Chemical Senses Advance Access published online on February 2, 2008
Chemical Senses, doi:10.1093/chemse/bjm092
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A Genome-Wide Screen for Hyposmia Susceptibility Loci
1 Section of Otolaryngology-Head and Neck Surgery, Department of Surgery, The Pritzker School of Medicine, The University of Chicago, Chicago, IL 60637, USA 3 Section of Genetic Medicine, Department of Medicine, The Pritzker School of Medicine, The University of Chicago, Chicago, IL 60637, USA 5 Department of Human Genetics, The Pritzker School of Medicine, The University of Chicago, Chicago, IL 60637, USA 2 Present address: Department of Otolaryngology, Khon Kaen University, Khon Kaen, 40002, Thailand 4 Present address: Division of Endocrinology, Metabolism, and Molecular Medicine, Feinberg School of Medicine Northwestern University, Tarry 15-751 CH S217, 303 East Chicago Avenue, Chicago, IL 60611-3008, USA
Correspondence to be sent to: Jayant M. Pinto, Section of Otolaryngology-Head and Neck Surgery, The University of Chicago, 5841 S. Maryland Avenue, MC 1035, Chicago, IL 60637, USA. e-mail: jpinto{at}surgery.bsd.uchicago.edu
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Abstract |
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Olfactory dysfunction is an important public health problem in the United States, with approximately 14 million elderly Americans having chronic olfactory impairment. We performed a genome-wide linkage scan for loci influencing susceptibility to hyposmia in the Hutterites, a founder population of European ancestry. Using interviews regarding the olfactory medical history and psychophysical smell testing, we identified 25 individuals with severe hyposmia. Elimination of subjects with confounding conditions yielded 7 hyposmics for analysis. A 52-member pedigree including all affected individuals was constructed from the larger, >1623-member pedigree, and a genome-wide screen for loci influencing the trait of hyposmia using 1123 markers was performed. The most significant evidence for linkage with hyposmia extended over a 45 cM region on chromosome 4q (P = 0.0013). Although this signal meets the criteria for suggestive linkage only and will require replication, these results offer the strongest data to date on the effects of genetic variation on olfactory dysfunction.
Key words: genome, hyposmia, linkage, olfaction
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingAcknowledgementsReferences |
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Olfactory dysfunction is an important public health problem in the United States, with the reported prevalence of olfactory impairment ranging from 0.8% to 24.5% (Wysocki and Gilbert 1989
; Hoffman et al. 1998; Murphy et al. 2002). This wide variation probably reflects differences in study design and in the populations examined and known discrepancies between self-reported and tested (albeit subjective depending on cooperation) measures of olfactory function (Nordin et al. 1995). In a recent study using testing, it was estimated that approximately 14 million Americans over the age of 55 years have chronic olfactory impairment (Murphy et al. 2002). Because olfactory function declines with age (Doty et al. 1984), the clinical impact of olfactory dysfunction is likely to increase as our population ages.
Olfactory disorders have been classified as conductive (peripheral transport problems), sensorineural (damage to neuroepithelium), and central impairment (central nervous system derangement). Olfactory dysfunction can arise from multiple etiologies, including certain medications, endocrine or metabolic disorders, congenital defects, industrial exposures, infections, nasal obstruction, neoplasms, neurologic or psychiatric disease, nutritional abnormalities, and pulmonary disease (Murphy et al. 2003). The most common causes of hyposmia or anosmia include prior upper respiratory tract infections, head trauma, and sinonasal disease; these etiologies account for up to two-thirds of patients seen in clinics who have complaints related to their sense of smell (Murphy et al. 2003). Additionally, in up to 18% of patients with hyposmia, there is no clear cause (Temmel et al. 2002). Although evidence of damage to the olfactory neuroepithelium (including scar formation, neurodegeneration, and inflammation) is observed for some of the many etiologies, common pathways that cause this damage or hinder its repair have not been elucidated. Currently, our ability to diagnose hyposmia or anosmia in patients precisely remains rudimentary, leaving most diagnoses to clinical suspicion.
