Treatment of Chryseobacterium indologenes ventilator-associated pneumonia in a critically ill trauma patient

OBJECTIVE

To report a case of Chryseobacterium indologenes ventilator-associated pneumonia (VAP) in a critically ill trauma patient.

CASE SUMMARY

This report describes a 66-year-old critically ill trauma patient who developed VAP, which was caused by C indologenes. The patient was injured in a riding lawn mower accident that trapped him underwater in a pond. The patient required surgery for intra-abdominal injuries and was mechanically ventilated in the trauma intensive care unit. On hospital day 5, the patient developed signs and symptoms of VAP. A diagnosis of C indologenes VAP was confirmed based on a quantitative culture from a bronchoscopic bronchoalveolar lavage. The patient's infection was successfully treated with moxifloxacin for 2 days followed by cefepime for 7 days.

DISCUSSION

Formally known as Flavobacterium indologenes, C indologenes is a Gram-negative bacillus normally found in plants, soil, foodstuffs, and fresh and marine water sources. Recently, worldwide reports of C indologenes infections in humans have been increasing, though reports from the United States are still rare. Bacteremia and pneumonia are the most commonly reported infections, and most patients are immunocompromised. The current case differs from most previous reports because this patient was in the United States and did not have any traditional immunocompromised states (eg, transplant, cancer, HIV/AIDS, or corticosteroid use).

CONCLUSION

This case report demonstrates that C indologenes can cause VAP in a trauma ICU patient.

Effectiveness of pseudoephedrine as adjunctive therapy for neurogenic shock after acute spinal cord injury: a case series

STUDY OBJECTIVE

To evaluate the effectiveness of pseudoephedrine as adjunctive therapy for neurogenic shock in patients with acute spinal cord injury (SCI).

DESIGN

Case series.

SETTING

Academic medical center.

PATIENTS

Thirty-eight patients admitted to the trauma intensive care unit between September 2005 and October 2012 with an acute SCI and who received more than 1 day of pseudoephedrine for one or more of the following: treatment of bradycardia (heart rate ≤ 50 beats/min), treatment of hypotension (systolic blood pressure < 90 mm Hg), or were receiving intravenous vasopressor support.

MEASUREMENTS AND MAIN RESULTS

The effect of adjunctive pseudoephedrine (PSE) was categorized as a success if vasopressors were discontinued after the initiation of PSE or improvement in the number of episodes of bradycardia was noted after the initiation of PSE as evidenced by decreased use of atropine. The effect of pseudoephedrine was categorized as a failure if it did not meet one of the criteria for success. The effect of pseudoephedrine was categorized as inconclusive if there were confounding factors such as vasopressors being restarted for another indication after initial discontinuation. Pseudoephedrine was successful in 31/38 (82%) patients, failed in 2/38 (5%) patients, and had inconclusive results in 5/38 (13%) patients. The mean ± SD time to successful weaning of intravenous vasopressors was 7 ± 7 days. Daily maximum pseudoephedrine doses ranged from 60-720 mg. Mean ± SD duration of pseudoephedrine therapy was 32 ± 23 days (range 2-135 days), with 64.5% of surviving patients discharged while receiving pseudoephedrine.

CONCLUSION

These data suggest that pseudoephedrine is an effective adjunctive therapy in facilitating the discontinuation of intravenous vasopressors and/or atropine in patients with acute SCI with neurogenic shock, although patients will typically require long durations of therapy.

