We have determined the frequency of the PPP1R3A ARE polymorphism in a large group (n = 1950) of approximately equal numbers of men and women with type 2 diabetes in Tayside, Scotland and considered association of genotype with age of diagnosis with type 2 diabetes, glycated haemoglobin and body mass index. 847 (43.4%) were ARE1 homozygotes, 891 (45.7%) ARE1/2 heterozygotes and 212 (10.9%) ARE2 homozygotes. Overall the ARE2 allele frequency was 0.337 (0.322–0.352). ARE genotype demonstrated a strong association with age of diagnosis that was both gene dosage and gender dependent (Table and Figure ). Thus whilst there was no difference between males and females in the mean age of diagnosis for individuals homozygous for the more common ARE1 allele (56.5 years vs 56.7 years respectively), males carrying the less common ARE2 allele (heterozygotes and ARE2 homozygotes) had an earlier mean age of diagnosis (55.1 years) compared with females carrying the ARE2 allele (58.6 years) a difference of 3.6 years which was highly significant (p < 0.000001). Furthermore female ARE2 carriers were on average 2 years older (p = 0.017) at diagnosis than female ARE1 homozygotes. On the other hand male ARE2 carriers were 1.5 years younger (p = 0.046) at diagnosis compared to male ARE1 homozygotes. Thus males and females who had the ARE2/2 genotype differed in mean age of diagnosis by on average 6.5 years (3.1–9.5, p = 0.0001). Over all there was also a weak association of the rare allele with body mass index after adjustment for age of diagnosis, age of BMI, and treatment, with homozygous ARE2 individuals having a BMI 0.81 lower than ARE1 homozygous individuals (p = 0.041). This association was however confined to females (BMI difference = 1.41, p = 0.021) with males showing no evidence of a difference (p = 0.939). There was also a weak association with glycated haemoglobin in the overall population and again this association was confined to females ARE2 homozygotes who had a glycated haemoglobin on average 1% lower than ARE1 homozygotes (p = 0.047).
Table 1 Characteristics of the Tayside Diabetic population by PPP1R3A genotype and sex. Parameters are given as corrected means ± Standard deviation, the corrected means are based on multiple measures through time and are adjusted for age diagnosed, age (more ...)
Age diagnosed with type 2 diabetes in males and females by PPP1R3A ARE genotype. Legend: Females dashed line open triangle, males solid line closed squares. (Error bars = standard error of the mean)
As we found an association of reduced BMI with genotype in females, and increased BMI is known to be the major risk for type 2 diabetes, we included BMI in our regression model for age of diagnosis. This completely removed any significance in the association (Table ). Interestingly, inclusion of BMI in the regression model in men did not negate the association with earlier age of diagnosis.
To further explore the association of this polymorphism with gender and age of diagnosis, we compared allele frequencies by gender in individuals diagnosed early and those diagnosed later. Early and later diagnosis were defined as being diagnosed at an age below and above the first quartile of the overall ages at diagnosis respectively. This corresponded to being diagnosed less than or greater than 48.2 years in this population. These frequencies were in turn compared with the background population frequency in a large cohort (495 males, 519 females) of healthy school children from the same region. The overall ARE2 allele frequency in the children was very close to the overall frequency in the diabetic population (Table and figure ). There was no difference in allele frequency between genders in the children (ARE1 allele frequency in males = 0.336, 0.307–0.366 vs. females 0.330, 0.302–0.359). As expected while there was no difference in ARE2 allele frequency between males and females diagnosed later (OR 0.927, 0.793–1.084 p = 0.333) the frequency in early diagnosed males and females was significantly different (OR 1.59, 1.20–2.12 p = 0.0009). The frequency in males was 0.383 (0.342 – 0.423) which was significantly higher (OR 1.24, 1.015–1.51, p = 0.030) than the background frequency in the children and in early diagnosed females it was 0.280 (0.237 – 0.324) which was in turn significantly lower (OR 0.78, 0.611–0.991, p = 0.037) than the background frequency in the children. The frequency in the women diagnosed early was also significantly lower than the frequency in those diagnosed later (OR 0.740, 0.576–0.947, p = 0.014). These frequencies were also similarly different in male type 2 diabetics (OR 1.269, 1.030–1.561 p = 0.021).
Genotype and ARE2 allele frequency (with 95% confidence intervals) in men and women diagnosed with early and late diagnosis of type 2 diabetes.
Odds ratios of the ARE2/2 genotype prevalence in type 2 diabetes. Shown are the Odds Ratios and the 95% confidence intervals.
Finally, analysis of the ratio of males to females in the total DARTS population and the genetic sub-study (Go-DARTS) showed a marked overall preponderance of early diagnosis for males that is very similar to that observed in previous epidemiological studies (figure ). This was supported by a χ2, test for trend, of 129.9, p < 0.0001, for the total DARTS population and a χ2 test for trend of 17.9, p < 0.0001, in the genotyped subpopulation. This trend resided exclusively in the ARE2 allele carriers with no such trend in ARE1 homozygotes (p = 0.003 for comparison of trends by genotype) (figure ).
Figure 3 Male preponderance in early diagnosis is associated with the ARE2 variant. A. Ratio of females to males in the total DARTS type 2 population (n= 8155) and in the genotyped subgroup (Go-DARTS, n = 1950) by quintile of age diagnosed B. Ratio of females (more ...)