Treatment options for olfactory impairment of either conductive or sensorineural origin are severely limited by our current poor understanding of the pathophysiology of impaired olfaction. Although medical and surgical treatments are available in cases with conductive causes, they are limited in scope and success (Holbrook and Leopold 2003). Understanding the factors that predispose patients to olfactory loss would provide important contributions to both our basic understanding of olfaction in human beings and advances in clinical care, especially regarding genetic factors that influence susceptibility to environmental insults. Despite the large number of etiologies, the precise pathology involved in these disorders is incompletely understood. Studies of olfactory physiology in man have been limited by access to tissue, influences of environmental factors, and the complexity of the neurosensory physiology. Therefore, novel approaches to investigating olfaction in human subjects are necessary for advances in our understanding of this area.
Better to understand the genetics of olfactory dysfunction, we performed a genome-wide screen for loci that influence hyposmia in the Hutterites, a founder population of European ancestry that lives on communal farms across the western United States and Canada. This population has several important advantages for genetic mapping studies. First, the Hutterites living in South Dakota (the subjects of our studies) are derived from only 62 ancestors (maximum number of independent chromosomes = 124) who were born in the early 1700s to the early 1800s. Thus, reduced genetic heterogeneity and fewer susceptibility alleles present at a "disease" locus are expected.
Second, their relatively recent origin results in relatively few meioses that have occurred since the population's founding and, therefore, relatively larger chromosomal regions flanking disease genes that have remained intact than in the general population. As a result, linkage disequilibrium extends over a greater distance than in outbred populations, making the Hutterites a population ideally suited for mapping of genes that influence common diseases, by means of both association tests and linkage studies (Wright et al. 1999; Abney et al. 2000).
Third, the Hutterites practice a communal lifestyle, thus reducing confounding from environmental variables. All Hutterites are exposed to similar environments, which are measurable and consistent. For example, smoking is prohibited and rare in the population, thereby eliminating an environmental exposure that may affect olfaction. The uniform environment also minimizes the effects of nongenetic factors and should enhance the relative effects of genetics on variation in disease expression.
Fourth, prior studies in the Hutterites have shown that nearly all the common alleles that are present in the outbred European population are also present in the Hutterites at the same relative frequencies and show similar associations with common diseases (Bourgain et al. 2003; Newman et al. 2004). Thus, the loci that influence olfactory function in the Hutterites should be the same loci that influence hyposmia in the outbred populations, as our studies of other phenotypes such as asthma indicate so far (Abney et al. 2000; Donfack et al. 2000; Ober et al. 2000; Weiss et al. 2005; Kurz et al. 2006).
Lastly, the Hutterites of South Dakota have been the subjects of complex trait mapping studies for more than 10 years. As part of these studies, about 750 individuals have been genotyped at more than 1000 loci, and association-based mapping statistics have been developed that harness the wealth of information present in this large, extended pedigree (Abney et al. 2000, 2002; Bourgain et al. 2003). This data set has already been used for mapping of genes for asthma and atopy (Ober et al. 1999; Kurz et al. 2006), triglycerides (Newman et al. 2003), morning serum cortisol (Kurina et al. 2005), whole-blood serotonin levels (Weiss et al. 2004), stuttering (Wittke-Thompson et al. 2006), and more than 15 additional quantitative traits (Weiss et al. 2006).
Here, we present the results of a genome-wide screen for loci influencing the trait of hyposmia in this population.
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Materials and methods |
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Subject recruitment
In March 2001, we conducted a population-based study of Hutterites who were the subject of prior studies of the genetics of complex traits in our laboratory (Ober et al. 1999, 2000, 2001; Weiss et al. 2006). We used the following inclusion criteria: age above 13 years, ability to comply with the questionnaire and smell test, and presence in the colony (communal farm) on the day of the study visit. The Institutional Review Board of The University of Chicago approved this study, and informed consent was obtained.
Evaluation of phenotype
Subjects answered the 12-item Cross-Cultural Smell Identification Test (CC-SIT, now renamed B-SIT: Sensonics, Inc.; Haddon Heights, NJ), a psychophysical test that measures olfactory function. This test, modified from the University of Pennsylvania Smell Identification Test (UPSIT), has been validated in cross-cultural populations (Liu et al. 1995; Doty et al. 1996). We selected this measure because of its ease of administration to large numbers of subjects, its extensive use and validation in other studies, and its ability to provide a score with age and sex norms. The technique for administration of the CC-SIT is described in detail elsewhere (Doty et al. 1996) but involves 12 items of forced preference smell identification in a scratch-and-sniff format. Smell tests were scored with use of age and sex norms (Doty et al. 1996).