Pooling/bootstrap-based GWAS (pbGWAS) identifies new loci modifying the age of onset in PSEN1 p.Glu280Ala Alzheimer's disease

The literature on GWAS (genome-wide association studies) data suggests that very large sample sizes (for example, 50,000 cases and 50,000 controls) may be required to detect significant associations of genomic regions for complex disorders such as Alzheimer's disease (AD). Because of the challenges of obtaining such large cohorts, we describe here a novel sequential strategy that combines pooling of DNA and bootstrapping (pbGWAS) in order to significantly increase the statistical power and exponentially reduce expenses. We applied this method to a very homogeneous sample of patients belonging to a unique and clinically well-characterized multigenerational pedigree with one of the most severe forms of early onset AD, carrying the PSEN1 p.Glu280Ala mutation (often referred to as E280A mutation), which originated as a consequence of a founder effect. In this cohort, we identified novel loci genome-wide significantly associated as modifiers of the age of onset of AD (CD44, rs187116, P=1.29 × 10⁻¹²; NPHP1, rs10173717, P=1.74 × 10⁻¹²; CADPS2, rs3757536, P=1.54 × 10⁻¹⁰; GREM2, rs12129547, P=1.69 × 10⁻¹³, among others) as well as other loci known to be associated with AD. Regions identified by pbGWAS were confirmed by subsequent individual genotyping. The pbGWAS methodology and the genes it targeted could provide important insights in determining the genetic causes of AD and other complex conditions.

The relationship between serum sodium and intracranial pressure when using hypertonic saline to target mild hypernatremia in patients with head trauma

INTRODUCTION

Limited data suggest mild hypernatremia may be related to lower intracranial pressure (ICP) in patients with traumatic brain injury (TBI). The practice at the study center has been to use hypertonic saline (HTS) to generate a targeted serum sodium of 145 to 155 mEq/l in patients with TBI. The purpose of this study was to determine the relationship between serum sodium values and ICP, and to evaluate the acute effect of HTS on ICP.

METHODS

A retrospective review of patients who were admitted to the trauma ICU for TBI, had an ICP monitor placed, and received at least one dose of HTS between January 2006 and March 2011 was performed. Data were collected for up to 120 hours after ICP monitor placement. The primary outcome was the relationship between serum sodium and maximum ICP. Secondary outcomes were the relationship between serum sodium and the mean number of daily interventions for ICP control, and the acute effect of HTS on ICP during the 6 hours after each dose. Linear regression was used to analyze the primary outcome. Analysis of variance on ranks and repeated measures analysis of variance were used to evaluate the number of interventions and the acute effect of HTS on ICP, respectively.

RESULTS

Eighty-one patients were enrolled with mean ± standard deviation age of 36 ± 15 years and median Glasgow Coma Scale score of 7 (interquartile range, 4 to 7). A total of 1,230 serum sodium values (range, 118 to 174 mEq/l) and 7,483 ICP values (range, 0 to 159 mmHg) were collected. There was no correlation between serum sodium and maximum ICP (R(2) = 0.0052). The overall mean ± standard deviation number of interventions for elevated ICP per day was 4.2 ± 2.9, 2.9 ± 2.0, and 2.6 ± 2.3 for patients with a mean serum sodium of < 145, 145 to 155, and > 155 mEq/l, respectively (P < 0.001). Regarding the acute effect of HTS on ICP, there was no statistical difference in mean ICP compared with baseline during hours 1 through 6 following HTS doses (baseline, 13.7 ± 8.4 mmHg; hour 1, 13.6 ± 8.3 mmHg; hour 2, 13.5 ± 8.8 mmHg; hour 3, 13.3 ± 8.7 mmHg; hour 4, 13.4 ± 8.7 mmHg; hour 5, 13.4 ± 8.3 mmHg; hour 6, 13.5 ± 8.3 mmHg; P = 0.84).

CONCLUSIONS

Serum sodium concentrations did not correlate with ICP values. These results warrant further evaluation and possible reassessment of sodium goals for ICP management in patients with TBI.

Clinical and microbiologic outcomes in trauma patients treated for Stenotrophomonas maltophilia ventilator-associated pneumonia

STUDY OBJECTIVES

To determine clinical and microbiologic plus clinical success rates in critically ill trauma patients who received treatment for Stenotrophomonas maltophilia ventilator-associated pneumonia (VAP).

