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1 From the Departments of Ophthalmology and Visual Sciences, 3 Physiology, and 4 Human Genetics, University of Michigan, Ann Arbor, Michigan.
| Abstract |
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METHODS. Two cDNA libraries (one amplified, the other unamplified) were constructed using RNA isolated from native human RPE sheets. The sequence from the 5' end was obtained for randomly selected clones from the two libraries. Of these, more than 2000 expressed sequence tags (ESTs) were analyzed for similarity to sequences and gene clusters in public databases.
RESULTS. EST analysis revealed several known RPE-expressed genes and more than 500 genes that have been characterized previously but were not known to be expressed in the RPE. Transthyretin and 90-kDa heat shock protein represent the most abundant transcripts identified in these RPE libraries. More than 200 novel ESTs and putative proteins were identified. An additional 344 sequences matched only the human genomic sequence.
CONCLUSIONS. High-complexity cDNA libraries were generated from native human RPE. Analysis of ESTs generated from these libraries has yielded a profile of genes expressed in the native RPE. Several of the identified genes are known to play a significant role in the RPE. Novel ESTs, putative proteins, and genomic hits may represent as yet unidentified RPE-expressed genes and many of these, mapping in the region of retinal disease loci, may serve as candidate genes. In addition, the nonredundant set of more than 1100 genes and ESTs described herein will be a valuable resource for generating gene microarrays, which can assist in delineating RPE expression profiles during human disease pathogenesis.
| Introduction |
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Expressed sequence tag (EST) analysis, together with the sequencing of human genome, has been a powerful tool for discovering novel and uniquely expressed genes.10 11 12 The collection of ESTs from a specific tissue or cell type provides an expression profile including information about the level and complexity of gene expression in that tissue or cell type.13 14 15 16 Several laboratories have published ESTs originating from eye tissues and cell lines.17 18 19 20 21 22 23 ESTs from RPE have been reported earlier,24 25 26 27 but these were derived from RPE cells in culture or RPE-choroid (available at neibank.nei.nih.gov), and therefore, are not necessarily representative of the native human RPE. In addition, the reported ESTs are not appropriately annotated, making their usage in microarray studies more difficult.
With the goal of generating a comprehensive profile of genes expressed in native human RPE, creating a nonredundant set of genes for custom RPE gene microarrays, and enhancing the pool of candidate genes for retinal and macular diseases, we constructed high-complexity cDNA libraries from freshly isolated human RPE sheets. Our analysis of random ESTs from these libraries provides a nonredundant set of more than 1100 genes. In addition to cDNAs for known RPE-expressed genes, we report a large compilation of ESTs for genes encoding proteins of yet unknown physiological functions.
| Materials and Methods |
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cDNA Library Construction
Human RPE poly(A)+ RNA (40 ng) was used
for generating the first cDNA library, with a kit (SMART cDNA Library
Construction Kit; Clontech, Palo Alto, CA), according to the
manufacturers instructions. Two microliters of cDNA from the
first-strand reactions was used for a template, together with primers
and reagents from the kit, in 24 cycles of amplification, using primers
and reagents from the kit. The double-stranded cDNA was ligated to
TriplEx2 vector arms (Clontech) and an aliquot of the ligation
mixture was packaged (Gigapack III Gold kit; Stratagene, La Jolla, CA)
to generate the cDNA library. Plating and titration of
TriplEx2
recombinants were performed on Escherichia coli strain
XL1-Blue MRF', as suggested by the manufacturer. Phagemids were
obtained through mass in vivo excision from the phage library using
E. coli strain BM25.8.
A second unamplified cDNA library was generated in the pSPORT1 vector
(SuperScript Plasmid System; Life Technologies) using total RNA.
Briefly, first-strand cDNA was synthesized with a
NotI-oligo(dT) primer-adapter. After second-strand synthesis
and ligation of SalI adapter, cDNA was digested with
NotI, generating cDNAs with SalI sites at the 5'
end and NotI sites at the 3' end. cDNAs were size
fractionated, and those of 0.5 to 2 kb were ligated to pSPORT1. The
ligated sample was used to transform E. coli (ElectroMax
DH-5
; Invitrogen, San Diego, CA) by electroporation.
