|Table of Contents|
|1. Cancer and CLL 2. miRNA & PTEN 3. DNA Origami 4. References|
1. CANCER AND CLL
2. miRNA & PTEN
Messenger RNA (mRNA) is transcribed from DNA and translated in the ribosomes within the cytoplasm to produce proteins, which regulate and catalyze all processes in the cell. However, in cancerous cells, proteins responsible for growth and division are overexpressed, while those responsible for inducing apoptosis and regulating growth are underexpressed. MicroRNAs (miRNAs) are small, single stranded RNA molecules which have a vital role in regulating almost all cellular processes, especially with respect to gene expression levels. They are imperfect complements to mRNA strands, allowing them to bind to mRNA at the 3’-UTR and prevent protein translation. The first miRNA strand, Lin-4 was discovered in 1993 where it was observed that the lin-4 gene could regulate the expression of another gene, lin-14 . It was also discovered that lin-4 didn’t code for a protein, and that a 22 nucleotide sequence of transcript was complementary to part of the lin-14 sequence. Therefore, it was hypothesized that the lin-4 transcript was blocking the expression of the lin-14 gene. This breakthrough led others to start studying how genes might regulate other genes, and thousands of subsequent miRNAs have been identified and characterized As reviewed in Jinju Han et al.’s report, microRNAs are coded by portions of the genome previously regarded as “junk DNA”. Certain miRNAs are encoded for by portions of the genome that do not code from proteins, while others are translated from introns which have been spliced from mature RNA. miRNAs which have their own genes are transcribed as long strands of primary miRNA (Pri-miRNA). Pri-miRNA can be several hundred nucleotides in length and can include several miRNA strands. Each individual miRNA has a hairpin-loop structure on the pri-miRNA. Two enzymes are responsible for recognizing, cutting and removing the hair-pin structures, which are known as pre-miRNA. The pre-miRNA are exported from the nucleus to the cytoplasm, where the RNase enzyme DICER cuts the loop from the pre-miRNA. Cutting the pre-miRNA results in two mostly complementary RNA strands, which are then processed by the RNA-induced silencing complex (RISC). The RISC selects an individual miRNA strand and the other strand is typically degraded. The strand that is less thermodynamically stable has been found to generally be selected for further processing. The RISC then uses an argonaute protein to bind with the single stranded miRNA and facilitate proper orientation to bind with the target mRNA. If the miRNA is a perfect complement to the target mRNA, the duplex will be destroyed by the argonaute protein. In the case of imperfect binding (more common), the miRNA simply blocks expression of the mRNA. MiRNA’s need not bind perfect complementary to block mRNA, so an individual miRNA can block the expression of several genes. This imperfect binding also suggests that miRNA might preferentially bind to a perfectly complementary nucleic acid strand over the mRNA target. MicroRNAs have been implicated in most cellular processes, including growth, development, and apoptosis. Because of their ubiquitous nature, miRNA dysregulation is a factor in most human disease. In cancer, some miRNA are overexpressed, while other are underexpressed. Predictably, miRNAs which should limit growth and proliferation are underexpressed in cancer, while those which limit apoptosis and cell cycle control are overexpressed. In Chronic Lymphocytic Leukemia (CLL), miR - 21 and miR - 155 are chronically overexpressed. miR - 21 targets genes such as PTEN and Bcl2, both of which are tumor suppressors. With elevated levels of miR - 21, PTEN and Bcl2 are suppressed, allowing cells to grow uncontrollably. Thus, miR-21 serves as an attractive target for antisense therapy in CLL cells. Phosphatase and Tensin Homolog (PTEN) is a classic tumor suppressor protein and plays an integral part in the Akt/PKB signalling pathway, which controls cell death. PTEN catalyzes the dephosphorylation of PIP3, a phospholipid in the cell membrane, resulting in the product PIP2. This reaction inhibits the Akt/PKB pathway, leading to cell death. In many types of cancer, including CLL, PTEN activity is low resulting in apoptotic evasion and cellular proliferation, resulting in a cancerous phenotype. PTEN has also been shown to induce apoptosis when it is overexpressed, as it activates a specific apoptotic pathway which detaches the cell from the extracellular matrix. When PTEN levels were elevated in breast cancer cells, their cell cycle was arrested and soon became apoptotic. Therefore, if PTEN levels could be elevated and restored in CLL cells due to antisense delivery via DNA nanostructures, it would be possible to induce apoptosis. Thus, we hypothesize that DNA nanostructures functionalized with miR-21 complementary oligomers will be effectively uptaken by CLL cells to induce apoptosis through elevation of PTEN expression levels. .
3. DNA OrigamiDNA origami is a technique which utilizes the Watson-Crick base-pairing of DNA strands to construct nanostructures with precise control of geometry. It was first developed in 2D by Paul Rothemund in 2006 and later expanded to 3D in 2009. DNA origami works by combining a multi-kilobase loop of phage DNA, known as the scaffold, with smaller, synthesized staple DNA strands (~40 nt). The staples bind to the scaffold in a piecewise manner, folding it into the desired shape (Figure 4). To ensure structural integrity, the staples must transition between helices, forming Holliday junctions.
As a nanoscale platform, DNA origami has several attractive features, including consistency and spatial control. DNA origami structures can be functionalized with a variety of molecules by incorporating single stranded DNA overhangs into the structural staples used to fold DNA origami. This can help to attach proteins, nanoparticles, or to control complex mechanisms. DNA origami also shows potential for therapeutic applications. The structures are inherently biocompatible and the size is appropriate for cellular endocytosis. Furthermore, the ability to functionalize molecules opens doors for biosensing. Theoretically, a targeting molecule such as folate could be attached to an origami structure, along with a fluorescent molecule and a chemotherapeutic drug. A promising example for therapeutic applications was shown recently with a DNA origami cage capable of releasing it’s cargo when exposed to a certain signal. Other recent studies have focused on delivering intercalating drugs, which naturally bind between DNA base-pairs. Anti-cancer intercalators are an ideal cargo for a DNA origami vehicle, and this approach has been used to successfully induce apoptosis in breast cancer cells.
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