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J Am Coll Cardiol, 2001; 38:1175-1180 © 2001 by the American College of Cardiology Foundation |
a Section of Cardiology, Department of Medicine, Baylor College of Medicine, Houston, Texas, USA
Manuscript received January 31, 2001; revised manuscript received June 11, 2001, accepted June 26, 2001.
Reprint requests and correspondence: Dr. A. J. Marian, Section of Cardiology, Baylor College of Medicine, One Baylor Plaza, 543E, Houston, Texas 77030
amarian{at}bcm.tmc.edu
| Abstract |
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The goal of this study was to identify genes upregulated in the heart in human patients with hypertrophic cardiomyopathy (HCM).
BACKGROUND
Hypertrophic cardiomyopathy is a genetic disease caused by mutations in contractile sarcomeric proteins. The molecular basis of diverse clinical and pathologic phenotypes in HCM remains unknown.
METHODS
We performed polymerase chain reaction-select complementary DNA subtraction between normal hearts and hearts with HCM and screened subtracted libraries by Southern blotting. We sequenced the differentially expressed clones and performed Northern blotting to detect increased expression levels.
RESULTS
We screened 288 independent clones, and 76 clones had less than twofold increase in the signal intensity and were considered upregulated. Sequence analysis identified 36 genes including those encoding the markers of pressure overload-induced ("secondary") cardiac hypertrophy, cytoskeletal proteins, protein synthesis, redox system, ion channels and those with unknown function. Northern blotting confirmed increased expression of skeletal muscle alpha-actin (ACTA1), myosin light chain 2a (MLC2a), GTP-binding protein Gs-alpha subunit (GNAS1), NADH ubiquinone oxidoreductase (NDUFB10), voltage-dependent anion channel 1 (VDAC1), four-and-a-half LIM domain protein 1 (FHL1) (also known as SLIM1), sarcosin (SARCOSIN) and heat shock 70kD protein 8 (HSPA8) by less than twofold. Expression levels of ACTA1, MLC2a and GNAS1 were increased in six additional and FHL1 in four additional hearts with HCM.
CONCLUSIONS
A diverse array of genes is upregulated in the heart in human patients with HCM, which could account for the diversity of clinical and pathologic phenotypes. Markers of secondary hypertrophy are also upregulated, suggesting commonality of pathways involved in HCM and the acquired forms of cardiac hypertrophy. Elucidation of the role of differentially expressed genes in HCM could provide for new therapeutic targets.
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We and others have proposed cardiac hypertrophy in HCM is a "compensatory" phenotype due to increased cardiac myocyte stress or altered Ca+2 sensitivity of the contractile apparatus imparted by the mutant contractile proteins (3). Accordingly, increased myocyte stress leads to expression of a variety of cardiac genes that activate the transcription machinery leading to hypertrophy and other phenotypes of HCM. Thus, the pathogenesis of hypertrophy, a common programmed response of the myocardium to any form of stress, whether caused by a genetic defect or by an acquired condition, involves common pathways. Similarly, pathogenesis of diverse cardiac phenotypes also results from upregulation of expression of a variety of genes in response to the primary impetus provided by the mutant contractile protein (3). To identify genes that are upregulated in the heart in HCM, as the initial step for delineating their role in induction of cardiac phenotypes, we performed subtraction hybridization between normal hearts and hearts with HCM followed by Northern blotting to confirm upregulation of expression of the differentially expressed genes in HCM.
| Methods |
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To confirm that the subtracted clones represented differentially expressed genes and to reduce the number of false positives, colonies were initially screened by PCR-select differential screening (CLONETECH). In brief, the cloned inserts were amplified by PCR using flanking primers, and 288 randomly selected clones in the forward-subtracted cDNA library were arrayed on three sets of 96-spot dot blot membranes in duplicates for subsequent hybridization. Probes were prepared by PCR amplification of the forward and reverse subtraction cDNA libraries, restriction enzyme digestion to remove the adapter sequences in order to reduce the background and radiolabeling with [32P] dCTP a specific activity of more than 109 cpm/µg. Each set of membranes was hybridized either with forward or reverse subtracted probe in ExpressHyb solution (CLONETECH). After washing in high stringency conditions, the membranes were exposed to X-ray films in the presence of intensifying screens for 24 h.
Sequencing. Differentially expressed clones were sequenced using Big Dye Terminator Cycle Sequencing Ready Reaction Kit on an ABI Genetic Analyzer 310 (PE Biosystem, Foster City, California). Sequences were analyzed using BLAST search of gene bank database.
