein is a protein that in humans is encoded by the LHFPL1 gene.
Lipoma HMGIC fusion partner-like 2 protein is a protein that in humans is encoded by the LHFPL2 gene.
Genome instability refers to a high frequency of mutations within the genome of a cellular lineage. These mutations can include changes in nucleic acid sequences, chromosomal rearrangements or aneuploidy. Genome instability does occur in bacteria. In multicellular organisms genome instability is central to carcinogenesis, and in humans it is also a factor in some neurodegenerative diseases such as amyotrophic lateral sclerosis or the neuromuscular disease myotonic dystrophy.
Cancer epigenetics is the study of epigenetic modifications to the DNA of cancer cells that do not involve a change in the nucleotide sequence, but instead involve a change in the way the genetic code is expressed. Epigenetic mechanisms are necessary to maintain normal sequences of tissue specific gene expression and are crucial for normal development. They may be just as important, if not even more important, than genetic mutations in a cell's transformation to cancer. The disturbance of epigenetic processes in cancers, can lead to a loss of expression of genes that occurs about 10 times more frequently by transcription silencing than by mutations. As Vogelstein et al. points out, in a colorectal cancer there are usually about 3 to 6 driver mutations and 33 to 66 hitchhiker or passenger mutations. However, in colon tumors compared to adjacent normal-appearing colonic mucosa, there are about 600 to 800 heavily methylated CpG islands in the promoters of genes in the tumors while these CpG islands are not methylated in the adjacent mucosa. Manipulation of epigenetic alterations holds great promise for cancer prevention, detection, and therapy. In different types of cancer, a variety of epigenetic mechanisms can be perturbed, such as the silencing of tumor suppressor genes and activation of oncogenes by altered CpG island methylation patterns, histone modifications, and dysregulation of DNA binding proteins. There are several medications which have epigenetic impact, that are now used in a number of these diseases.
DNA methylation in cancer plays a variety of roles, helping to change the healthy cells by regulation of gene expression to a cancer cells or a diseased cells disease pattern. One of the most widely studied DNA methylation dysregulation is the promoter hypermethylation where the CPGs islands in the promoter regions are methylated contributing or causing genes to be silenced.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.