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1From the Center for Corneal Genetics, Cornea and External Disease Service, The Wilmer Eye Institute, Baltimore, Maryland; the 2Genome Technology Branch, National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland; and 3The Johns Hopkins Oncology Center Molecular Genetics Laboratory, The Johns Hopkins School of Medicine, Baltimore, Maryland.
| Abstract |
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METHODS. Three pairs of normal human corneas were obtained from eye banks. Thirteen bisected Fuchs corneal buttons were processed at the time of corneal transplantation. The endothelia of normal and Fuchs-affected corneas were stripped, and total RNA was isolated. Serial analysis of gene expression (SAGE) was performed to identify and quantify gene transcripts. Genes over- and underexpressed by Fuchs endothelium were limited to P < 0.01 by the method of Audic and Claverie.
RESULTS. A total of 19,136 tags were identified with 9,530 from normal and 9,606 from Fuchs endothelium. The expression of 18 transcripts was upregulated, and 36 transcripts were downregulated in Fuchs endothelium compared with normal tissue. Upregulated transcripts included serum amyloid A1 and A2, metallothionein, and apolipoprotein D. Of the downregulated transcripts, 26 matched known genes, 3 matched expressed sequence tags (ESTs), and 7 were unknown to current databases. One downregulated transcript involved a newly reported bicarbonate transporter. Decreased transcripts related to antioxidants and proteins conferring protection against toxic stress were noted in Fuchs versus normal endothelium including nuclear ferritin, glutathione S-transferase-
, and heat shock 70-kDa protein. Nine different SAGE tags matching mitochondrial sequences accounted for 25% of the ESTs that were decreased in Fuchs endothelium.
CONCLUSIONS. SAGE analysis comparing normal to Fuchs endothelium demonstrates diminished expression of mitochondrial, pump function, and antiapoptotic cell defense genes.
The functional activities of a specific cell are characterized by a set of expressed genes (the transcriptome). Pathologic changes in cell function can be associated with alterations of gene expression patterns where identification of differentially expressed genes can be useful to recognize disease processes. For example, genes exhibiting the greatest difference in expression between normal and diseased endothelium are likely to be relevant to the disease processes. Currently, oligonucleotide or cDNA microarrays6 7 and serial analysis of gene expression (SAGE)8 9 10 are the techniques most commonly used to compare global gene expression profile changes in normal or diseased tissues. One difference between these two technologies is that a microarray is limited to a finite number of gene sequences localized to the chip. However, SAGE allows the generation of expressed sequenced tags (ESTs), which are solely dependent on gene expression in the tissue. This allows for the identification of novel genes expressed in a given tissue. SAGE has the additional advantage of providing quantitative information about the level of gene expression.8 9 10 SAGE depends on the generation of short sequence tags at a specific location within a transcript. These tags, which contain sufficient information to identify a gene uniquely, are generated, concatenated, and sequenced. By identifying corresponding genes for each tag and tabulating the frequency for generating each tag, the number of genes expressed and their expression level can be estimated. In the present study, we used SAGE to analyze the differences in gene expression between normal endothelium and endothelium from Fuchs dystrophy.