Because there are no instruments specifically designed for this purpose, we developed a survey containing closed- and open-ended questions regarding sinonasal and olfactory symptoms and medical history. This questionnaire was modified to meet Hutterite cultural norms and was administered through personal interview overseen by J.M.P. at the South Dakota Hutterite colonies. The questionnaires asked about perception of smell, the morbidities for olfactory dysfunction (current or recent [within the last 3 weeks] common colds, history of nasal trauma, history of rhinosinusitis/polyps/allergies, nasal/paranasal sinus surgery), and nasal symptoms (congestion, postnasal drip, facial pressure, facial pain, headache, and nasal obstruction). Subjects did not know the result of the smell test before answering the smell perception question. Presence of atopy was determined by skin prick testing performed during prior visits (Ober et al. 2000).
The diagnosis of hyposmia was based on the following criteria: 1) a CC-SIT score demonstrating loss of smell according to normative values (
5th percentile) and 2) absence of confounding medical conditions. Setting our threshold for hyposmia low (5th percentile), the phenotype of hyposmia represents a generalized hyposmia. Because of the inherent limitations of the test (12 items, no ability to test threshold) and of phenotyping in the field, and the large number of possible odorants, we cannot accurately further delineate the phenotype into hyposmia to a subset of odorants. It was not possible to repeat testing of the individuals involved. However, the test–rest reliability of the CC-SIT is reported to be relatively high (0.71), and this 12-item test was found to predict accurately the performance on the 40-item UPSIT (Doty et al. 1995).
Genotyping
DNA was extracted from whole blood by standard methods. Genotyping of 658 autosomal microsatellite markers (screening sets 9 and 51) was performed at the Mammalian Genotyping Service in Marshfield, WI (http://research.marshfieldclinic.org/genetics/home/index.asp) by means of a scanning fluorescence detector system developed for high-throughput genotyping. An additional 226 microsatellites and 239 single-nucleotide polymorphisms (SNPs) or insertion/deletions were gentoyped in our laboratory in these individuals in selected regions according to standard methods. The 1123 microsatellites and SNP markers have an average spacing of 2.6 cM. The deCODE genetic map was used for reference in this analysis (www.decode.com) (Kong et al. 2002).
Statistical analysis
Agreement between the perception of smell and the result of the CC-SIT was analyzed by means of kappa statistics. Logistic regression was used for analysis of the associations between the comorbidities and olfactory dysfunction. Such comorbidities included allergic rhinitis, upper respiratory tract infection (cold), and a history of trauma to the nose/paranasal sinuses.
Because the pedigree that included all the hyposmics was too large for programs using algorithms of exact methods, we performed a multipoint nonparametric linkage (NPL) analysis by using Simwalk 2.83 (Sobel and Lange 1996). This program uses a Markov Chain Monte Carlo approach to sample possible allele descent trees according to their likelihood. Two identity by descent (IBD) statistics that measure the degree of allele sharing among affected individuals were calculated and P values determined by comparison to a null distribution of these statistics when only the pedigree structure and the affection status of each person was available. NPLpairs is the extent of allele sharing among all affected pairs, and NPLall is the extent of allele sharing among all affected individuals. Both statistics are intended to detect linkage to traits in an additive model.
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Demographics
There were 297 subjects who participated in the study. Data were available for 285 subjects for the smell test and for 291 subjects for the sinonasal/olfactory questionnaire. Of the 285 subjects, 114 were male and 171 were female, with ages ranging from 14 to 74 years (mean, 33.4 years). Relevant comorbidities, including the common cold, allergic rhinitis, or a history of trauma to the nose or nasal/paranasal sinus surgery, are shown in Table 1.
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The overall prevalence of an abnormal CC-SIT was 8.8% (95% confidence interval [CI]: 5.8–12.7%). This is lower than the 24.5% prevalence reported in a community-based study (Murphy et al. 2002
) and may reflect population differences, risk exposures such as pollution or tobacco smoke that are absent in this population, or the severity of our cutoff (5th percentile).