DESIGN

Retrospective medical record review.

SETTING

Level I trauma intensive care unit of a large academic medical center.

PATIENTS

A total of 101 patients who developed S. maltophilia VAP between January 1997 and December 2007.

MEASUREMENTS AND MAIN RESULTS

Patients' baseline demographic and clinical characteristics, as well as characteristics of their VAP, were documented. The primary study outcome was the rate of clinical success in patients with S. maltophilia VAP; a secondary outcome was microbiologic plus clinical success rate in these patients. Standard definitions were employed to determine these outcomes related to VAP treatment. The study population had higher injury severity scores and a higher rate of traumatic brain injury than is typically observed in the study's intensive care unit. The median time to diagnosis of S. maltophilia VAP was 15 days (interquartile range 11-24 days). Stenotrophomonas maltophilia was the sole organism isolated in 34% of patients; the other patients had polymicrobial VAP. Despite inadequate empiric antibiotic therapy being administered to 97% of the patients, the overall clinical success rate was 87%. The microbiologic plus clinical success rate was 82%. The most common treatments for S. maltophilia VAP were trimethoprim-sulfamethoxazole (77 patients received monotherapy, 9 received combination therapy) and ciprofloxacin (6 patients received monotherapy, 8 received combination therapy); all-cause and VAP-related mortality rates were 13% and 7%, respectively.

CONCLUSION

Critically ill trauma patients with S. maltophilia VAP responded well to therapy despite high rates of inadequate empiric antibiotic administration. Trimethoprim-sulfamethoxazole was the most common therapy, but clinical success rates did not differ significantly based on antibiotic selection. This study adds significantly to the available S. maltophilia VAP outcomes data.

Treatment of recurrent Stenotrophomonas maltophilia ventilator-associated pneumonia with doxycycline and aerosolized colistin

OBJECTIVE

To report a case of recurrent Stenotrophomonas maltophilia ventilator-associated pneumonia (VAP) that was successfully treated with doxycycline and aerosolized colistin.

CASE SUMMARY

A 28-year-old male was admitted with a severe head injury and required mechanical ventilation. The patient developed S. maltophilia VAP on hospital day 17, which was cured after 7 days of treatment with high-dose intravenous trimethoprim/sulfamethoxazole (TMP/SMX). However, on day 34, the patient developed recurrent S. maltophilia VAP that did not respond clinically or demonstrate eradication on follow-up culture after 10 days of TMP/SMX. At that time, TMP/SMX was discontinued and treatment was initiated with intravenous doxycycline and aerosolized colistin. The VAP episode was cured after 14 days of treatment with doxycycline/aerosolized colistin.

DISCUSSION

S. maltophilia is an emerging cause of VAP in some centers. This organism is associated with high mortality rates and has few treatment options because it is intrinsically resistant to most drug classes. Recent data suggest that doxycycline and aerosolized colistin each are effective in treatment of other multidrug-resistant organisms, such as Pseudomonas aeruginosa and Acinetobacter baumannii. However, this is the first report describing the use of this antibiotic regimen for S. maltophilia. High-dose TMP/SMX is considered to be the drug of choice primarily based on excellent in vitro activity. Few data exist on how to treat patients who fail therapy with TMP/SMX or cannot receive that drug because of resistance, allergy, or adverse events. Thus, it is important to report alternative methods for treating this infection.

CONCLUSIONS

The positive clinical response to doxycycline and aerosolized colistin seen in the patient described here suggests that this combination may be an alternative treatment in patients who fail initial treatment or cannot receive standard therapies.

Adjunctive aerosolized antibiotics for treatment of ventilator-associated pneumonia

STUDY OBJECTIVE

To determine clinical and microbiologic success in patients receiving adjunctive aerosolized antibiotics for the treatment of ventilator-associated pneumonia (VAP).

DESIGN

Retrospective medical record review.

SETTING

Level I trauma intensive care unit of a large academic medical center.