| Materials and Methods |
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Generation of SAGE Libraries
The Fuchs SAGE library was constructed according to the SAGE protocol8 (available at http://www.sagenet.org/sage_protocol.htm, provided in the public domain by the Johns Hopkins Oncology Center, Molecular Genetics Laboratory, Baltimore, MD). The normal SAGE library was constructed as previously described.9
SAGE Data Analysis
All sequence data were analyzed with the Phred base-calling algorithm.11 The eSAGE ver. 1.2a computer program (http://research.nhgri.nih.gov/eSAGE/, provided in the public domain by the National Human Genome Research Institute [NHGRI], National Institutes of Health, Bethesda, MD) was used to extract and analyze the SAGE data.12 SAGE tags were extracted from high-quality sequence data by analyzing the Phred-generated PHD-formatted output files (*.phd.1) with eSAGE, which was programmed to exclude automatically the sequences with Phred quality values less than 20. Analysis of SAGE tag GC content was performed as described in Margulies et al.13
Statistical Tests for Identifying Differentially Expressed SAGE Tags
Before determining whether specific pair-wise differences exist between the two populations of SAGE tags, a formal statistical test was used to validate an overall difference between the normal and Fuchs data sets. Because the
2 test of independence performs poorly when tag counts are less than five,14 a Monte Carlo simulation approach was used to determine the overall difference between the two populations of SAGE data.15 This overall test was also used to determine the reproducibility of subpopulations of data from a single SAGE library.13 In this approach, data sets were generated randomly, keeping the row and column totals of the observed data set fixed. Values were then calculated from each randomly generated data set. This process was repeated 200 times to obtain a distribution of two values under the null hypothesis of no difference between two populations of SAGE data. An empiric probability was calculated by comparing the observed 2 value with the values generated under the null hypothesis. A program was written in a commercial software program (S-Plus 2000; Insightful Corp., Seattle, WA, available at http://www.insightful.com) to perform this Monte Carlo simulation and is available on request. The probability for specific pair-wise gene expression differences was calculated using the test statistic developed by Audic and Claverie.16
SAGE Tag Matching
The ehm-Tag-Mapping method (http://genome.nhgri.nih.gov/ehmTagMapping, provided in the public domain by NHGRI) was used to match SAGE tags with specific UniGene clusters.15 This method is implemented through the use of several Perl scripts designed to extract tag-to-UniGene cluster information from the UniGene flatfiles available at the National Center for Biotechnology Information (NCBI; http://www.ncbi.nlm.nih.gov/UniGene). Briefly, the ehm-Tag-Mapping method extracts a SAGE tag from each sequence in a UniGene cluster, only if the orientation and 3' end of the sequence can be confirmed by identifying poly(A) signals and/or tails. To minimize the extraction of SAGE tags from entries with potential sequencing errors, SAGE tags not representing at least 20% of all tags extracted from a given UniGene cluster are removed from the final ehm-Tag-Mapping flatfile.
Because mitochondrial transcripts are not represented in UniGene, a Perl script was used to extract SAGE tags from a complete annotated sequence of the human mitochondrial genome (GenBank accession number NC_001807).
| Results |
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2 statistics that fall within the null distribution. We were also able to use this method to identify statistically an overall difference between the normal and Fuchs SAGE libraries (Fig. 1C) , because the observed
2 value fell outside the null distribution.
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, and heat shock 70-kDa protein.
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| Discussion |
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Our data indicate that underexpressed transcripts exceed the overexpressed genes in Fuchs endothelium. Mitochondrial transcripts account for the majority of these downregulated genes. Mitochondrial genes encode for the enzymes essential for energy metabolism. Our data are consistent with early findings that the numbers of mitochondria and pump site densities are significantly decreased in Fuchs dystrophy.20 21 These data suggest that the basis for endothelial cellular dysfunction is related to energy-dependent activity.
Mitochondria are the major source of cellular reactive oxygen species (ROS). Mitochondrial DNA (mtDNA) is especially vulnerable to oxidative damage because of its proximity to the inner mitochondrial membrane, where electron transport systems generate ROS. Recent evidence indicates that oxidative DNA damage is a major cause of aging.22 A number of age-related neuromuscular degenerative diseases have been associated with point mutations and deletions in mtDNA, and progressive accumulation over time of oxidative damage in mtDNA from neuronal tissues has been demonstrated.23 Fuchs dystrophy is a progressive disease that does not manifest clinically until the third or fourth decade.1 Gradual oxidative mtDNA damage may occur over time in Fuchs endothelial cells. The progressive oxidative damage would probably increase the number of mtDNA mutations, which would eventually interfere with polypeptides encoded by mutated genes and lead to a degradation in enzyme activity. Biochemical findings that support this hypothesis include the demonstration that respiratory enzyme complexes gradually decline in Fuchs endothelial cells.22 23 In addition, a patient with Fuchs dystrophy has been reported to have missense mutations in the mtDNA at position 15,257 of the cytochrome b subunit of complex III and at position 4213 of the ND1 subunit of complex I.24 However, further study is necessary to determine whether mitochondrial defects could be the root cause of this dystrophy.
