Heart disease and its manifestations, including coronary artery disease, myocardial infarction, congestive heart failure and cardiac hypertrophy, clearly presents a major health risk in the United States today. The cost to diagnose, treat and support patients suffering from these diseases is well into the billions of dollars. Two particularly severe manifestations of heart disease are myocardial infarction and cardiac hypertrophy. With respect to myocardial infarction, typically an acute thrombocytic coronary occlusion occurs in a coronary artery as a result of atherosclerosis and causes myocardial cell death. Because cardiomyocytes, the heart muscle cells, are terminally differentiated and generally incapable of cell division, they are generally replaced by scar tissue when they die during the course of an acute myocardial infarction. Scar tissue is not contractile, fails to contribute to cardiac function, and often plays a detrimental role in heart function by expanding during cardiac contraction, or by increasing the size and effective radius of the ventricle, for example, becoming hypertrophic. Although initial collagen deposition is required for infarct healing and to prevent cardiac rupture, the continuous production of collagen by fibroblasts induces interstitial fibrosis surrounding the myocytes in the infarct borderzone and remote myocardium of the infracted heart. This fibrosis induces stiffness, diastolic dysfunction, and cardiomyocyte hypertrophy due to the increase in stress and can also lead to arrythmias.
Cardiac hypertrophy is an adaptive response of the heart to virtually all forms of cardiac disease, including those arising from hypertension, mechanical load, myocardial infarction, cardiac arrhythmias, endocrine disorders, and genetic mutations in cardiac contractile protein genes. While the hypertrophic response is initially a compensatory mechanism that augments cardiac output, sustained hypertrophy can lead to dilated cardiomyopathy (DCM), heart failure, and sudden death. In the United States, approximately half a million individuals are diagnosed with heart failure each year, with a mortality rate approaching 50%. The causes and effects of cardiac hypertrophy have been extensively documented, but the underlying molecular mechanisms have not been elucidated. Understanding these mechanisms is a major concern in the prevention and treatment of cardiac disease and will be crucial as a therapeutic modality in designing new drugs that specifically target cardiac hypertrophy and cardiac heart failure.
Treatment with pharmacological agents represents the primary mechanism for reducing or eliminating the manifestations of heart failure. Diuretics constitute the first line of treatment for mild-to-moderate heart failure. If diuretics are ineffective, vasodilatory agents, such as angiotensin converting enzyme (ACE) inhibitors (e.g., enalopril and lisinopril) or inotropic agent therapy (i.e., a drug that improves cardiac output by increasing the force of myocardial muscle contraction) may be used. Unfortunately, many of these standard therapies have numerous adverse effects and are contraindicated in some patients. Thus, the currently used pharmacological agents have severe shortcomings in particular patient populations. The availability of new, safe and effective agents would undoubtedly benefit patients who either cannot use the pharmacological modalities presently available, or who do not receive adequate relief from those modalities.
Cardiac myocytes are normally surrounded by a fine network of collagen fibers. In response to pathological stress, cardiac fibroblasts and extracellular matrix proteins accumulate disproportionately and excessively. Myocardial fibrosis, a characteristic of all forms of pathological hypertrophy, leads to mechanical stiffness, which contributes to contractile dysfunction (Abraham et al., 2002). Another hallmark of pathological hypertrophy and heart failure is the re-activation of a set of fetal cardiac genes, including those encoding atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP) and fetal isoforms of contractile proteins, such as skeletal α-actin and β-myosin heavy chain (MHC). These genes are typically repressed post-natally and replaced by the expression of a set of adult cardiac genes (McKinsey and Olson, 2005). The consequences of fetal gene expression on cardiac function and remodeling (e.g., fibrosis) are not completely understood. However, the up-regulation of β-MHC, a slow ATPase, and down-regulation of α-MHC, a fast contracting ATPase, in response to stress has been implicated in the diminution of cardiac function (Bartel, 2004) and BNP is known to play a dominant role in cardiac fibrosis.
In addition to cardiac fibrosis, there are a number of disorders or conditions that are associated with fibrosis of various tissues. Congenital hepatic fibrosis, an autosomal recessive disease, is a rare genetic disease that affects both the liver and kidneys. The disease is characterized by liver abnormalities, such as hepatomegaly, portal hypertension, and fiber-like connective tissue that spreads over and through the liver (hepatic fibrosis). Pulmonary fibrosis, or scarring of the lung, results from the gradual replacement of normal lung air sacs with fibrotic tissue. When the scar forms, the tissue becomes thicker, causing an irreversible loss of the tissue's ability to transfer oxygen into the bloodstream. The most current thinking is that the fibrotic process in pulmonary tissue is a reaction (predisposed by genetics) to microscopic injury to the lung. While the exact cause remains unknown, associations have been made with inhaled environmental and occupational pollutants, cigarette smoking, diseases such as scleroderma, rheumatoid arthritis, lupus and sarcoidosis, certain medications and therapeutic radiation.