Sequencing
Double-stranded phagemid DNA templates were prepared for
sequencing from randomly selected clones (CONCERT Rapid Plasmid
Miniprep System; Life Technologies). The recombinant clones from
library 1 were sequenced (Cy5/Cy5.5 Dye Primer Cycle Sequencing Kit;
Visible Genetics, Toronto, Canada) with 5'TriplEx-labeled primer, and
the products were analyzed by an automated system (OpenGene; Visible
Genetics). For the clones from library 2, DNA sequencing was performed
with a high-throughput automated sequencer (Applied Biosystems, Inc.
Foster City, CA). BLAST search was performed against GenBank and dbEST
for each sequence (BLAST, GenBank, and dbEST are provided in the public
domain by the National Center for Biotechnology Information
[NCBI], Bethesda, MD, and are available at
http://www.ncbi.nlm.nih.gov/BLAST/, /genbank/, and /dbEST/,
respectively).
| Results |
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-complementation system.
Screening of cDNAs by PCR amplification (using
TriplEx2 LD-Insert
Screening Amplimers; Clontech) revealed that more than 90% of the
clones contain inserts in the range of 400 to 1100 bp. Library 2 clones are 60% recombinant by restriction enzyme analysis, with inserts in the size range of 500 to 2000 bp.
Sequence Analysis and EST Identification
Typically, 200 to 400 bp of unambiguous sequence data were
obtained from 487 independent cDNA clones from the amplified RPE
library 1. A summary of the comparison of these sequences to
those in NCBI databases is provided in Table 1
. Sequence comparison analysis resulted in identification of 322 known
genes. Among these, transthyretin (26 clones) and 90-kDa heat shock
protein (HSP90; 13 clones) appear to represent abundant transcripts in
the RPE. Several known RPE-expressed genes were also identified,
including retinol-binding protein (five clones), RPE65 (two
clones), pigment epitheliumderived factor (PEDF; four clones), and
cystatin C (three clones). In addition, several genes known to be
functionally important in the RPE, such as cellular retinaldehyde
binding protein (CRALBP), and TIMP3, were
detected. Of the remaining 111 clones, 82 represented ESTs identified
in other cDNA libraries, including brain and retina or matched human
genomic sequences. Twenty-nine ESTs did not show any significant
sequence homology with current database entries or to each other. These
sequences may represent novel genes expressed in the RPE. Sequences
from all clones have been deposited to GenBank (accession numbers
BG108623BG108956).
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retina/RPE.html), which
includes an alphabetical list of the known genes along with their
accession numbers, chromosomal locations, and the number of times they
were detected in the unamplified library (quantitative expression
level). | Discussion |
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The identification of RPE- and retina-expressed genes will enhance the success and the pace of studies attempting to identify the genes responsible for many retinal and macular disorders. Herein, we report the isolation of a large number of ESTs from native RPE that match anonymous sequences or clusters identified from other cDNA libraries or the human genomic sequence. Several of these map to the chromosomal regions of previously localized disease loci (Table 2) and thus qualify as candidate disease genes. In addition, we identified numerous novel ESTs, some of which may represent RPE-specific transcripts. Their identification should provide valuable insight into the molecules necessary for normal RPE function.
We identified more than 500 genes that had previously been
characterized. Some of these are known RPE-specific or RPE-expressed
genes and include genes involved in visual pigment transport and
metabolism (RPE65, CRALBP, retinol-binding
protein); growth and development (PEDF and cystatin C); and
transport (monocarboxylate transporter-1 [MCT1] and -3
[MCT3],28
GABA transporter,
I and ß I
subunits of Na,K-ATPase, and Kir7.129
). In the unamplified
library, a substantial fraction of the known genes represents genes
involved in metabolism (
21%) and in gene regulation and protein
expression (
22%). Approximately 13% of the reported genes are
involved in cell-signaling pathways or are growth factors and membrane
proteins; 7% are involved in cell structure, cell surface, and
motility; 7% in protein trafficking and remodeling; and 5% in
membrane transport (Fig. 1)
. This is consistent with the role of RPE in
synthesis of various enzymes, growth factors, and pigments, transport
of nutrients and ions, and maintenance of the interphotoreceptor
matrix.