PATIENTS

Forty-nine patients (mean +/- SD age 42 +/- 19 yrs) who received aerosolized antibiotics for the treatment of a total of 60 episodes of VAP caused by Pseudomonas aeruginosa and/or Acinetobacter baumannii between January 2001 and July 2007.

MEASUREMENTS AND MAIN RESULTS

Patients were identified by using an existing database of patients with documented VAP at the study center. To receive a diagnosis of VAP, patients had to have bacterial growth of 10(5) or more colony-forming units/ml from a bronchoscopic bronchoalveolar lavage and new or changing infiltrate on chest radiograph, plus at least two of the following: abnormal body temperature (> 38 degrees C or < 36 degrees C), abnormal white blood cell count (> 10 or < 4 x 10(3)/mm(3), or > 10% immature bands), or macroscopically purulent sputum. By reviewing patient data, we evaluated clinical and microbiologic success using standard definitions. The median (interquartile range) Injury Severity Score and admission Acute Physiology and Chronic Health Evaluation II score were 40 (29-45) and 17 (9-21), respectively. Pseudomonas aeruginosa, A. baumannii, or both were isolated in 45, 14, and 1 episode(s), respectively. Eighteen VAP episodes included additional bacteria. Aerosolized tobramycin, amikacin, and colistimethate were used in 44, 9, and 9 episodes, respectively. Systemic antibiotics were used in 59 (98%) of the 60 episodes. Clinical success was achieved in 36 (73%) of the 49 first episodes of VAP, 8 (73%) of 11 subsequent episodes, 17 (85%) of 20 episodes that were failing intravenous monotherapy, and 30 (79%) of 38 episodes with multidrug-resistant P. aeruginosa or A. baumannii. Microbiologic success was achieved in 29 (71%) of 41 evaluable episodes. Six patients died from VAP.

CONCLUSION

Treatment with adjunctive aerosolized antibiotics was associated with a good response rate in critically ill trauma patients with VAP due to nonfermenting gram-negative bacilli. It is noteworthy that episodes of VAP that followed intravenous therapy failure and/or that were due to multidrug-resistant organisms responded well.

Evaluation of glycemic control following discontinuation of an intensive insulin protocol

BACKGROUND

Intensive insulin protocols (IIPs) have been demonstrated to reduce morbidity and mortality in critically ill patients. Currently, there are no published studies evaluating glycemic control after discontinuation of an IIP.

OBJECTIVE

The purpose of this study was to compare blood glucose (BG) control during an IIP and for 5 days following its discontinuation (follow-up period).

METHODS

The study was a retrospective review of intensive care unit patients who received an IIP for >or=24 hours. Data were collected during the last 12 hours of the IIP and subsequent follow-up period.

RESULTS

For all 65 included patients, the mean +/- standard deviation for BG on the IIP was 123 +/- 26 mg/dL versus 168 +/- 50 mg/dL following discontinuation of the IIP (P < 0.001). The median (interquartile range) insulin that was administered decreased from 40 (22-65) units on the IIP to 8 (0-18) units after the IIP was stopped (P < 0.001). The mean daily BG during the follow-up period was significantly higher than that during the IIP (P < 0.001). Additionally, an insulin requirement of >20 units during the last 12 hours of the IIP was identified as a risk factor for poor glycemic control during the follow-up period (odds ratio: 4.62; 95% confidence interval: 1.17-18.17).

CONCLUSIONS

This study demonstrates a significant increase in BG following discontinuation of an IIP. Higher insulin requirements during the last 12 hours of an IIP were identified as an independent risk factor for poor glycemic control following the IIP. A standardized insulin transition protocol may help better control BG after discontinuation of an IIP.

