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1From the Department of Ophthalmology, Fukuoka University School of Medicine, Fukuoka, Japan; the 2Division of Genome Analysis, Research Center for Genetic Information, Medical Institute of Bioregulation, Kyushu University, Fukuoka, Japan; and the 3Department of Ophthalmology and Visual Science, Graduate School of Medicine, Chiba University, Chiba, Japan.
| Abstract |
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METHODS. Forty di-, tri-, or tetra-nucleotide repeats tightly linked to 10 genes known to be responsible for adRP were identified from the human genome sequence and used as markers in multiplex amplification and genotyping, followed by linkage analysis. Discordance of cosegregation of markers and the disease excluded the majority of the examined genes as candidates, and mutation screening for the remaining genes was performed.
RESULTS. With this strategy, examination of an adRP-affected family indicated that 3 of 10 candidate genes segregated concordantly with the disease. Further searches for mutations revealed a novel insertion and deletion in the last exon of a splicing factor gene, PRPF8.
CONCLUSIONS. This systematic approach facilitates the molecular diagnosis of adRP, which is known to have a highly heterogeneous genetic background.
Based on the large prevalence survey by Sohocki et al.4 who analyzed 500 probands of RP and other inherited retinopathies against five known candidate genes, the disease-causing mutation was identified in one third of the subjects with adRP in Canada and the United States. Another six genes for adRP were cloned shortly after the survey, with the number of genes to be analyzed therefore doubling and the number of exons to be screened increasing sevenfold.11 12 13 14 15 16 17 To facilitate the molecular diagnosis for such highly heterogeneous disorders, a systematic diagnostic approach is necessary.
Genetic linkage analysis has offered a substantial role in identifying novel genes responsible for genetic disease. Even for diseases with known genes, linkage analysis is an essential tool for molecular diagnosis when more than two loci are candidates.20 However, linkage analysis has an intrinsic weakness, because the possibility of meiotic crossover between gene loci and the available genetic markers remains. Furthermore, analyzing multiple gene loci requires a considerable amount of time and laboratory resources. Nonetheless, we believe that examining the genomic information in the vicinity of the RP-causative genes by linkage analysis provides substantial information for a molecular diagnosis. To achieve the best results from limited numbers of individuals in a given pedigree, many tightly linked genetic markers with high heterozygosity must be examined, to maximize the chance of analyzing informative markers within a given family.
In the present study, we showed the feasibility of a hierarchical approach for molecular diagnosis of adRP. This approach involved collecting a set of genetic markers that were tightly linked to each of the candidate adRP genes. These markers were then amplified by multiplex polymerase chain reaction (PCR) and subjected to linkage analysis. Based on concordance or discordance of segregation for the markers and disease, candidate genes were selected and mutation screening for the candidate genes was performed.
| Methods |
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Search of Short Tandem Repeats and Primer Design
For each gene, except FSCN2, the human draft sequence data were retrieved from GenBank. The accession numbers of sequence contigs are NT_006025.4 (RHO), NT_023409.5 (RDS), NT_008101.5 (RP1), NT_019583.5 (NRL), NT_011190.5 (CRX), NT_010692.6 (PRPF8), NT_011148.6 (PRPF31), NT_029226.4 (HPRP3P), and NT_007933.8 (IMPDH1). Short tandem repeats (STRs) were chosen from the gene, basically within 300 kb, if the number of repetitions was more than 13 for dinucleotide repeats and more than 7 for tri- and tetranucleotide repeats. We could not accurately locate the FSCN2 gene on the draft sequence, although the gene locus has been established to be at 17q25. As an alternative way to search for novel markers, six known polymorphic microsatellite markers near the locus: AFMc008we1, AFMa133yg9, GATA65G11, AFM107ye3, AFM210xa5, and AFM163yg1, encompassing approximately 6 cM, were selected from the Gènèthon (www.genethon.fr; provided in the public domain by the French Association against Myopathies, Evry, France)22 and The Cooperative Human Linkage Center (CHLC; http://gai.nci.nih.gov/provided in the public domain by the Laboratory for Population Genetics, Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health) databases.23
Four to 12 candidate STRs were selected for each gene, and primer sequences were designed using Primer3 (http://www-genome.wi.mit.edu; provided in the public domain by the Whitehead Institute, Massachusetts Institute of Technology, Cambridge, MA) after masking interspersed repetitive elements with RepeatMasker (http://ftp.genome.washington.edu/cgi-bin/RepeatMasker/provided in the public domain by the University of Washington Genome Center, Seattle, WA). One of the 5'-ends of each primer pair was modified for post-PCR fluorescent labeling24 to contain either a GTT or ATT for the purpose of a 3'-exchange labeling reaction with N,N'-diethyl-2',7'-dimethyl-6-carboxyrhodamine labeled deoxycytidine 5'-triphosphate (R6G-dCTP; Applied Biosystems, Foster City, CA) or with 6-carboxyrhodamine labeled deoxyuridine 5'-triphosphate (R110-dUTP; Applied Biosystems), respectively. The 5' end of the other primer was modified to contain TCC to prohibit labeling. By including the two different dyes (R6G and R110) in the same labeling reaction tubes, we could label several PCR products differentially, with two colors. The PCR primers from the same gene were designed so that the size of the fragments using the same color did not overlap each other in the multiplex analysis (i.e., allowing simultaneous separation in one run of electrophoresis).
