(Hypertension. 2000;35:135.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
Human Na+/H+ Exchanger Genes
Identification of Polymorphisms by Radiation Hybrid Mapping and Analysis of Linkage in End-Stage Renal Disease
From the Departments of Biochemistry (H.Y., D.W.B.), Internal Medicine/Nephrology (B.I.F.), and Public Health Sciences (S.S.R.), Wake Forest University School of Medicine, Winston-Salem, NC.
Correspondence to Barry I. Freedman, MD, Department of Internal Medicine/Nephrology, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1053. E-mail bfreedma{at}wfubmc.edu
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Abstract—The Na+/H+ exchangers (NHEs) are membrane-bound transporters that catalyze the electro-neutral movement of extracellular Na+ for intracellular H+. NHE genes play a critical role in pH homeostasis and cellular volume regulation and can be considered candidate genes for essential hypertension and renal disease. This study was performed to determine whether the NHE genes contributed to genetic susceptibility in end-stage renal disease (ESRD). To date, 5 isoforms of NHE have been cloned in mammals (NHE1 to NHE5). The complementary DNA (cDNA) sequences of NHE1 to NHE3 and NHE5 are known in humans. Because the chromosomal structure of the NHE genes is unknown, we used cDNA sequences to design polymerase chain reaction primers for use in radiation hybrid mapping. Radiation hybrid mapping of NHE genes identified nearby polymorphic markers for NHE1 to NHE3 (NHE1: D1S197, D1S2677; NHE2: D2S373, D2S1789; and NHE3: D5S678, D5S2005). We used these markers, and other previously identified polymorphic markers for NHE5, in linkage and association analyses of ESRD. The NHE1 to NHE3 and NHE5 loci did not demonstrate evidence for linkage to ESRD. However, NHE5 showed significant evidence for association (P
1.0x10-4). The strongest evidence for
association was observed with allele 6 of NHE5
(P0.001 to 0.01). Allele 6 appeared to have a
renoprotective effect, with a frequency of 0.15 in the control
population and 0.06 to 0.09 in patients with ESRD. The combined
approach of designing primers from cDNA and radiation hybrid mapping
has proven successful in identifying polymorphisms for human genes
of which only cDNA sequences were previously available. The NHE primers
and associated polymorphic loci identified in this study can be
used in genomic, linkage, and association analysis of NHE genes
in future genetic studies of hypertension and renal failure. Given the
allelic association, further evaluation of the role of NHE5 in ESRD
susceptibility appears warranted.
Key Words: genes • renal disease • genetics • blacks • chromosome mapping
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Introduction |
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The Na+/H+ exchangers (NHEs), also called antiporters or solute carrier family 9 (SLC9A), are membrane-bound transporters present in all cells. In eukaryotic cells, NHEs catalyze the electro-neutral transport of extracellular sodium for intracellular protons with a stoichiometry of 1:1.1 2 They are also a major cellular proton-secreting system, which is induced by increasing concentrations of extracellular sodium.3 4
The first complementary DNA (cDNA) for a NHE (NHE1; SLC9A1: SLC9, isoform a1) was cloned in humans and expressed in NHE-deficient mouse fibroblast mutant cells.4 On the basis of sequence homology, this NHE and other NHEs have been cloned in human and mammalian cells, such as rabbit, rat, pig, and Chinese hamster.5 6 These NHEs include NHE2 (SLC9A2: SLC9, isoform a2),7 8 NHE3 (SLC9A3: SLC9, isoform a3),9 and NHE5 (SLC9A5: SLC9, isoform a5)10 in humans. Although the cDNA clone of NHE4 (SLC9A4) is available in the rat,11 it has not yet been cloned in humans.
