Tabulation of Genotype Frequencies of 3 ADRB2 Polymorphisms in Cases and Controls and calculated Chi Square statistics:
|
Case |
Control |
OR |
Chi Square |
p |
C/C |
6 |
6 |
0.99 |
|
|
C/T |
97 |
102 |
0.942 |
|
|
T/T |
400 |
396 |
1 |
0.144 |
ns |
|
|
|
|
|
|
G/G |
106 |
86 |
1.54 |
|
|
G/R |
236 |
217 |
1.36 |
|
|
R/R |
161 |
201 |
1 |
7.3 |
< .05 |
|
|
|
|
|
|
E/E |
8 |
6 |
1.2 |
|
|
E/Q |
39 |
88 |
0.4 |
|
|
Q/Q |
456 |
410 |
1 |
21.6 |
< .001 |
(1) no difference in frequency of C allele among groups, suggesting no association of -47C/T polymorphism with hypertension;
(2) among cases, there are more individuals homozygous or heterozygous for Gly16 alleles, suggesting an association of Arg16/Gly with hypertension (p <.05);
(3) among cases, there are more individuals with Gln27 allele mainly due to higher percentage of cases homozygous for Gln27 (p<.001).
Note amino acid abbreviations:
G=gly, R=arg, E=glu, Q=gln
Determine if Genotype is a Predictor of Hypertension Status (dichotomous variable) by Stepwise Logistic Regression Analysis:
(1) significant positive contribution from Gly16 allele (OR=1.46, 95% C.I. of 1.11-1.93);
(2) significant negative contribution from Gln27 allele (OR=.42, 95% C.I. of .28-.62).
(3) however, genotypes at Arg16/Gly and Gln27/Glu do not predict hypertension completely as BMI, glucose, triglyceride, creatinine, and drinking status were found to contribute to prediction using models already incorporating Arg16/Gly and Gln27/Glu genotypes.
There is no data on allele or genotype frequencies at the population level.
Population allele frequencies as estimated by allele frequencies (%) among controls:
- C allele: 11.3; T allele: 88.7
- Gly allele: 38.6; Arg allele: 61.4
- Glu allele: 9.9; Gln allele: 90.1
--Therefore, the most highly associated alleles in this study, Gly16 and Gln27, are highly prevalent in the population. Thus, disease penetrance of these alleles appears to be low, as expected in multigenic disorders.
Determine if Genotype is a Predictor of Blood Pressure (continous variable)
The mean systolic blood pressures (SBP) of genotypes associated with the Arg16/Gly polymorphic locus as follows:
(1) Gly/Gly 150.25 +/- 36.3 mmHg
(2) Gly/Arg 149.49 +/- 36.6 mmHg
(3) Arg/Arg 142.98 +/- 36.7 mmHg
--Multivariate analysis of above shows significant difference (p=.025).
--The mean blood pressure for Gly16 homozygote (Gly/Gly) is not statistically different from Gly16 heterozygote (Gly/Arg). Hence, there does not appear to be a dose effect.
The mean SBP of genotypes associated with the Gln27/Glu polymorphic locus are as follows:
(1) Glu/Glu and Glu/Gln combined 136.25 +/- 30.5 mmHg
(2) Gln/Gln 149.1 +/- 37.3 mmHg
--The above difference is significant (p < .001)
--The mean blood pressure of the Glu/Glu and Glu/Gln were combined when compared to Gln/Gln. However, Table 2 shows that the odds ratios of Glu homo- and heterozygotes are 1.20 and .40, respectively (relative to the Gln/Gln referent group), which is not consistent with a gene dose effect.
Implications of Findings for Underlying Population
Genotypes with significantly elevated blood pressures are Gly/Gly, Gly/Arg, and Gln/Gln. The prevalence of these genotypes is not known but may be estimated by prevalence in the control group (%) as follows:
- Gly/Gly (G/G): 17.06
- Gly/Arg (G/R): 43.06
- Gln/Gln (Q/Q): 81.34
- Gln/Glu (Q/E): 8.9
- Glu/Glu (E/E): .98
However, the estimates above may not be valid because genotypes at the Arg16/Gly site may not be in Hardy Weinberg equilibrium (see below).
Estimation of Attributable Risk Fraction for hypertension due to Gly16 -containing genotypes or Gln27 homozygosity.
Ordinarily, Attributable Risk Fraction (ARF) can be calculated based on odds ratio of a particular genotype and the estimated prevalence of the genotype in the population. However, such calculations are probably not valid in this case because the association of these polymorphisms with hypertension remains questionable despite the present study, "penetrance" of the culpable genotypes seems far from complete, the definition of control group is problematic, and Hardy Weinberg equilibrium (HWE) may not be present (see below).
Determine if genotype distribution follow Hardy Weinberg Equilibrium
AMONG CASES: |
|
|
|
Genotype |
Observed |
Expected |
Chi Square |
p |
C/C |
6 |
6 |
0 |
|
C/T |
97 |
97 |
|
|
T/T |
400 |
400 |
|
|
|
|
|
|
|
G/G |
106 |
100 |
0.59 |
ns |
G/R |
236 |
248 |
|
|
R/R |
161 |
155 |
|
|
|
|
|
|
|
E/E |
8 |
1.5 |
6.4 |
<.05 |
E/Q |
39 |
52 |
|
|
Q/Q |
456 |
449 |
|
|
AMONG CONROTLS: |
|
|
|
Genotype |
Observed |
Expected |
Chi Square |
p |
C/C |
6 |
6 |
|
ns |
C/T |
102 |
101 |
|
|
T/T |
396 |
397 |
|
|
|
|
|
|
|
E/E |
6 |
5 |
0.11 |
ns |
E/Q |
88 |
90 |
|
|
Q/Q |
410 |
409 |
|
|
|
|
|
|
|
G/G |
86 |
75 |
2.1 |
ns |
G/R |
217 |
239 |
|
|
R/R |
201 |
190 |
|
|
Determine if Haplotype Data Improve Prediction of Hypertension
The haplotype frequencies shown in Table 4 are calculated based on allele frequencies reported in Table 2. The authors noted that:
(1) The -47C/T and Arg16/Gly polymorphisms has the largest LD coefficient (D`=.66, p<.0001).
(2) T-Gly is preferrentially linked to Gln in cases and to Glu in controls.
(3) C/C and Glu/Glu are preferentially linked to Gly/Gly.
(4) There is linkage disequilibrium between any of the two polymorphism combinations.
Given the uncertainty of HWE equilibrium in both cases and controls, it may not be legitimate to estimate haplotype frequencies based on allele frequencies.
Cautionary Note Regarding Inference:
The genotyping method is not well documented and the definition of hypertension has shifted, introducing potential error in inferences drawn from the results of this study.
|