Reduction in Inadequate Empiric Antibiotic Therapy for Ventilator-Associated Pneumonia: Impact of a Unit-Specific Treatment Pathway

Empiric antibiotic therapy is routinely initiated for patients with presumed ventilator-associated pneumonia (VAP). Reported mortality rates for inadequate empiric antibiotic therapy (IEAT) for VAP range from 45 to 91 per cent. The purpose of this study was to determine the effect of a unit-specific pathway for the empiric management of VAP on reducing IEAT episodes and improving outcomes in trauma patients. Patients admitted with VAP over 36-months were identified and stratified by gender, age, severity of shock, and injury severity. Outcomes included number of IEAT episodes, ventilator days, intensive care unit days, hospital days, and mortality. Three hundred and ninety-three patients with 668 VAP episodes were identified. There were 144 (22%) IEAT episodes: significantly reduced compared with our previous study (39%) ( P < 0.001). Patients were classified by number of IEAT episodes: 0 (n = 271), 1 (n = 98) and ≥ 2 (n = 24). Mortality was 12 per cent, 13 per cent, and 38 per cent ( P < 0.001), respectively. Multivariable logistic regression identified multiple IEAT episodes as an independent predictor of mortality (odds ratio = 4.7; 95% confidence interval: 1.684–13.162). Multiple IEAT episodes were also associated with prolonged mechanical ventilation and intensive care unit stay ( P < 0.001). Trauma patients with multiple IEAT episodes for VAP have increased morbidity and mortality. Adherence to a unit-specific pathway for the empiric management of VAP reduces multiple IEAT episodes. By limiting IEAT episodes, resource utilization and hospital mortality are significantly decreased.

Reduction in inadequate empiric antibiotic therapy for ventilator-associated pneumonia: impact of a unit-specific treatment pathway

Empiric antibiotic therapy is routinely initiated for patients with presumed ventilator-associated pneumonia (VAP). Reported mortality rates for inadequate empiric antibiotic therapy (IEAT) for VAP range from 45 to 91 per cent. The purpose of this study was to determine the effect of a unit-specific pathway for the empiric management of VAP on reducing IEAT episodes and improving outcomes in trauma patients. Patients admitted with VAP over 36-months were identified and stratified by gender, age, severity of shock, and injury severity. Outcomes included number of IEAT episodes, ventilator days, intensive care unit days, hospital days, and mortality. Three hundred and ninety-three patients with 668 VAP episodes were identified. There were 144 (22%) IEAT episodes: significantly reduced compared with our previous study (39%) (P < 0.001). Patients were classified by number of IEAT episodes: 0 (n = 271), 1 (n = 98) and > or = 2 (n = 24). Mortality was 12 per cent, 13 per cent, and 38 per cent (P < 0.001), respectively. Multivariable logistic regression identified multiple IEAT episodes as an independent predictor of mortality (odds ratio = 4.7; 95% confidence interval: 1.684-13.162). Multiple IEAT episodes were also associated with prolonged mechanical ventilation and intensive care unit stay (P < 0.001). Trauma patients with multiple IEAT episodes for VAP have increased morbidity and mortality. Adherence to a unit-specific pathway for the empiric management of VAP reduces multiple IEAT episodes. By limiting IEAT episodes, resource utilization and hospital mortality are significantly decreased.

Heparin-Induced Thrombocytopenia

Unfractionated heparin is widely used for numerous clinical situations. A well-known adverse effect of heparin exposure is thrombocytopenia. Heparin-induced thrombocytopenia is a clinicopathologic syndrome that can be associated with severe complications and significant mortality. The pathophysiology of heparin-induced thrombocytopenia includes an immune-mediated reaction to heparin that activates platelets and results in an acquired hypercoagulability. Diagnosis of heparin-induced thrombocytopenia should incorporate clinical signs and symptoms and laboratory testing for heparin-induced thrombocytopenia antibodies. Therapy should include discontinuation of heparin, initiation of a direct thrombin inhibitor, and eventually therapy with warfarin (only after the platelet count is at least 100 x 10(9)/L).