NHE1 is highly conserved among many mammalian species, with >90% amino acid identity. It is expressed in most organs and plays a critical role in the maintenance of cellular pH homeostasis and volume.5 6 12 NHE2 and NHE4 closely resemble each other and their functions are unknown.5 12 NHE3 differs in sequence from other isoforms of the NHE and is involved in transepithelial sodium absorption. NHE5 resembles NHE3 in its amino acid sequence in humans (73% identity).11 Its functions are also unknown, but Klanke et al10 proposed that NHE5 may be an amiloride-insensitive NHE in the brain. NHE1 to NHE3 are present in the renal tubule, with NHE1 in the basolateral membrane of most renal tubule segments and NHE2 and NHE3 in the apical membrane of renal tubule segments.13 Although NHE3 is expressed in many tissues, Northern blot analysis indicates that most NHE3 is expressed in the kidney; thus, making it a candidate gene for susceptibility to renal disease.9
The physiological roles of NHEs in renal disease
are well known. In rats fed adriamycin, NHE1 expression
increased with the degree of glomerulosclerosis
and interstitial fibrosis,14 and the
acid-activation of NHE3 suggested that NHE isoforms could be involved
in the pathogenesis of renal tubular acidosis.15 Increased
NHE activity in the peripheral blood cells of diabetics and
hypertensives may also play a role in progressive renal
impairment.16 17 18 To date, genetic studies of the role
of NHEs in renal disease are limited. In mice, linkage analyses
point to NHE genes as candidates for type 1 diabetes
mellitus.19 Consequently, we believe that NHE genes are
possible candidate genes for human essential hypertension, diabetes
mellitus, and their associated end-stage renal disease (ESRD). Our
group has previously demonstrated that the familial predisposition to
ESRD in blacks with type 2 diabetes mellitus is more pronounced
than that in whites with type 1 diabetes.20 We determined
that the 
s for ESRD in blacks with type 2 diabetes approach 10
compared with 1 to 2 in whites with type 1 diabetes.21
Of the 4 NHE genes studied here, 3 have no previously reported associated polymorphisms (NHE1, NHE2, and NHE3). In this study, we used the radiation hybrid mapping technique to identify polymorphisms for these NHE genes. We then used these polymorphisms as genetic markers in linkage and association analyses of ESRD. We performed linkage and association analysis for NHE5 with previously identified polymorphisms.10
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Methods |
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Patient Populations
The study population represents an expansion of the data on black families that have multiple members with ESRD previously studied.22 23 24 Families were recruited through the "Family History of ESRD" database of incident dialysis patients in ESRD Network 6 (North Carolina, South Carolina, and Georgia). For this study, 367 individuals from 172 black families were evaluated (comprising 206 sibling pairs concordant for ESRD). These families and sib pairs were further characterized by the primary causes of ESRD. Ninety sib pairs were concordant for ESRD and type 2 (non–insulin dependent) diabetes mellitus. The remaining 116 sib pairs had ESRD associated with chronic glomerulonephritis, hypertension, and disparate and undetermined cause. DNA from 88 healthy blacks employed by the North Carolina Baptist Hospital (Winston-Salem, NC) were used as controls for determining allele frequencies with an allele-counting method for the black population and for the association studies. Families that have members with ESRD attributed to Alport’s syndrome, inherited cystic diseases (ie, autosomal dominant polycystic kidney disease), and urologic causes were excluded from the analyses.
Radiation Hybrid Mapping
NHE1, NHE2, and NHE3 genes were mapped with the Stanford G3
radiation hybrid map (version 2).25 Genomic DNA for the G3
radiation hybrid mapping panel was purchased from Research Genetics.
The polymerase chain reaction (PCR) methods used for amplification of
NHE gene-specific sequences and in the radiation hybrid mapping have
been previously described.24 The DNA sequences of human
NHE genes were retrieved from Genbank (http://www.ncbi.nlm.nih.gov).
