Bipolar disorder patients with comorbid alcohol use disorder show a higher frequency of a serotonin transporter gene polymorphism than their peers with no alcohol comorbidity, research shows.

The effect was only seen in female patients, however, and disappeared after correction for multiple comparisons, prompting the researchers to call for further studies to confirm the link.

“Comorbidity in bipolar disorder often complicates the course of the disease as patients use more healthcare resources and experience greater rates of impairment, disability, and suicide,” Usoa Busto (Centre for Addiction and Mental Health, Toronto, Ontario, Canada) and colleagues comment in the journal Psychiatry Research.

Previous research has demonstrated that both bipolar disorder and alcohol use disorder show strong genetic linkage in family, twin, and adoption studies. In addition, some studies have shown that polymorphisms in dopamine and serotonin pathway genes are linked to these two disorders separately.

But few studies have looked at genetic vulnerability to the comorbid condition of bipolar disorder with alcohol use disorder.

To address this, Busto and colleagues performed a retrospective study of a genomic database comprising 278 bipolar disorder patients (64% female, average age 43 years). The prevalence of alcohol use disorder comorbidity was 25.5%, while the prevalence of cannabis use was 23.0%.

They analyzed single nucleotide polymorphisms (SNP) in several dopamine receptor genes (DRD1, DRD2 and DRD3) and serotonin receptor/transporter genes (5HTTLPR, 5HT1B, 5HT2A, 5HT2C).

In female bipolar patients, there was a significant difference in genotype frequency between the bipolar patients with comorbid alcohol use disorder and non-comorbid bipolar patients for the Ser23Cys (rs6318) polymorphism of the 5HT2C gene.

Busto et al then performed logistic regression analysis with comorbid alcoholism as a categorical dependent variable and each of the SNPs as categorical covariates, but found no significant results.

They comment: “These results support the possible involvement of the serotonin genes in the concurrent development of bipolar disorder and alcohol use disorder.

“Further study with large, well-characterized populations of bipolar patients is needed to confirm these findings.”

MedWire (www.medwire-news.md) is an independent clinical news service provided by Current Medicine Group, a trading division of Springer Healthcare Limited. © Springer Healthcare Ltd; 2010

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