Screening of Polymorphic STRs
Genomic DNA from five unrelated healthy individuals (Japanese) served as an initial screen for polymorphisms in the STRs. STR markers were amplified by 30 cycles of three-step PCR, as described previously.24 An aliquot of each PCR product was examined by agarose gel electrophoresis to confirm specific amplification. The remainder of the amplification products served as substrates for post-PCR fluorescent labeling and fragment length analysis, as described previously.24 STRs, for which at least one of five individuals were heterozygous, were regarded to be sufficiently polymorphic and were used in further analyses.
Optimization of Multiplex PCR
A set of amplicons for multiplex PCR were created with primers for the STRs of each gene. Initially, primers were simply mixed at equal concentration and a 40-cycle PCR performed, followed by labeling and electrophoresis. Based on the results, primer sets for unsuccessful STR amplifications were eliminated and the PCR was repeated. Concentrations of the primers were then adjusted to obtain peak heights of the same magnitude for all loci in the multiplex analysis.
Fragment Length Analysis
Capillary electrophoresis was performed with a gene sequencer (ABI 310 or 3700; Applied Biosystems) under the conditions suggested by the manufacturer. The dye matrix for the analysis using the 3700 Gene Analyzer was modified and optimized for the dyes used in this study. Data collection and allele identification was performed on computer (GeneScan and Genotyper software; Applied Biosystems).
Genotyping of adRP Family
Seven individuals of the adRP family participated in this study (Fig. 2) . DNA was isolated from peripheral blood using a DNA extraction kit (QiaAmp; Qiagen, Chatsworth, CA). Genotyping was performed as described earlier, and haplotypes were determined with GeneHunter25 (http://linkage.rockefeller.edu/soft/gh/provided in the public domain by Rockefeller University, New York, NY).
Mutation Screening
Based on the results of the linkage analysis for each gene locus, remaining candidate genes were analyzed by denaturing high-performance liquid chromatography (DHPLC).26 The corresponding genomic sequences of the genes were retrieved, and oligonucleotide primers were designed by Primer3, so that each exon was bracketed by the intronic primers. Before analyzing the patients samples, the optimal annealing temperature for PCR was determined empirically. After 30 cycles of PCR in a total volume of 20 µL, the amplification products were applied to DHPLC. Once the DNA fragments for which heterogeneous mobility in DHPLC were segregated with disease trait, direct sequencing was performed with a kit (BigDye Terminator Sequencing; Applied Biosystems) after treatment with shrimp alkaline phosphatase (Amersham Pharmacia Biotech, Parsippany, NJ) and exonuclease I (Amersham). The samples were denatured and analyzed with a DNA sequencer (3700 Gene Analyzer; Applied Biosystems). Mutation in exon 42 of PRPF8 was analyzed with a 217-bp PCR fragment generated with the primers 5'-ATTAGCAGGAGGCAGGGAAAC-3' and 5'-GTTGCCCTGTTAACATTGGCTGT-3', to which the tagged sequences (5'-ATT and 5'-GTT) were added for postlabeling purposes.24
| Results |
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Among the 58 STRs of which DNA fragments were successfully amplified, 53 (91%) showed polymorphisms among the five normal individuals. Poor amplification was observed for some of the markers under multiplex PCR conditions. Subsequently, 40 (69%) of 53 polymorphic STRs were used for multiplex amplification. Three to six STRs were used for each subset of multiplex PCR for each of the 10 genes. PCR product size, deduced from draft sequence data, ranged from 107 to 308 bp (average, 176 bp). The 40 markers consisted of 33 dinucleotide, 2 trinucleotide, and 5 tetranucleotide markers, of which 18 markers were labeled by R6G and 22 markers by R110. On the basis of five normal unrelated individuals analyzed, the number of alleles varied from two to six (median, three) and heterozygosity from 0.18 to 0.82 (average, 0.57).
Genotyping of the adRP-Affected Family
Ten PCRs for 10 gene loci were performed for each individual. A computer program (GeneScan; Applied Biosystems) produced clear chromatograms (Fig. 3) . Although fluorescein signals for the amplifications differed among STRs, all alleles were successfully genotyped by computer (Genotyper software; Applied Biosystems). Among the 40 STRs genotyped, all but two markers were consistent with Mendelian inheritance. One allele in marker 4 of RHO was not amplified in some individuals. We assumed this was due to the presence of a null allele. One allele of marker 4 of FSCN2 gene was regarded as a mutation in individual II-6, as shown in Figure 2 .