The accession numbers are M81768 for NHE1, S81591 and S83549 for NHE2,
and U28043 for NHE3. We designed primers from these cDNA sequences and
used them as markers for the NHE genes in radiation hybrid mapping. NHE
primers were designed with a computer program called
PRIMER.26 Because the genomic sequences of NHE
genes have not been determined, it was necessary to design several sets
of primers from each cDNA to obtain 1 set that would efficiently
amplify human genomic DNA. The annealing temperature was set at 55°C
for all primers. Primer oligos were synthesized by the DNA Synthesis
Core Facility of the Comprehensive Cancer Center of Wake Forest
University School of Medicine.
Primers specific for each NHE gene were used to amplify genomic DNA from the Stanford G3 radiation hybrid panel. The results of PCR amplification of test PCR or PCR amplification of genomic DNA that encompassed the G3 radiation hybrid-mapping panel were visualized by electrophoresis of the products through 2% agarose gels (1% regular agarose and 1% low-melting agarose) followed by staining with ethidium bromide. For each NHE gene, the amplified PCR product was scored as present, absent, or ambiguous for the 83 radiation hybrid clones, and the resulting data were analyzed as described in the Results section.
Genotyping
The purpose of radiation hybrid mapping was to identify
polymorphic genetic markers closely linked to the NHE genes and to
use them as surrogates for the NHE genes in genotyping. Preferred
markers are those close to, and flanking, the NHE genes (<3 cM) and
highly polymorphic (heterozygosity >0.6). For each NHE gene, we
used 2 microsatellite markers identified in radiation hybrid mapping to
genotype our ESRD sib pair population. Primers for the markers
were either purchased from Research Genetics or synthesized by the DNA
Synthesis Core Facility of the Comprehensive Cancer Center of Wake
Forest University School of Medicine. The annealing temperature was
55°C for all primers. The methods of genotyping by PCR were
previously described.22 23
Cloning and Sequencing
The PCR product of the sequence tagged site designed for the
NHE2 gene was cloned and sequenced to verify its identity. The PCR
product amplified from the human Caucasian CEPH 1331-01 DNA was
purified by electrophoresis through an agarose gel. The appropriate
size fragment was cut out, and DNA was purified with the GeneClean II
(Bio 101). This product was cloned with vector pGEM T Easy
(Promega). Sequencing of this product from both ends was performed
with the use of the ABI 377 automated sequencer (PE Applied
Biosystems).
Data Analysis
Sequence Analysis
For each NHE gene, the genomic DNA sequence of the PCR
product was aligned with the cDNA sequence from which PCR primers
were derived. The alignment was performed by the comparative algorithm
of the computer program Sequence Navigator (Applied Biosystems Inc).
The human partial NHE2 cDNA sequence (Genbank accession S83549) was
also aligned to the human full NHE2 cDNA sequence (Genbank accession
S81591) and to the rat full NHE2 cDNA sequence (Genbank accession
L11004). Alignments were performed with the BLAST 2-sequence algorithm,
available from the Genbank site (http://www.ncbi.nlm.nih.gov).
Radiation Hybrid Analysis
The radiation hybrid data for each NHE gene were sent to
RHServer at the Stanford Human Genome Center by E-mail
(rhserver@shgc.stanford.edu). Results of the analysis were
returned by E-mail, which listed any linked marker on the G3 map
maintained by the Stanford Human Genome Center, along with the log of
the odds (LOD) score, and distance (in centiRay [cR]) of the linked
marker with the NHE gene. Raw mapping data with the G3-mapping panel
for several markers that flanked the linked marker were then downloaded
from the RHServer site (http://www-shgc.stanford.edu) and
analyzed together with the NHE data. We mapped each NHE gene
next to the linked marker on the G3 map by comparing the 2 point LOD
scores and distances of the NHE gene with the markers on the G3 map.