The demographic and clinical features of the 25 people with abnormal CC-SIT are shown in Tables 2 and 3. In an unadjusted analysis, female sex (odds ratio [OR] = 1.8), a recent or current cold (OR = 2.88), and past nasal trauma (OR = 1.16) were associated with hyposmia (Table 4). However, in a multivariate analysis, only female sex (adjusted OR = 3.07, 95% CI = 1.01–9.39, P = 0.05) and the presence of a common cold (adjusted OR = 2.99, 95% CI = 1.14–7.87, P = 0.03) remained significantly associated with olfactory dysfunction in the Hutterites (Table 4). Although a history of trauma to nose and paranasal sinuses was more common in the hyposmics (adjusted OR = 2.24), this was not statistically significant (95% CI = 0.55–9.07, P = 0.26). Surprisingly, age was not associated with olfactory dysfunction in this population, and there was a higher percentage of subjects in the younger (less than 50 years) age group with hyposmia. The correlation of hyposmia by CC-SIT and reported normal or abnormal smell perception was 0.08 (95% CI = –0.04–0.2). This is comparable to the results of other studies showing discrepancies between self- reported and tested loss of smell and emphasizes the need for testing (Doty et al. 1986; Nordin et al. 1995; Temmel et al. 2002).
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Selection of cases of hyposmia
To minimize the heterogeneity of the hyposmic phenotype, we focused our studies on subjects who were identified by CC-SIT and we excluded those with comorbidities. Therefore, 120 individuals with a current or recent cold (within the last 3 weeks) as well as 31 individuals with a history of nasal or paranasal surgery or facial trauma were excluded. Subjects with atopy were included in the study because atopy was not associated with hyposmia in this sample. This resulted in a final sample of 7 Hutterites with hyposmia in the absence of these related factors. Review of questionnaires from these subjects did not reveal any other relevant clinical information; specifically, these individuals had no acute or chronic medical conditions, they did not take any medication, and they gave no other medical history. For the genetic studies, we then broke all inbreeding loops in the pedigree and constructed a 52-member pedigree by using PedHunter (Agarwala et al. 1998), which was the smallest pedigree that included the 7 affected individuals (Figure 1).
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Genome screen for hyposmia
NPL analysis is a powerful tool for genetic mapping (Kruglyak et al. 1996). This technique is independent of specific models for the inheritance of the phenotype and is based only on IBD measurements at the marker loci. IBD refers to the probabilities that affected individuals share marker alleles identical by descent, that is, inherited from a common ancestor. If a marker is linked to a disease locus, one expects to see a clustering, among the affected individuals, of a few marker alleles descended from the pedigree founders.
The P values for the NPLall statistic across the 22 autosomes are plotted in Figure 2. The NPLpair results were virtually indistinguishable from the NPLall results and therefore are not shown. The strongest evidence for linkage to hyposmia in the pedigree was observed on chromosome 4q with marker D4S3333 at 137.4 cM from the p terminus (p ter) (P = 0.0013). Additional regions showing nominal evidence for linkage include chromosome 12 (P = 0.047 at 52.54 cM from p ter and P = 0.037 at 110.24 cM from p ter), chromosome 16 (P = 0.046 at 114.00 cM from p ter), and chromosome 17 (P = 0.048 at 21.40 cM from p ter and P = 0.044 at 56.48 cM from p ter).
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The strongest linkage signal (D4S3333) meets the criteria for suggestive linkage (P < 1.7 x 10–3) following Lander and Kruglyak (1995)
. Whereas it is not computationally feasible to conduct the simulations required for determining if our results meet genome-wide significance, it is important to note that estimates of the empirical P value using Simwalk2 were found to be conservative of the true P value when compared with exact results for large simulated pedigrees where the exact results were known and for pedigrees small enough so that exact P values could be calculated (Sobel and Lange 1996). Therefore, we are likely to be underestimating the true significance of our findings in these analyses. We are much less confident of the robustness of the weaker linkage signals, which marginally meet nominal significance at best. These could easily represent false positive results. Hence, we focused subsequent investigations on our most significant linkage peak only.
To define a critical region for further investigation, we set a P < 0.01 significance threshold boundary of 102–147 cM (Figure 3), corresponding to a 53-Mb physical distance mapping to the 
95–148 Mb position on chromosome 4 (4q22.3–4q31.22) (NCBI Build 36.2, September 2006). Within this critical region, 2 subpeaks are evident in addition to the most significant peak at 137.4 cM; they correspond to D4S1647 (P = 0.0034 at 104.9 cM; peak C in Figure 3) and D4S2394 (P = 0.0017 at 129.2 cM; peak B in Figure 3). This 52-Mb critical region contains 288 genes (183 known RefSeq genes, 105 hypothetical genes), none of which are olfactory receptor genes.