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Amplified DNA of exon 42 of PRPF8 showed a heterogeneous mobility in DHPLC in all affected individuals. Fragment size analysis revealed an extra peak 5 bp longer than the normal allele (Fig. 4 , left). Sequence analysis revealed a 6-bp deletion and 11-bp insertion of the coding nucleotides at nucleotides 6972 to 6977 (Fig. 4 , right). Because of the frame shift of the coding sequence, the last 11 amino acids (codons 2325 to 2335: VYSADREDLYA) were replaced by a longer putative sequence (TLCSLRIGRTCMPDRFPASCFSLPRPKPQPLQTGR) in the mutated allele. The deleted residues contain highly conserved sequence (DRED) across homologous genes in Mus musculus, Drosophila melanogaster, Caenorhabditis elegans, and Arabidopsis thaliana.
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| Discussion |
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With highly accurate human genome sequence data now available in the GenBank public database, it is possible to collect informative markers, such as microsatellites, that are tightly linked to most of the genes. By performance of linkage analysis with these markers, many of the candidate genes responsible for genetically heterogeneous hereditary diseases can be efficiently excluded. The efficiency of exclusion depends on the number of meioses (n) within the examined family. In the case of highly penetrant autosomal dominant diseases such as adRP, incidence of false loci remaining by chance after the exclusion test is given by (1/2)n, if the phase is known (i.e., if the DNA of grandparents of the affected parent are available for the study).29 For a nuclear family comprising parents and offspring, the chance is (1/2)n-1, because the phase is unknown. Thus, for a nuclear family with four children, the probability of failing to exclude nonresponsive genes under such phase-unknown conditions is 1 in 8, if markers associated with the candidate genes are fully informative. In this study, seven individuals (four plus two meioses with unknown phase) were genotyped, and one (or two) false and one true locus remained, after examination of 10 loci, a number close to that expected.
To facilitate the genotyping, we multiplexed the analysis by PCR. In general, optimization of multiplex PCR is a tedious step because of the interaction of primer sets, which may drastically affect product yield, even if nonspecific amplification and formation of primer dimers are avoided.30 However, once the optimization is reached, multiplex PCR is a highly efficient method of genotyping many loci. To analyze 10 gene loci, this method requires 10 PCRs and postlabeling per individual, thus the whole genotyping was accomplished by 70 reactions in this study. As an alternative, direct sequencing and/or DHPLC of the common adRP genes in a single proband may be comparable in cost and labor. However, such common genes differ widely in different ethnicities, as between the United States and Japan, and remain unknown in most ethnicities.11 An advantage of postlabeling in the multiplex genotyping is that sets of markers can be easily revised when the genomic sequence is updated (e.g., FSCN2 and HPRP3P). Toward more efficient use of this method, other candidate genes will be incorporated, such as PIM1K. Although we used 5 individuals to test the polymorphism of microsatellite markers, 50 individuals would give a more accurate representation of the number of different alleles in people and of the heterozygosity fraction. In this study, most markers were located within 500 kb of the candidate genes, so that they are also suitable markers for use in linkage disequilibrium analysis in the future.31
McKie et al.15 identified the gene in the RP13 locus to be PRPF8, a human homologue of the yeast PRP8. PRP8 is expressed ubiquitously and is a component of the spliceosome, an RNA-protein complex consisting of at least 50 proteins and five small nuclear RNAs that are involved in removal of introns of pre-mRNA.32 The association of the defects in the splicing process with RP is unknown. PRP8 exhibits striking phylogenetic conservation but has no obvious functional motifs, precluding testable predictions for its activity.32 So far, mutations in RP13 have been shown clustered within a 14-codon stretch in the last exon of PRPF8.15 20 The mutations reported previously were all missense and associated with a relatively severe phenotype, suggesting a dominant negative mechanism for disease causation.15 33 In this study, we identified a novel mutation of the PRPF8 gene (i.e., a frame-shift mutation that affects C-terminal 11-amino-acid residues). The additional amino acids at the C terminus due to the frame shift could also be responsible for altering the protein in such a way as to cause a dominant negative effect.
| Acknowledgements |
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| Footnotes |
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Submitted for publication September 2, 2002; revised October 7, 2002; accepted October 10, 2002.
Disclosure: H. Kondo, None; T. Tahira, None; A. Mizota, None; E. Adachi-Usami, None; K. Oshima, None; K. Hayashi, None
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: Hiroyuki Kondo, Department of Ophthalmology, Fukuoka University School of Medicine, 7-45-1, Nanakuma, Jonan-ku, Fukuoka, 814-0180, Japan; hkondo{at}fukuoka-u.ac.jp.
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