The 2-point analysis was conducted by RH2PT analysis by
the computer program package RHMAP.27
Linkage Analysis
Two types of linkage analyses were performed with the
affected ESRD sib pairs. One was to calculate the proportion of
siblings identical by state (IBS) with the computer program SIBPAL (sib
pair analysis) by SAGE.28 Because we use IBS, our
results are not as strong as if we had parental data with identity by
descent. The other analysis was the linkage and exclusion
analysis with MAPMAKER/SIBS (Whitehead Institute for Biomedical
Research).29 We have previously described the details of
these analyses.24
Association Analysis
The initial association analysis was performed with the
RXC program (George Carmody, University of Ottawa, Canada). This
program performs Monte Carlo simulations to calculate the statistical
significance of the 
2 contingency
tables.30 31 This program performs an overall comparison
of allele frequencies between the case and control populations. For
the marker NHE5, which showed evidence of association to ESRD, a
further analysis was performed with the ASSRELPREF option of
the Genetic Analysis System (GAS) package, version 2.0 (Alan
Young, Oxford University, 1995). This program evaluates data by the
Relative Predispositional Effect (RPE) method.32 The RPE
method identifies the associations between alleles and disease
sequentially according to their strength. As a consequence, each
allele can be evaluated individually and the relative contribution
to disease susceptibility can be estimated.
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Results |
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New STSs for NHE Genes
At the outset of this study, polymorphic markers for the analysis of human samples had not been identified for the NHE1, NHE2, and NHE3 genes. Several highly polymorphic markers have been reported for the NHE5 gene.10
For the NHE1 gene, primers NHE1a and NHE1c (Table 1) were designed to amplify the cDNA sequence between base pairs 2220 to 2491 from total human genomic DNA.4 The PCR product from total genomic DNA was the expected length of 272 bp. In other words, these primers were amplifying a portion of an NHE1 exon and could be used as the marker for the NHE1 gene. Similarly, primers NHE3a.for and NHE3a.rev (Table 1), which were designed to amplify the NHE3 cDNA between position 337 and 440, resulted in a human DNA product of the expected 104 bp.9
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The sequences from full-length8 and partial cDNAs7 (Genbank accession numbers: S81591 and S83549, respectively) were used to design NHE2 STS primers. Initially, 9 pairs of primers were designed from different positions along the full-length cDNA. None of the primers successfully amplified genomic DNA. PCR amplification of human genomic DNA was obtained from primers NHE2pc.for and NHE2pc.rev (Table 1) that were based on the partial cDNA sequence between position 400 and 517.7 The expected length was 118 bp, but amplification of genomic DNA from the NHE2pc.for and NHE2pc.rev primers yielded a PCR product of 1286 bp in length. The PCR product was cloned, sequenced, and found to contain an intron within the sequence (Figure 1). The GT-AG rule33 was obeyed at the 5' and 3' termini of the intervening sequence. The exonic portions of the sequence matched the cDNA sequence exactly.
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Because there are 2 human cDNA sequences for NHE2 in GenBank, and none of the 9 sets of primers designed from the full-length cDNA8 amplified human genomic DNA, the 2 sequences were further evaluated. They were compared with the rat NHE2 cDNA sequence (Genbank accession L11004).34 We performed sequence alignment by the BLAST 2-sequence analysis method. The human partial cDNA was aligned to positions 1653 to 2277 on the human full-length cDNA and to positions 1778 to 2320 on the rat full-length cDNA. The partial human cDNA had 90 and 87 nucleotides different from the human and the rat full-length cDNA sequences, respectively (Figure 2). However, only 4 nucleotides were different between the human and the rat full-length cDNA sequences. In addition, the full-length alignment between the latter 2 sequences produced only 14 mismatches, a negligible number considering that both cDNA are almost 3-Kb long (data not shown). Because the NHE2 partial cDNA reported by Dudeja et al7 maps to the correct location for NHE2 (see below), it is possible that the reported cloning of the full-length human cDNA might instead be a contaminant of rat cDNA.