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Discussion |
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Demographics of olfactory dysfunction in the Hutterites
The prevalence of olfactory dysfunction among the Hutterites, as defined by the performance on the CC-SIT, was 8.8% (95% CI = 5.8–12.7%). This prevalence was higher than in some previous community-based studies (Wysocki and Gilbert 1989; Hoffman et al. 1998) but lower than others (Murphy et al. 2002) that may reflect differences in study design (e.g., tested vs. self-reported olfactory determination, self vs. monitored administration). The agreement between the awareness of olfactory loss and the CC-SIT results was quite poor in this study. That people usually are unaware of their smell loss despite having olfactory dysfunction implies a lack of perception, minimal loss of smell, or misinterpretation by the subject of the test. Likewise, people believing their olfactory sense was abnormal had, in fact, normal olfaction by CC-SIT, implying that the test is not capturing the perceived abnormality. Therefore, self-reported smell function may provide misleading information, and olfactory testing is necessary for assessment of olfactory function. Doty et al. (1988) found that, in the case of Parkinson's disease, approximately 90% of patients had a demonstrable olfactory deficit, even though only 28% of such patients were aware of their deficit. Studies in other clinical settings noted similar observations (Doty et al. 1986; Nordin et al. 1995; Temmel et al. 2002), including the use of threshold testing (Philpott et al. 2006). Murphy et al. (2002) also showed that self-report in older adults significantly underestimated the prevalence rates obtained by olfactory testing.
The common cold and female sex were found to be 2 factors contributing to olfactory dysfunction in the Hutterites, with the adjusted ORs of 2.99 for presence of the common cold (95% CI = 1.14–7.87) and 3.07 for female sex (95% CI = 1.01–9.39), respectively. Age above 50 years, allergic rhinitis, and a history of nasal/facial trauma including nasal/paranasal sinus surgery, after adjusting for other effects, did not show any statistically significant effect on olfactory dysfunction in this study. Although there was a trend toward increased olfactory dysfunction with a history of trauma to the nose/paranasal sinuses (adjusted OR = 2.24 with a wide 95% CI of 0.55–9.07), this was not statistically significant, perhaps as a result of the lack of severe head trauma in this rural population (there were no motor vehicle accidents or assaults).
The common cold has been postulated to be one of the leading causes of olfactory dysfunction (Temmel et al. 2002). This could occur as a result of the conduction problems caused by nasal congestion or inflammatory effects on the olfactory pathway (Cowart et al. 1993). Rydzewski et al. (2000) demonstrated that the prevalence of hyposmia in allergic rhinitis was 21.4%, higher than the prevalence in our study of 7.9%, and may reflect the stringency of the definition of hyposmia in our study. Simola and Malmberg (1998) compared olfactory thresholds in 105 rhinitis patients and 104 healthy controls to analyze possible relationships between the sense of smell and rhinitis, age, sex, smoking, prick-test results, nasal resistance, and history of nasal or paranasal surgery. They found that age and rhinitis were the only variables with a significant effect on the olfactory threshold. Neither smoking habits nor a history of nasal or paranasal surgery were related to olfactory thresholds in that study. The differences between our results and those of other studies might be due to the differences in population (urban vs. rural), study design (hospital based vs. population based), different measurement of olfactory function (threshold vs. identification vs. subjective report), or the different status of allergic rhinitis at the time of test administration (all testing in our study was done in the winter months, outside of certain allergen seasons). In particular, the lifelong absence of exposure to environmental tobacco smoke (which is known to impair olfactory function [Frye et al. 1990; Murphy et al. 2002]) in the Hutterites may contribute both to the overall lower prevalence of hyposmia and the lack of an association with age.