Scleroderma is a chronic disease characterized by excessive deposits of collagen in the skin or other organs. The localized type of the disease, while disabling, tends not to be fatal. The systemic type or systemic sclerosis, which is the generalized type of the disease, can be fatal as a result of heart, kidney, lung or intestinal damage. Scleroderma affects the skin, and in more serious cases it can affect the blood vessels and internal organs.
Skeletal muscle fibrosis is a phenomenon which frequently occurs in diseased or damaged muscle. It is characterized by the excessive growth of fibrous tissue which usually results from the body's attempt to recover from injury. Fibrosis impairs muscle function and causes weakness. The extent of loss of muscle function generally increases with the extent of fibrosis. Victims of muscular dystrophies, particularly Becker muscular dystrophy (BMD) and the more severely penetrating allelic manifestation, Duchenne muscular dystrophy (DMD), frequently suffer from increasing skeletal muscle fibrosis as the disease progresses. Other afflictions such as denervation atrophy are known to produce skeletal muscle fibrosis, as well as neuromuscular diseases, such as acute polyneuritis, poliomyelitis, Werdig/Hoffman disease, amyotrophic lateral sclerosis (Lou Gehrig's Disease), and progressive bulbar atrophy disease.
MicroRNAs have recently been implicated in a number of biological processes including regulation of developmental timing, apoptosis, fat metabolism, and hematopoietic cell differentiation among others. MicroRNAs (miRs) are small, non-protein coding RNAs of about 18 to about 25 nucleotides in length that are derived from individual miRNA genes, from introns of protein coding genes, or from poly-cistronic transcripts that often encode multiple, closely related miRNAs. See review of Carrington et al. (2003). MiRs act as repressors of target mRNAs by promoting their degradation, when their sequences are perfectly complementary, or by inhibiting translation, when their sequences contain mismatches.
miRNAs are transcribed by RNA polymerase II (pol II) or RNA polymerase III (pol III; see Qi et al. (2006) Cellular & Molecular Immunology Vol. 3:411-419) and arise from initial transcripts, termed primary miRNA transcripts (pri-miRNAs), that are generally several thousand bases long. Pri-miRNAs are processed in the nucleus by the RNase Drosha into about 70- to about 100-nucleotide hairpin-shaped precursors (pre-miRNAs). Following transport to the cytoplasm, the hairpin pre-miRNA is further processed by Dicer to produce a double-stranded miRNA. The mature miRNA strand is then incorporated into the RNA-induced silencing complex (RISC), where it associates with its target mRNAs by base-pair complementarity. In the relatively rare cases in which a miRNA base pairs perfectly with an mRNA target, it promotes mRNA degradation. More commonly, miRNAs form imperfect heteroduplexes with target mRNAs, affecting either mRNA stability or inhibiting mRNA translation.
The 5′ portion of a miRNA spanning bases 2-8, termed the ‘seed’ region, is especially important for target recognition (Krenz and Robbins, 2004; Kiriazis and Krania, 2000). The sequence of the seed, together with phylogenetic conservation of the target sequence, forms the basis for many current target prediction models. Although increasingly sophisticated computational approaches to predict miRNAs and their targets are becoming available, target prediction remains a major challenge and requires experimental validation. Ascribing the functions of miRNAs to the regulation of specific mRNA targets is further complicated by the ability of individual miRNAs to base pair with hundreds of potential high and low affinity mRNA targets and by the targeting of multiple miRNAs to individual mRNAs. Enhanced understanding of the functions of miRNAs will undoubtedly reveal regulatory networks that contribute to normal development, differentiation, inter- and intra-cellular communication, cell cycle, angiogenesis, apoptosis, and many other cellular processes. Recently, the inventors reported a cardiac-specific microRNA, miR-208, which is encoded by an intron of the α-myosin heavy chain (MHC) gene, and is required for up-regulation of β-MHC expression in response to cardiac stress and for repression of fast skeletal muscle genes in the heart (see co-pending application WO2008/016924, which is herein incorporated by reference in its entirety). The present invention expands on the involvement of microRNAs in the heart as well as other tissues.