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Radiation Hybrid Mapping
Radiation hybrid mapping of NHE genes was performed by PCR
amplification of genomic DNAs from the radiation hybrid mapping panels
with primers specific for each NHE gene. Results obtained from the
RHServer for the NHE1a and NHE1c primers indicated that the linked
marker was D1S3143, with the LOD score of 4.37 at a distance of 53.7
cR, which corresponds to a distance of 1611 kb.35
Two-point analysis with RH2PT showed that NHE1 mapped between
bin 28 and bin 29 on chromosome 1, next to D1S3143 on the Stanford G3
radiation hybrid map, version 2 (Figure 3A).
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Similarly, mapping of NHE2 with primers NHE2pc.for and NHE2pc.rev identified D2S373 as a linked marker for NHE2, with the LOD score of 12.4 at a distance of 8.6 cR, which corresponded to a distance of 258 kb. Two-point analysis showed that NHE2 was in bin 57 of chromosome 2 on the Stanford G3 map and could be placed between D2S373 and SHGC-13273 (Figure 3B). D2S373 and markers in neighboring bins, such as D2S135 in bin 58 and IL1A in bin 62, were mapped to 2q11.2 to 2q13 (CHLC marker map, version 4, http://www.chlc.org). Therefore, with primers derived from the sequence of Dudeja et al,7 NHE2 was mapped to the long arm of chromosome 2 (2q11.2 to 13), which was consistent with the somatic cell hybrid analysis performed by Szpirer et al.36
Mapping of NHE3 with NHE3a.for and NHE3a.rev (Table 1) identified D5S678 as a linked marker for NHE3. The LOD score and the distance were 9.66 and 21.17 cR (corresponding to a distance of 635 kb), respectively. The 2-point analysis placed NHE3 in bin 1 of the G3 map, on the short arm of chromosome 5. NHE3 is between the most telomeric cDNA marker SHGC-36639 and the polymorphic markers D5S678 and D5S2005 (Figure 3C).
Linkage Analysis with ESRD
On the basis of the results from radiation hybrid mapping, the 2
closest, preferably flanking, polymorphic markers were chosen to
genotype NHE genes: D1S197 and D1S2677 for NHE1, D2S373 and
D2S1789 for NHE2, and D5S678 and D5S2005 for NHE3 (Figure 3).
The distances obtained from the 2-point analysis were 106.6 cR
between D1S197 and D1S2677, 28.7 cR between D2S373 and D2S1789, and 0
cR between D5S678 and D5S2005. These distances correspond to 3.3 Mb, 1
Mb, and 0 Mb with the cR/Kb35 and chromosome-specific
cM/Mb conversion ratios.25 For convenience, we used
genetic distances of 3.0, 1.0, and 0.1 cM, respectively, in the
multipoint linkage analysis of these markers. For NHE5, we used
a dinucleotide polymorphic marker identified within a
cosmid subclone of the NHE5 gene and another highly polymorphic
marker D16S421.10 The distance between these 2 markers was
approximately 1 cM.10
Linkage analysis to evaluate cosegregation of the NHE genes with ESRD by SIBPAL indicated that none of the 8 markers showed evidence of linkage to ESRD (Table 2). The calculated mean identity by state (IBS2) did not deviate significantly from the expected (0.5) in either the total ESRD population or subpopulations on the basis of causes of ESRD (diabetic or non-diabetic).
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We evaluated the magnitude of the risk to siblings
(S) as a way to estimate how large a genetic
contribution to ESRD could be excluded on the basis of our data. In the
multipoint exclusion analysis performed with the program
MAPMAKER/SIBS (shown in Table 3), nearly
all of the markers could be excluded (LOD score -2.0) from
contributing a S of 2 for ESRD (ie, 2-fold
increased risk to siblings relative to the general population). This
was true in either the total population of all sib pairs or the
stratified population on the basis of the diabetes status (Table 3). Only D1S2677 was excluded at the relative risk of 3 in the
diabetic sib pairs. Thus, the markers for NHE1, NHE2, NHE3, and NHE5
could be excluded from contributing a substantial risk toward
developing ESRD in our study population on the basis of the numbers of
sib pairs available. However, we could not exclude them from
contributing a more modest level of risk.