The endothelial pump is an essential requirement for maintaining proper fluid transport and hydration in the cornea. Na+,K+,ATPase has been identified as an important pump in the endothelial plasma membrane requiring adenosine triphosphate (ATP). SAGE has identified several Na+,K+,ATPases.9 Although comparisons of SAGE Fuchs versus normal endothelial transcripts did not show a significant difference between the Na+,K+,ATPases of the two libraries, the substantial decrease in other mitochondrial gene transcripts may have compromised ATP production to such a low level that proper Na+,K+ pump function could not be maintained. A comparison of the two libraries revealed a significant decrease in the expression of a newly reported bicarbonate transporter, bicarbonate transporterrelated protein-1 (BTR1).25 Although the precise transport activity of BTR1 is yet to be determined, this bicarbonate transporter is believed to be responsible for as yet unexplained anion transport identified in several human tissues including the kidney and salivary glands.
There are many ways to activate apoptosis in a cell, but in general two main pathways predominate: an intrinsic pathway involving mitochondria and an extrinsic pathway initiated by cell surface receptors for growth factors, cytokines, or drugs.26 Apoptosis has been shown to play a role in endothelial cell death in Fuchs dystrophy.27 Which apoptotic pathway is executed in endothelial cell death in Fuchs dystrophy is not known, but our data favor the intrinsic pathway. Mitochondria play a central role in apoptosis, as cytochrome c released in the cytoplasm opens permeability transition pores and leads to the apoptotic cascade.28 The sensitive balance between ROS production and antioxidant defenses determines the degree of oxidative stress. Consequences of this stress include modification of cellular proteins, lipids, and DNA that could result in cell death. Our data indicate that Fuchs endothelial cells exhibit decreased abundance of transcripts related to antioxidants and proteins conferring protection against toxic stress. For examples, nuclear ferritin protects DNA against UV damage.29 Glutathione S-transferase-
catalyzes intracellular detoxification reactions by conjugating glutathione with electrophilic compounds resulting in the elimination of toxic compounds.30 31 Heat shock 70-kDa protein is a major cytoplasmic chaperone that protects cells against apoptosis.32 Downregulation of these genes may in turn increase the production of ROS and toxic molecules that may have an especially detrimental impact, because corneal endothelial cells are terminally differentiated and will not be replaced.
Apolipoprotein D (apoD) is a lipophilic protein that has an important function in nerve regeneration. A large increase of apoD concentration has been found in the cerebrospinal fluid of patients with Alzheimers disease and other neuropathologic conditions.33 Our study shows that ApoD gene is expressed at moderate level in normal endothelium, but its expression markedly increases in Fuchs dystrophy. Overexpression of apoD may represent tissue injury repair in Fuchs dystrophy. An unexpected result was the differences in transcript abundance among the ribosomal proteins between normal and Fuchs endothelium.
Our observation that 11 differentially expressed SAGE tags do not match any known sequences suggests that a number of human genes may remain untagged, despite the large number of SAGE tags currently available and despite the data in the Human Genome Project. These no-match tags may represent novel genes putatively involved in maintaining endothelial functions and may provide insight into essential missing links in the molecular pathogenic pathways of Fuchs dystrophy. Furthermore, these no-match tags sequences can be used to further identify the corresponding uncharacterized genes.
In summary, we have applied SAGE technology to identify differentially expressed tags corresponding to known and unknown genes possibly involved in the pathophysiology of Fuchs dystrophy. At least three possible roles for these differentially expressed genes in this endothelial degeneration may be hypothesized: diminished pump function capacity due to under expression of key transporter components and/or decreased mitochondrial delivery of adequate energy sources to maintain pump; an apoptotic cascade of events leading to endothelial cell death as a result of oxidative stress, mitochondrial damage, and/or diminished levels of antioxidants; and overexpression of defensive genes to inhibit apoptosis. Our study demonstrates that analysis of global gene expression profiles can identify a group of genes that function in a common pathway to illuminate the underlying mechanism of a complex disease process in Fuchs dystrophy.
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Submitted for publication March 29, 2002; revised May 15, 2002; accepted May 23, 2002.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: John D. Gottsch, The Wilmer Eye Institute, Johns Hopkins Hospital, 600 N. Wolfe Street, Maumenee #321, Baltimore, MD 21287; jgottsch{at}jhmi.edu.
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