Although cDNA libraries have previously been constructed from cultured fetal RPE cells26 and from RPE cell lines,24 25 this is the first report of a library derived from native human RPE sheets. Based on previous studies,30 31 32 33 the gene expression profile of native RPE might be expected to be different from that of cultured RPE cells. Indeed, the analysis of ESTs indicates that many RPE-specific genes are not represented in two cDNA libraries derived from cultured RPE.24 26
It is estimated that 35,000 to 120,000 genes are encoded by the human genome,34 35 and only a fraction of these (10%20%) may be expressed in any single differentiated cell type. As the first step toward defining the expression profile of native human RPE, we describe at least 1100 genes expressed in the RPE. We compared the gene profile of native RPE to that of the ciliary epithelium.36 These two epithelia share a common embryonic origin and therefore similarities in gene expression might be expected. Of the 220 nonredundant clones from library 1 that match genes in databases, 12% were similar to those in the subtracted library from ciliary body. These include genes for extracellular proteins, antioxidants, apoptotic as well as stress proteins, and transcription and translation factors. Specifically, PEDF, 90-kDa heat shock protein, prostaglandin D2 synthase, calmodulin, inositol-1,4,5-triphosphate receptor, and cathepsin D are present in both libraries. In contrast, a comparison of the native RPE gene profile with that of the corneal epithelium18 shows little similarity. Although apolipoprotein J, a frequently detected gene in the corneal epithelium library is detected in the second RPE library, overall a different set of receptors, transporters, and secretory and cytoskeletal proteins are present in the two epithelia. Not surprisingly, the native RPE gene profile is also distinct from that of the neural retina.19 23 37 These findings suggest that the highly specialized functions of the RPE required for the maintenance of photoreceptor integrity are the result of a unique gene expression pattern. The similarity between the RPE library and libraries from the retina and the corneal epithelium is limited primarily to the presence of housekeeping genes including ribosomal proteins and those involved in metabolism and energy production.
The comparison of RPE profile with genes expressed in nonocular tissues, such as cochlear15 and hematopoietic tissues,12 reveals that only constitutively expressed genes, such as those involved in protein synthesis and metabolism, are shared between these tissues. However, many differences among the libraries are notable: For example, different types of collagen are among the most frequently identified genes in the cochlea, whereas no collagen form has been observed among the RPE clones thus far. Furthermore, approximately 13% of the known genes in the unamplified RPE library appear to be in the cell-signaling and growth factor category, whereas only approximately 5% of the genes from the cochlear library belong to this category.
The identification of RPE-expressed genes will be beneficial to future studies of gene expression profiles in RPE and generation of DNA microarrays that are fast becoming the technology of choice for identifying global changes in gene expression patterns during development and disease.38 39 40 Gene arrays of specific tissues or cell type of interest should greatly facilitate such investigations because of greater focus, relatively low expense, and better methods for data analysis. Mouse gene microarrays that contain more than 3000 ESTs from mouse eyeretina cDNA libraries have already been generated.41 The nonredundant set of more than 1100 human RPE cDNAs described herein provides a starting point for producing human gene microarrays that include RPE-expressed genes. Investigations using such microarrays will lead to better understanding of the genes and pathways involved in the maintenance of photoreceptor function and will provide candidate genes for retinal and macular diseases.
| Acknowledgements |
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| Footnotes |
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Supported by grants from the Macula Vision Research Foundation, Bala Cynwyd, PA (AS); National Institutes of Health Grants EY07961 and EY11115 and an administrative supplement for the microarray facility (AS) and Core Grant EY07003 and Grant M01-RR00042 to the General Clinical Research Center; The Foundation Fighting Blindness (AS, BH); and Research to Prevent Blindness. AS is recipient of a Research to Prevent Blindness Lew R. Wasserman Merit Award.
Submitted for publication February 6, 2001; revised November 12, 2001; accepted November 27, 2001.
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: Anand Swaroop, University of Michigan, W. K. Kellogg Eye Center; 1000 Wall Street, Ann Arbor, MI 48105; swaroop{at}umich.edu
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