Genetics of olfaction
A genetic perspective to understanding olfactory dysfunction should provide insight into chemosensory physiology. Studies employing a genetic approach to olfaction in man are limited, although animal studies suggest a genetic basis for a variety of related phenotypes (reviewed in Segal et al. 1995). Limited data from human studies suggests that genetic factors may affect the sense of smell. For example, idiopathic hypogonadotrophic hypogonadism with anosmia, or Kallmann's syndrome (KS), is a rare genetic disorder with variable expressivity (Kallmann et al. 1944). Family members have a variety of phenotypes, including anosmia alone. This condition is genetically heterogeneous, with both X-linked, autosomal dominant, and autosomal recessive transmission reported (Kallmann et al. 1944; Hockaday 1966; White et al. 1983). Mutations in the KS1 locus (KAL1) on chromosome Xp22.3 have been identified in the X-linked form (Franco et al. 1991; Legouis et al. 1991; Bick et al. 1992; Hardelin et al. 1992), and mutations in the fibroblast growth factor receptor 1 locus (KAL2) on chromosome 8p11.2–12 cause one autosomal dominant form of the disease (Dode et al. 2003). A recent study also reported linkage of KS to chromosome19p13, with loss-of-function mutations in the GPR54 locus causing KS (de Roux et al. 2003).
Studies of nonsyndromic forms of anosmia or hyposmia also support a genetic basis of olfactory function. Forrai et al. (1981) examined the genetic influence on the ability to smell ketones by using a twin pair study design and observed genetic effects on the detection of acetone. Wysocki and Beauchamp (1984) studied sensitivity to androstenone and pyridine by using twin pairs, showing that all the monozygotic twin pairs, but only 61% of dizygotic pairs, were concordant for sensitivity to androstenone. A genetic influence on androstenone detection was confirmed by Gross-Isserhoff et al. (1992) who also used a twin pair design and showing a genetic effect on sensitivity to isoamyl nitrate. Using the UPSIT, Doty et al. (1984) and Segal et al. (1995) found a higher correlation of scores between monozygotic twins (r = 0.31) than between dizygotic twins (r = 0.15). Using a different measure of preference ratings of UPSIT odors, Topolski reported a genetic influence on certain items (Segal et al. 1995). Also, in a twin pair study, Finkel et al. (2001) found moderate heritability for several olfactory measures, including odor identification, intensity, detection, and pleasantness.
Knaapila et al. (2007) recently reported the first genome-wide screen of olfactory phenotypes. They did not find any significant results for linkage with the phenotype of identification by CC-SIT, although there were methodologic differences in their analytic strategy (phenotyping all subjects without exclusions; quantitative mapping based on total score on the B-SIT vs. qualitative analysis based on selecting severe hyposmia by age/sex norms in our study) and marker density (350 markers vs. 1123 in our study). Additionally, this population sample of 146 individuals from 26 families was recruited through a study of migraine in Finland. Population, diet, allele frequency, or environmental differences, inclusion of smokers, and/or the presence of migraine may explain the discrepancy between their results and ours. Interestingly, they did show a significant linkage with the phenotype of pleasantness of cinnamon odor (logarithm of the odds = 3.01) on chromosome 4q33.3 at 163.65 cM, just outside our region.
Recently, subjects with Bardet–Biedl syndrome (BBS), which encompasses retinal degeneration, truncal obesity, renal and limb malformations, and developmental delay, have been shown to have partial or complete anosmia (Kulaga et al. 2004). To date, 12 BBS loci have been identified (reviewed in Li and Wong 2001; Mykytyn et al. 2001; Nishimura et al. 2001; Sheffield et al. 2001; Mykytyn et al. 2002; Ansley et al. 2003; Badano et al. 2003; Chiang et al. 2004; Fan et al. 2004; Nishimura et al. 2005; Chiang et al. 2006; Stoetzel et al. 2006, 2007). In fact, the loci for 2 members of this novel group of genes, BBS7 (Badano et al. 2003) and BBS12 (Stoetzel et al. 2007), lie within our most significant region on chromosome 4. This finding may indicate that more subtle perturbations in BBS7 and/or BBS12 function may underlie olfaction in nonsyndromic olfactory dysfunction and make these loci excellent candidates for association with hyposmia.
Although the large number of candidate genes precludes extensive discussion here, other promising candidate genes include SCYE1 (Shalak et al. 2007), a cytokine generated in apotosis, and CASP6, a caspase that is activated in human neurons upon an apoptotic insult. Interestingly, apoptosis has been implicated in olfactory pathology (Kern et al. 2004). Additionally, another candidate in the region, UNC5C, a receptor involved in both neural migration and development, has also been reported to be involved in apoptosis (Thiebault et al. 2003).
