Differential Gene Expression Profiling in a Small Animal Model of Progressively Pacing-Induced Heart Failure

Donald Evan Selby, Differential Gene Expression Profiling in a Small Animal Model of Progressively Pacing-Induced Heart Failure. Doctor of Philosophy (Biomedical Sciences), July 2006, 235 pp, 4 tables, 35 illustrations, references, 328 titles. Pacing induced tachycardia (PIT), in mammals, is known to cause a change from normal heart function to early left ventricular dysfunction. Progression to heart failure in experimental animals, such as dogs, pigs, and sheep, takes place in a relatively short period of time compared to the disease development observed in humans. Due to the cost and nature of using such animals, there is a need for a small animal model of PIT, which would delineate the etiology of the disease state by impairing the systolic function. The mode of action of overpacing inducement of cardiomyopathy, as the data suggests, may be through a sarcomere stretch sensor and its length-dependent signaling mechanism. In this study, an internal electrical-overpacing of an isogenic rabbit strain over a 52-day period was used to initiate a pathology consistent with human CHF. The data presented demonstrated that PIT causes alterations in the systolic ability of the heart, observed as reduced fractional shortening of the heart. This is seen in changes of the message pool population for proteins of the contractile architecture. Initially the heart is being paced rapidly and therefore there is insufficient time to get blood into the chamber. Thus, the data suggests that a mechanical stretch sensor is the process by which overpacing the heart leads to changes in gene expression which ultimately cause a compounding cellular condition which exists during heart failure. The data shows that there are gene isoform ratio changes that occur as the disease develops these include changes in differential expression of cardiac titin alternative splicing isoforms. The data suggests that there is also isoform switching occurring with alternative splicing of the gene encoding for SERCA2a, the probe 1587641_at shows a moderate decrease in expression and using BLAST for this probe this sequence is homologous to an alternative splicing variant of SERCA2a of the rabbit accession number J04703. The data shows that ferritin heavy chain also has an alternative splicing variant that are differentially regulated, this dysregulation of the isoform ratio may be linked to ADAMSTS1, a disintegrin and metalloproteinase isoform 1, which is seen to be downregulated in the data, these play a role in negative regulation of cellular proliferation. In addition to these detected isoform changes in the ratios of alternative splice variants changes are seen in genes linked to sarcomere integrity such as dystrophin probe 1582958_at is significantly increased in its expression, also integrin beta-1 probe 1584175)at shows a marginal increase in expression. The protease calpain probe 1604384)at, which uses a substrate the aforementioned integrin, dystrophin, and titin is also significantly upregulated in the data. Interestingly calpastatin, probe 1591603_at the inhibitor of calpain is marginally increased in its expression. Only recently has titin become to be appreciated as the protein that is responsible for the Frank Starling law as it undergoes an isoform ratio change as heart failure develops. These changes are initially caused by changes in ion concentration and stress upon the contractile proteins but as seen in the study, leads to altered gene expression. In this model, these gene alterations lead to diastolic dysfunction and the compounded problems constitute heart failure. This work shows that heart failure induced by over-pacing creates physical demands upon the framework of the heart and these physical stresses are transmitted through mechanical sensors leading to differential expression of the message pools for proteins involved in the way the heart contracts, and fills upon relaxation which ultimately ends in a heart that can do neither, thus leading to death.