Study Patients

This study was conducted by the Taiwan Bipolar Consortium, which was established in 2003 with members from the Institute of Biomedical Sciences, Academia Sinica, and 25 psychiatric departments of general hospitals and psychiatric institutions in Taiwan.16 The consortium initially set out to understand genetic susceptibility to bipolar I disorder and broadened its scope to the pharmacogenetic study of mood stabilizers. The first part of the study has been described previously.16 In brief, unrelated patients, 20 to 65 years of age, who had bipolar I disorder were recruited from the psychiatric departments and institutions of the Taiwan Bipolar Consortium. Bipolar I disorder with recurrent episodes of mania with or without depressive episodes had been diagnosed in all the patients, according to criteria of the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV).17 We excluded patients with other psychotic and affective disorders.

Psychiatric nurses and psychiatrists evaluated the study patients using a cross-culturally validated Chinese version of the Schedules for Clinical Assessment in Neuropsychiatry (SCAN),18 supplemented by available medical records and reports from family members and treating psychiatrists. Only patients of Han Chinese descent were considered for the study, with ancestry determined on the basis of oral report by the patients to members of the research team. From March 2003 through May 2012, we recruited 1761 patients with bipolar I disorder.

Study Design and Oversight

We performed a discovery genomewide association study and two tests of replication. Using array methods, we genotyped 1647 patients with bipolar I disorder who were consecutively recruited from outpatient clinics and inpatient units of the 25 psychiatric departments and institutions in the Taiwan Bipolar Consortium. For the genomewide association study, we identified 294 (17.9%) of these patients (discovery cohort) who had received lithium treatment with good adherence for at least 2 years. The remainder of the patients in the series either did not receive such treatment or did not have good adherence for at least 2 years. We identified genetic regions associated with a response to lithium treatment and then performed a replication test using SNPs marking these loci in an independent group of 100 patients with bipolar I disorder (first replication cohort). These 100 patients were selected from 114 patients with bipolar I disorder (distinct from the discovery cohort) who were referred to us by staff psychiatrists who had treated them for more than 10 years with lithium and observed good adherence to the therapy. The remaining 14 patients were excluded because they did not fulfill the inclusion criteria.

In a second test of replication, we genotyped an independent series of 24 patients who had received lithium monotherapy for at least 2 years through May 2012 (second replication cohort). The inclusion criteria for these patients were based on a life chart that was constructed for all 1761 patients with bipolar I disorder. Each of these 24 patients had had a history of good adherence to mood stabilizers other than lithium before they initiated lithium monotherapy but had had an unsatisfactory response to such mood stabilizers.

The study was approved by the institutional review board at each participating hospital and at Academia Sinica, Taiwan. All the patients provided written informed consent.

Phenotype Definition and Assessment

To assess the response to long-term lithium treatment in bipolar I disorder, we prepared a life chart with a graphic depiction of a lifetime clinical course for each of the 1761 patients recruited before June 2012. This life chart included all manic, hypomanic, and depressive episodes with the date of onset (year and month), duration, and severity (including the extent of functional disability, hospitalization, and the presence of psychotic features); all doses of and duration of treatment with psychotropic drugs and mood stabilizers that were known have been prescribed; drug adherence, as recorded in medical charts for all visits at outpatient clinics; all recorded blood levels of mood stabilizers; and any adverse drug reactions. We presented this information graphically on the basis of integrated information gathered from direct interview with the patients and their family members, interviews with in-charge psychiatrists, and a thorough medical-chart review. On the basis of this life chart, we determined whether each patient had a history of good drug adherence.