Aerosolised Antibacterials for the Prevention and Treatment of Hospital-Acquired Pneumonia

Aerosolised administration of antibacterials remains theoretically attractive for the prevention and treatment of hospital-acquired pneumonia (HAP) because of the ability to generate high drug concentrations at the site of infection. There is renewed interest in this area because of the shortcomings of current therapies and increasing multidrug resistance in Gram-negative organisms. Clinical trials of aerosolised or endotracheally administered antibacterials for HAP prevention have generally been positive; however, early trials were hampered by the development of resistance related to indiscriminate use. More recent trials have shown efficacy at HAP prevention without adverse effects on microflora as a result of more limited usage. However, prophylactic aerosolised antibacterials still need to be studied in large randomised trials before they could enter widespread use. The treatment of HAP with aerosolised antibacterials has mostly been reported in case series without control groups. Both early reports with aminoglycosides and the more recent use of colistin have reported very good response rates; even with organisms such as Pseudomonas aeruginosa and Acinetobacter baumannii. Aerosolised antibacterials were almost always added to intravenous therapy. On the basis of these reports, the current HAP guidelines allow the addition of aerosolised antibacterials in selected patients with multidrug-resistant organisms. This seems to be a reasonable recommendation until large trials are performed. Overall, toxicity was relatively low in the publications reviewed. Aerosolised drug administration in mechanically ventilated patients requires attention to a number of factors in order to maximise drug deposition in the lung.

Pharmacokinetic changes in critical illness

Physiologic alterations in critically ill patients can significantly affect the pharmacokinetics of drugs used in the critically ill patient population. Understanding these pharmacokinetic changes is essential relative to optimizing drug therapy. This article outlines the major differences seen in the absorption, distribution, metabolism, and excretion of drugs in critically ill patients. Important strategies for drug therapy dosing and monitoring in these patients are also addressed.

The futility of the clinical pulmonary infection score in trauma patients

INTRODUCTION

The Clinical Pulmonary Infection Score (CPIS) has received much attention recently. Advocates have touted its use for the diagnosis and duration of therapy in patients with ventilator-associated pneumonia (VAP). However, little has been written about its utility in trauma patients. The clinical, physiologic, and radiologic components of the CPIS may be difficult to differentiate from the systemic effects of injury. Quantitative cultures of the lower airway have been shown to be efficacious in differentiating VAP from the systemic inflammatory response syndrome (SIRS). In this study, we evaluated the potential use of CPIS as the sole means for diagnosis of VAP in critically injured patients.

METHODS

Patients were identified from the VAP database maintained in our Level I trauma center. Only those who had CPIS calculated at the time of bronchoscopy with BAL were included. VAP required >or=10 colonies/mL on quantitative BAL for diagnosis. Antibiotic therapy was based on quantitative BAL results. Patients with <10 colonies/mL were diagnosed with SIRS. Sensitivity and specificity of a CPIS>6 for VAP diagnosis (confirmed by BAL) were calculated.

RESULTS

In all, 158 patients underwent 285 BALs. The overall incidence for VAP was 42%. Patients with episodes of VAP and SIRS were well matched for age, Injury Severity Score, APACHE II score, and Glasgow Coma Scale score. The average CPIS was 6.8 in patients with SIRS and 6.9 for those with VAP. Using a CPIS>6 as the threshold for VAP only yielded a sensitivity of 61% and a specificity of 43%.

CONCLUSIONS

CPIS cannot differentiate VAP from SIRS in critically injured patients. Using CPIS to initiate antibiotic therapy in trauma patients could be harmful. Whether CPIS is useful to determine duration of antibiotic therapy is unknown.