Northern blotting. To confirm differential expression of the putative candidate genes in HCM, Northern blotting was performed on mRNA extracts from hearts with HCM and control hearts. In brief, 10 µg aliquots of total RNA extracts were loaded onto a formaldehyde-agarose gel, subjected to electrophoresis and were transferred to a nylon membrane (Bio-Rad Laboratories, Cambridge, Massachusetts). The forward-subtracted cDNA probes were radiolabeled with [32P] dCTP and hybridized to mRNAs on the membranes in the presence of Denhardts reagent (0.1% ficol, 0.1% polyvinylpyrrolidone, 0.1% bovine serum albumin, 100 µg/ml of denatured fragmented salmon sperm DNA) in hybridization solution (6 x sodium chloride-sodium citrate, 0.5% sodium dodecylsulfate). After washing, the membrane was exposed to an X-ray film for 24 to 48 h.
To determine whether the differentially expressed clones were also upregulated in additional hearts with HCM, Northern blotting was performed on mRNA extracts from six additional hearts with HCM and two control hearts.
| Results |
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| Discussion |
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We have confirmed increased expression of selected differentially expressed genes (eight genes) by Northern blotting in the myomectomy sample that was used for subtraction subsequently and subsequently for four genes in multiple hearts with HCM, further validating the observed results. We used PCR-select cDNA subtraction hybridization, which is considered a robust technique for identification of differentially expressed genes (4,5). Unlike microarray DNA chips, which are restricted to known sequences and are subject to bias for the selection of putative genes, subtraction hybridization affords the opportunity to screen all differentially expressed genes without a selection bias or a priori knowledge of their sequences. However, it is a tedious method, and, in each set of subtraction experiments, a fraction of total differentially expressed genes are identified, and the yield decreases with each additional subtraction. We performed three sets of colony screening from a single cDNA subtraction experiment and selected genes that showed at least a twofold increase in the expression levels. Therefore, the total number of differentially expressed genes in the heart in HCM could be greater. In addition, expression of many genes could be decreased in HCM, and, since we did not screen the reverse subtracted library, they were not detected. Furthermore, cDNA subtraction hybridization detects gene expression at the mRNA stage, which may not correspond to protein expression. Moreover, expression profiling detects only expression of the genes and not the activity of their proteins, which, for many proteins, such as the intracellular signaling kinases, is the primary determinant of protein function. Therefore, it not surprising that upregulation of expression of intracellular signaling kinases, which play fundamental roles in induction of acquired forms of cardiac hypertrophy (6) and, likewise, are expected to modulate cardiac hypertrophy in HCM, were not detected. Furthermore, expression profiling is an initial step that provides a basis for future studies to delineate the role of differentially expressed genes in the pathogenesis of cardiac phenotype.
A variety of genes including those encoding for contractile sarcomeric proteins, cytoskeletal proteins, ion channels, intracellular signal tranducers, protein maintaining the redox state of the myocardium, along with transcriptional and translation machinery were upregulated in HCM. The most common upregulated genes were the markers of "secondary" cardiac hypertrophy, such as skeletal alpha-actin, isoforms of myosin light chain and, less commonly, brain natriuretic factor, also known to be activated in pressure-overload induced (secondary) cardiac hypertrophy. Many of the upregulated genes have not been previously implicated in cardiac hypertrophy, and function of a few, such as sarcosin (7) and SLIM1 (8), is unknown. In addition, structure and function of proteins encoded by HSPC040 (9) and C1ORF8 (10), which have been identified by expressed sequence tags analysis and comparative proteomics, respectively, are unknown. Among the genes with the highest upregulation were HSPA8, also known as HSP73, which is a member of HSP70 multigene family with chaperoning roles for nascent polypeptide facilitating their correct folding, translocation and degradation (11). Expression of NDUFB10 encoding for the first enzyme complex in the electron transport chain of mitochondria (12) was also upregulated significantly. Upregulation of the expression of HSPA8 and NDUFB10 (12), along with increased expression of many ribosomal proteins is in accord with the increased protein synthesis and mitochondria function in hypertrophic states and signifies their role as potential modulators of cardiac phenotype in HCM. Other notable genes included GNAS1, encodes for the GTP-binding protein Gs and couples the hormone-receptor binding to adenylyl cyclase activation (13); VDAC1, encodes for VDAC1 that forms the major pathway for movement of adenine nucleotides through the outer membrane of mitochondria (14) and sodium hydrogen exchanger, which is a ubiquitous membrane-bound enzyme involved in pH regulation and activated by a variety of signals including growth factors (15).
In summary, we have identified a variety of genes that are upregulated in human patients with HCM that could contribute to the diversity of clinical and pathologic phenotypes of HCM. Increased expression of genes encoding for the markers of "secondary" cardiac hypertrophy supports the hypothesis that hypertrophy in HCM, a genetic disease, is also a "secondary" phenotype and involves pathways that are similar to those implicated in the acquired forms. Characterization of the role of upregulated gene in HCM could provide clues to the diversity of clinical and pathologic phenotypes and for new therapeutic and preventive targets.
| Footnotes |
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| References |
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