Our results further support a genetic etiology for hyposmia, and they suggest that genetic variation influences the olfactory system. We selected our phenotype and threshold in order carefully to reduce the issue of confounding environmental and host factors as much as possible to focus on idiopathic olfactory decline. Nevertheless, it is possible that our affected individuals had an environmental exposure influencing their susceptibility to olfactory loss. As viral illness is one of the most common reported causes of the loss of the sense of smell, it is possible that postviral damage is the etiology of the olfactory loss in our affected individuals. In olfactory clinics, some patients note the loss of smell to be temporally associated with an especially severe viral upper respiratory tract infection (URI), but some do not notice the insult. Moreover, injury from a URI that is subtotal or predisposing to further loss by apoptosis or other mechanisms without patient perception is entirely possible. In this case, the phenotype being mapped in our study might be an example of a gene–environment interaction where genetically susceptible individuals who experience the appropriate viral insult are predisposed to loss of smell. Nevertheless, mapping of genes that underlie such a phenomenon would provide information on the mechanisms involved in this process. Hence, our data would still provide useful information on susceptibility to olfactory loss, although perhaps not on nonvirally mediated decline.
Another possibility is that the affected individuals share a common diagnosis of isolated, nonsyndromal congenital hyposmia. We believe that it is unlikely that our subjects had congenital hyposmia, given the lack of a supportive medical history and the absence of behaviors (inability to detect body odor, complaints of flavorless food, inability to notice farm odors, spoiled milk, etc.) that would be associated with this disease. Although a number of our affected subjects were in their teens, we believe that our demographics reflects the age structure of our population, which has a large number of younger individuals because of large family size.
Lastly, isolated congenital hyposmia is extremely rare and has been mapped in one small family study to a different region of the genome (chromosome 18p) (Ghadami et al. 2004). We believe that a number of factors influence the complex trait of hyposmia, both environmental and genetic, and teasing out this interaction remains a challenge. Although the injuries might be heterogeneous, there may be a common genetic susceptibility to a variety of insults that lead to a common outcome of olfactory decline. Our studies will eventually require replication in other population samples for analysis of these issues.
The ability of the nervous system to distinguish among many odors and the variability in olfactory function among individuals are under intense scrutiny. To understand how genetic factors may explain the variability in human olfaction between individuals, Menashe et al. (2002, 2003) examined the prevalence of segregating pseudogenes in the olfactory receptor gene family and proposed that the combination of these polymorphisms could underlie variability in olfaction between individuals and populations. Indeed, Keller et al. (2007) recently reported evidence that supports this concept, finding that variation in an olfactory receptor gene, OR7D4, affects odorant perception in human subjects and is associated with differential binding to its odorant in vitro. Additionally, 2 of the common deletion polymorphisms in the human genome are in olfactory receptor genes (McCarroll et al. 2006). These are intriguing ideas that attempt to explain how olfactory function varies among individuals. However, although specific olfactory receptor genes have been implicated in specific anosmia in mouse models (Griff and Reed 1995; Zhang and Firestein 2002), our phenotype of generalized hyposmia is perhaps less likely to be caused by variation in olfactory receptor genes. Indeed, there are no olfactory receptor genes in the linked region on chromosome 4q, indicating that variations in nonolfactory receptor genes also influence smell perception.
This study was the first step in identifying novel genes that contribute to variation in olfaction in man. Subsequent studies to identify the specific gene within our linked region are currently under way.
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Funding |
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The University of Chicago General Clinical Research Center (T32 DC 00058-04 to J.M.P., T32 HL 007605 to M.G.H., HL56399 to C.O., HL66533 to C.O., M01 RR00055); Mammalian Genotyping Service (HV-48141).
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Acknowledgements |
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The authors thank Harvey Dytch, Robert Stanaker, Rebecca Andersen, Dr Darrel J. Waggoner, Erika Sorensen, Daniel Schneider, Natasha Phillips, Marcella DeTineo, Jamie Phillips, and Justin Nguyen for technical assistance; Dr Nancy J. Cox for helpful discussion (The University of Chicago); and Dr Rodney Parry for help in planning and coordinating field trips to Hutterite colonies (University of South Dakota).
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References |
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Accepted 7 December 2007
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