Sequence variants of the DRD4 gene in autism: further evidence that rare DRD4 7R haplotypes are ADHD specific

A high prevalence of rare dopamine receptor D4 (DRD4) alleles in children diagnosed with attention-deficit hyperactivity disorder (ADHD) has been reported [Grady et al., 2003]. In this prior study, extensive resequencing/haplotype data of the DRD4 locus was used to suggest that population stratification was not the explanation for the high prevalence of rare alleles. In the current study, DNA resequencing/haplotyping was conducted on 136 DRD4 alleles obtained from autism probands, collected from the same geographic population as the prior ADHD probands (Orange County, CA). A number of studies have suggested that the susceptibility genes underlying these two disorders might partially overlap. Rare DRD4 variants were not uncovered in this autism sample beyond that expected by chance. These results suggest strongly that the high prevalence of rare DRD4 alleles in ADHD probands is due to ascertainment of the sample by diagnosis of ADHD.

Dopamine in drug abuse and addiction: results from imaging studies and treatment implications

The involvement of dopamine in drug reinforcement is well recognized but its role in drug addiction is much less clear. Imaging studies have shown that the reinforcing effects of drugs of abuse in humans are contingent upon large and fast increases in dopamine that mimic but exceed in the intensity and duration those induced by dopamine cell firing to environmental events. In addition, imaging studies have also documented a role of dopamine in motivation, which appears to be encoded both by fast as well as smooth DA increases. Since dopamine cells fire in response to salient stimuli, the supraphysiological activation by drugs is likely to be experienced as highly salient (driving attention, arousal conditioned learning and motivation) and may also reset the thresholds required for environmental events to activate dopamine cells. Indeed, imaging studies have shown that in drug-addicted subjects, dopamine function is markedly disrupted (decreases in dopamine release and in dopamine D2 receptors in striatum) and this is associated with reduced activity of the orbitofrontal cortex (neuroanatomical region involved with salience attribution and motivation and implicated in compulsive behaviors) and the cingulate gyrus (neuroanatomical region involved with inhibitory control and attention and implicated in impulsivity). However, when addicted subjects are exposed to drug-related stimuli, these hypoactive regions become hyperactive in proportion to the expressed desire for the drug. We postulate that decreased dopamine function in addicted subjects results in decreased sensitivity to nondrug-related stimuli (including natural reinforcers) and disrupts frontal inhibition, both of which contribute to compulsive drug intake and impaired inhibitory control. These findings suggest new strategies for pharmacological and behavioral treatments, which focus on enhancing DA function and restoring brain circuits disrupted by chronic drug use to help motivate the addicted subject in activities that provide alternative sources of reinforcement, counteract conditioned responses, enhance their ability to control their drive to take drugs and interfere with their compulsive administration.

The genetic architecture of selection at the human dopamine receptor D4 (DRD4) gene locus

Associations of the seven-repeat (7R) allele of the human dopamine receptor D4 (DRD4) gene with both the personality trait of novelty seeking and attention deficit/hyperactivity disorder have been reported. Recently, on the basis of the unusual DNA sequence organization of the DRD4 7R 48-bp tandem repeat (VNTR), we proposed that the 7R allele originated as a rare mutational event that increased to high frequency by positive selection. We now have resequenced the entire DRD4 locus from 103 individuals homozygous for 2R, 4R, or 7R variants of the VNTR, a method developed to directly estimate haplotype diversity. DNA from individuals of African, European, Asian, North and South American, and Pacific Island ancestry were used. 4R/4R homozygotes exhibit little linkage disequilibrium (LD) over the region examined, with more polymorphisms observed in DNA samples from African individuals. In contrast, the evidence for strong LD surrounding the 7R allele is dramatic, with all 7R/7R individuals (including those from Africa) exhibiting the same alleles at most polymorphic sites. By intra-allelic comparison at 18 high-heterozygosity sites spanning the locus, we estimate that the 7R allele arose prior to the upper Paleolithic era (approximately 40000-50000 years ago). Further, the pattern of recombination at these polymorphic sites is the pattern expected for selection acting at the 7R VNTR itself, rather than at an adjacent site. We propose a model for selection at the DRD4 locus consistent with these observed LD patterns and with the known biochemical and physiological differences between receptor variants.