We assessed the phenotype of lithium response on the basis of the life chart, using the Retrospective Criteria of Long-Term Treatment Response in Research Subjects with Bipolar Disorder developed by Martin Alda and colleagues (Alda scale) (see the Methods section in the Supplementary Appendix, available with the full text of this article at NEJM.org).12 The Alda scale has two criteria. Criterion A measures the extent of clinical improvement (the reduction in illness activity) and takes into account the frequency, duration, and severity of episodes during periods of lithium treatment considered to be adequate in duration and dose, as compared with the frequency, duration, and severity of episodes during periods in which the patient was not receiving lithium treatment. Criterion A is scored on a scale from 0 (no change in disease severity or exacerbation) to 10 (complete remission). Criterion B (divided into B1 through B5, with each part scored as 0, 1, or 2 points) is used to determine whether there is a causal relationship between clinical improvement and treatment. B1 and B2 are measures of the number and frequency of episodes, respectively, while the patient is not receiving lithium treatment (with higher scores indicating lower number and frequency of episodes), B3 is a measure of the duration of lithium treatment (with higher scores indicating lower duration), B4 is a measure of adherence to lithium treatment (with higher scores indicating worse adherence), and B5 is a measure of concomitant use of psychotropic medications during periods of stability (with higher scores indicating higher dose and longer duration of use). The total score is obtained by subtracting the sum of the B scores from the A score.

In investigating a causal relationship between lithium treatment and clinical improvement in individual patients, it is important to ensure the following three factors: the ability to compare the clinical course (number, frequency, and severity of episodes) between periods in which the patients were receiving lithium and those in which they were not, satisfactory drug adherence, and the minimization of influence from additional medications (hypnotics, antidepressants, antipsychotics, and other mood stabilizers). We determined inclusion criteria accordingly in order to minimize the misclassification of patients who had a response to lithium and those who did not have a response. For example, we included patients who had a poor response to lithium combined with prolonged use of antipsychotics or additional mood stabilizers and excluded those with a good response to lithium combined either with the use of an additional mood stabilizer throughout the course or with prolonged use of high-dose antipsychotic drugs (see the Methods section in the Supplementary Appendix).

For patients in the second replication cohort, we performed regular follow-up evaluations at outpatient clinics (usually monthly or at least once every 3 months) for at least 2 years. The evaluations included a lithium assay to assess drug adherence and a SCAN interview to assess the patient's clinical condition.

We tested the interrater reliability of the Alda scale by evaluating 18 randomly selected patients with bipolar I disorder from the discovery cohort. Three senior psychiatrists performed ratings that were based on the life chart. We observed an intraclass correlation among the three evaluators of 0.904 for the total score (0 to 10) (Table S1 in the Supplementary Appendix).

Outcomes

In previous studies that used the Alda scale, investigators adopted a total score of 6 to 7 as the best cutoff point between patients with no response to lithium treatment (0 to 6) and those with a response (7 to 10).12,19 For patients in the discovery cohort, we selected four potential cutoff points that were associated with a reduction in illness activity (4 to 5 points, >50% reduction; 5 to 6 points, >65% reduction; 6 to 7 points, >80% reduction; and 7 to 8 points, >90% reduction) for classifying patients according to their response to lithium treatment.

Genotyping, Imputation, and Sequencing

We genotyped samples obtained from the 1647 patients in the discovery cohort using the Illumina HumanHap550-Duo BeadChip and the HumanOmni1-Quad BeadChip and integrated the two data sets through imputation with HapMap phase 2 data. Quality-control procedures were applied to the genotype data and the imputed data (for details, see the Methods section in the Supplementary Appendix). We genotyped the top SNPs in the two replication cohorts using the Sequenom MassARRAY platform and then used the Applied Biosystems 3730 DNA Analyzer to sequence GADL1 in samples obtained from 94 patients with a response to lithium treatment and 94 patients without a response who were randomly selected from the discovery cohort (see the Methods section in the Supplementary Appendix).

Statistical Analysis

We compared the prevalence of alleles that were implicated by results of the genomewide association study in patients with a response to lithium treatment and those without a response using the Cochran–Armitage test for trend. The threshold P value was set at 6.9×10−9 after a Bonferroni correction for the number of SNPs (1,814,186) and for the four different cutoff points. We examined P-value distributions using quantile–quantile (Q-Q) plots (Fig. S2 in the Supplementary Appendix). We analyzed the data from the genomewide association study according to the four cutoff points used to classify patients with a response to lithium treatment and those without a response. We used PLINK software, version 1.07,20 to evaluate the top hits with adjustments for psychotic features (delusion and hallucination), a history of bipolar I disorder in at least one first-degree relative, rapid cycling, age at onset, sex, and history of alcoholism.