The major circadian pacemaker ARNT-like protein-1 (BMAL1) is associated with susceptibility to gestational diabetes mellitus

Abstract

Aims

Recently a relationship between circadian clock function and the risk for type 2 diabetes (T2D) has been shown. BMAL1 is a key component of the mammalian molecular clock. Two SNPs in the BMAL1 gene have been identified to confer T2D susceptibility. In the present study we investigated for the first time the association between the BMAL1 gene and the risk for GDM, in a Greek population.

Methods

We studied 185 women with GDM and 161 non-diabetic controls for BMAL1 polymorphisms. For BMAL1 mRNA expression, peripheral leukocytes were harvested from 20 GDM and 20 control women, harboring different genotypes for the tested polymorphisms, using real-time quantitative PCR.

Results

The minor allele (A) of the BMAL1 rs7950226 (G > A) polymorphism was found to be significantly associated with an increased risk of GDM (P = 0.025). Analysis of the second BMAL1 rs11022775 (T > C) polymorphism, showed that the C-allele frequency was strongly increased in women with GDM (P = 4.455e−06). The CC genotype was also significantly overrepresented in GDM vs. controls (P = 0.00001). Additionally, the rs7950226G/rs11022775C and rs7950226A/rs11022775C haplotypes were also found to be associated with increased susceptibility to GDM. Furthermore, the expression levels of BMAL1 mRNA were significantly lower in GDM patients than in controls.

Conclusion

These data suggest that the impairment of the BMAL1 clock gene expression is closely associated with GDM susceptibility.

Introduction

Gestational diabetes mellitus (GDM) is defined as glucose intolerance that is first diagnosed during pregnancy [1]. GDM occurs in about 2–5% of all pregnancies, following failure of insulin secretion to overcome the additive effect of insulin resistance, which usually develops during GDM pregnancy [2]. However, the prevalence varies among populations [3]. It is well documented that women developing GDM are at high risk for presenting type 2 diabetes (T2D) at a later stage [4]. Specifically, current data project that those women who have had GDM, exhibit a 35–60% chance of developing diabetes within the next 10–20 years [5]. Furthermore, GDM shares many cardinal features with T2D, such as glucose intolerance, insulin resistance and impaired insulin secretion [6], while both entities have exhibited a dramatic increase in recent years worldwide.

Despite years of investigations, very little is known about the genetic background and the predisposition for GDM. A series of population studies have provided support that susceptibility to GDM involves a genetic component, though without excluding the multigenic nature of the disease, as it is also the case for T2D. Few studies have also evaluated the heritability of GDM, suggesting that GDM clusters within families and is associated with a history of T2D [7]. We [2] and others [8], [9], [10] have documented recently that several candidate genes and their polymorphisms are actually shared both by GDM and T2D. This represents a significant step towards the understanding of the genetic basis of GDM and T2D forms. Advances in our knowledge of the human genome and in genome-wide association technologies to query variation across the genome, have further enabled the discovery of variants in a number of genes responsible for both diabetes syndromes and for conferring increased risk for typical T2D. How these same variants affect susceptibility to GDM and whether distinct variants in these or other genes influence GDM has been less studied. Understanding the genetic susceptibility of GDM may lead to new biomarkers to safely predict the actual risk and thus permit early diagnosis and prevention in order to minimize pregnancy-related complications.

Recently, global and unbiased genetic analyses have produced a wealth of new information about the pathogenesis of T2D [8], while the link between circadian clock function and metabolic diseases, including T2D diabetes has attracted considerable attention [11]. The molecular machinery of circadian rhythm consists of oscillating loops of transcription factors expression. Central to this machinery, are the transcription factors circadian locomotor output cycles kaput (CLOCK) and brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like 1 (BMAL1). In humans, genetic variations in the BMAL1 gene are reported to be associated with susceptibility to T2D and hypertension [12]. Furthermore, specific CLOCK haplotypes are associated with metabolic syndrome and non-alcoholic fatty liver disease [13], [14]. Taken together, these findings strongly indicate that dysfunction of the circadian clock may be a critical parameter contributing to the development of T2D diabetes.

Given the fact that (a) GDM shares many features with T2D, (b) the growing interest in the role of circadian rhythms in metabolic regulation and the development of diabetes, and (c) the lack of any studies on the role and alterations of clock genes and circadian rhythms in GDM, in the present study we investigated the putative association between the BMAL1 polymorphisms, its expression and the risk for development of GDM, in a Greek population of pregnant women.