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Association Analysis
The genotype data from NHE-linked polymorphic markers
was also evaluated for the presence of allelic associations with ESRD.
It has been observed that association studies may have more power than
conventional linkage studies in the context of closely spaced markers
or candidate genes.37 In addition to genotyping the
collection of ESRD sib pairs, we also genotyped a collection of
88 DNA samples from healthy black controls with each marker. The
results of this analysis of black controls are shown in Table 4. The allele frequencies in this
control population for each marker were compared with the allele
frequencies of the marker in the ESRD families. For the ESRD sib pairs,
data from a single affected individual (the proband) from each family
was used for the analysis. Initially, the overall distribution
of allele frequencies between controls and cases was evaluated with
the RXC program. Significant evidence of association was observed
between the NHE5 microsatellite and all cause ESRD
(P1.0x10-5), as well as when the
ESRD cases were stratified into NIDDM-associated ESRD
(P7.0x10-3) and non-diabetic ESRD
(P1.0x10-5). The allele
frequencies for NHE5 in cases and controls are summarized in Table 5.
With the observation that the allele frequency distributions for
NHE5 differed in the ESRD cases and normal controls, we evaluated the
contribution of each NHE allele to the association with the RPE
methodology.32 As shown in Table 5, NHE5
polymorphism alleles 6, 8, 9, and 10 show significant
association with ESRD in 
1 of the tested population groups. Among
these, allele 6 had an allele frequency >0.1. This is the
threshold allele frequency that is commonly used in association
analysis for identifying important allelic
association.38 The association is protective, rather than
with ESRD susceptibility, because the frequency of this allele is
lower in ESRD populations (varied between 0.061 and 0.087) than in the
normal controls (0.153).
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Discussion |
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In this study, we used radiation hybrid mapping to identify novel polymorphisms for the NHE1, NHE2, and NHE3 genes. These markers, and preexisting markers for NHE5, were tested for genetic linkage and association with the common causes of ESRD in blacks. Allele 6 of NHE5 was significantly associated with all-cause ESRD. None of the NHE markers demonstrated evidence for linkage to ESRD. Our inability to observe linkage may relate to a lack of power because of the number of families studied or other factors. Failure to find genetic linkage or association from NHE1 to NHE3 and ESRD could also occur because of potential heterogeneity. Our analysis suggests, however, that we can exclude major genetic contributions for the NHE1, NHE2, and NHE3 candidate genes to ESRD.
The observation of NHE5 alleles, which are associated with a reduced risk of ESRD, is intriguing. Enhanced erythrocyte NHE appears to be a sensitive and specific predictor of nephropathy in type 1 diabetes mellitus17 and predisposition to hypertension.39 NHE5, with an as yet unknown function, may be an amiloride-insensitive NHE in the brain.10 Central mechanisms are clearly important in the development of essential hypertension. Predisposition to hypertension could potentially explain the association observed between the NHE5 alleles and renal disease attributed to both hypertension and non–insulin dependent diabetes mellitus. We anticipate that it would probably be a protective effect because of the higher frequency of alleles 6 and 8 in healthy controls. Unfortunately, we were unable to measure functional characteristics of the NHEs in our patient population. This related to the variable levels of renal function (some patients received renal transplants) and different modalities of renal replacement therapy and medications to control blood pressure.
