HMGA2

Key takeaways

HMGA2 belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect HMGA2 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HMGA2 from memory before moving on to harder problems.

Reference excerpt

High-mobility group AT-hook 2, also known as HMGA2, is a protein that, in humans, is encoded by the HMGA2 gene.

Function 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. 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. The microRNA let-7 is largely responsible for this time-dependent regulation of HMGA2. 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), and genome-wide association studies linking HMGA2-associated SNPs to variation in human height.

Regulation by let-7 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). 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.

Clinical significance

Relationship with cancer 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. The same mutations that lead to pituitary adenomas in mice can be found in similar cancers in humans. Its presence is associated with poor prognosis for the patient, but also with sensitization of the cancer cells to certain forms of cancer therapy. 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. 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. A very common finding in HMGA2-high cancers is the under-expression of let-7. 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.

ERCC1 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. (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., 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. ERCC1 protein expression was also reduced in a diet-related mouse model of colon cancer. 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).

Chromatin immunoprecipitation 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. 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.

Non-homologous end joining DNA repair 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. showed that

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with HMGA2

Start with the simplest possible case. Write down what HMGA2 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to HMGA2 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about HMGA2 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of HMGA2

In research: HMGA2 appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses HMGA2 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: HMGA2 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Genes on human chromosome 12, Transcription factors, so understanding it makes those chapters shorter.
In everyday life: Look for HMGA2 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study HMGA2 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HMGA2 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain HMGA2 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HMGA2 in simple terms?

High-mobility group AT-hook 2, also known as HMGA2, is a protein that, in humans, is encoded by the HMGA2 gene. Function This gene encodes a protein that belongs to the non-histone chromosomal high-mobility group (HMG) protein family.

Why does HMGA2 matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study HMGA2?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on HMGA2.