High-density linkage and physical maps are now available for the human
genome. For example, the 1993 to 1994 Genethon map consisted of 2066
(CA/GT)n repeat markers, which spanned 3690 cM of the human
genome.40 Most of these markers were spaced at 1 cM. The
1996 map consisted of 5264 (CA/GT)n repeat markers.41 Only
1% of these markers were in intervals >10 cM. Several radiation
hybrid maps have also been developed. The G3 map (version 2) covered
5994 STSs, with an average spacing of only 500 Kb.25 This
amount of genomic information suggests that any known gene will have a
nearby highly polymorphic microsatellite marker. These developments
allowed us to develop the novel polymorphic markers for the NHE
genes contained in this report.
The primers developed in this study represent novel STS markers for NHE genes. They were useful in radiation hybrid mapping for the identification of closely linked, highly polymorphic genetic markers. In this study, we used these polymorphisms identified in radiation hybrid mapping in linkage analysis of ESRD. These STSs can also be used for identifying yeast artificial chromosomes and bacterial artificial chromosomes that contain NHE genes. Cloning of large genomic DNA segments that contain these genes may enable investigators to evaluate the regulatory elements associated with the expression of these genes in humans.
Other than the NHE5 gene, there were no previously identified hypervariable genetic markers associated with the NHE genes, although the chromosomal positions of the human NHE1 and NHE3 genes were known. With our radiation hybrid mapping approach, we have been able to identify new polymorphic markers and to precisely map these genes on physical maps of the chromosomes. NHE1 was previously localized near the Rh (Rhesus) blood group locus on chromosome 1 by in situ hybridization with a cDNA probe and linkage analysis with a cDNA probe (1p36.2-p34).42 43 44 Only low-heterozygosity restriction fragment length polymorphisms and denaturing gradient gel electrophoresis polymorphisms were available for this gene.42 With the use of rat isoform-specific cDNA probes, the genomic positions for NHE2 and NHE4 have been determined on rat chromosome 9 and human chromosome 2.36 The exact locations, however, have not been determined for these 2 genes. In this study, we mapped human NHE2 to the long arm of chromosome 2 (2q11.2 to 13). Our analysis of NHE2 sequences suggests that one of the reported human cDNA clones, Genbank S81591, may be a rat-derived cDNA rather than the human sequence. NHE3 has been localized to human chromosome 5p15.3.36 44 Brant et al45 suggested that it might be the most telomeric gene identified on the short arm of chromosome 5. However, on the basis of our results of radiation hybrid mapping, another cDNA marker SHGC-36639 was more telomeric (Figure 2C), although its gene function is unknown. NHE5 was mapped to 16q21 to 22.10
As the incidence rates and mortality from diseases such as human immunodeficiency virus infection, cancer, and cardiovascular diseases decline, ESRD is increasingly important. This is especially true in the black population. The high incidence of diabetes and hypertension in blacks renders this population vulnerable to chronic renal disease. Little is known about the genetics of this disease. We have studied a number of candidate genes.22 23 24 At this time, none of these appear to be major genetic contributors to ESRD susceptibility, although alleles of plasma kallikrein (KLK3) and the interleukin-1 receptor antagonist (IL1RN) show association with ESRD.22 24 These alleles, and NHE5, may represent minor genetic contributors to ESRD susceptibility.
This study of NHE candidate genes complements an ongoing total genome screen of sib pairs with ESRD currently under way in our laboratory.46 The complementary approaches of evaluating known genes and searching for previously unidentified genetic components of ESRD should provide novel insights in the genetic contributions to ESRD in blacks. Because of the allelic association of NHE5 with ESRD, further evaluation of the role of NHE5 in ESRD susceptibility appears warranted.
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Acknowledgments |
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This work was supported by RO1 HL-56266 awarded to B.I.F. and R01 DK53591 awarded to D.W.B. Some of the results of this paper were obtained with the program package SAGE, which is supported by a US Public Health Service resource grant (1 P41 RR03655) from the National Center for Research Resources. We also thank the patients and staff of the dialysis facilities in ESRD Network 6 for their participation and support of this study.
Received June 27, 1999; first decision August 10, 1999; accepted August 18, 1999.
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