Association of HLA-DRB1 amino acid residues with giant cell arteritis: genetic association study, meta-analysis and geo-epidemiological investigation

Table 5 Results of tests for association of amino acid at the most significant positions: 9, 11, 13, 33, and 37

Position in HLA-DRB1	Amino acid residue	Frequency		Univariable		Stepwise model (entry P = 0.05) applied separately at each position
Position in HLA-DRB1	Amino acid residue	Cases	Controls	OR	P value	OR	P value
9	E	0.698	0.565	1.78	7.3×10⁻⁸
	K	0.011	0.011	1.03	0.96
	W	0.291	0.424	0.56	5.8×10⁻⁸	0.56	5.85×10⁻⁸
11	D	0.011	0.011	1.03	0.96
	G	0.127	0.142	0.88	0.38
	L	0.060	0.123	0.46	3.6×10⁻⁵	0.64	0.040
	P	0.104	0.159	0.63	0.0024
	S	0.391	0.371	1.09	0.41	1.37	0.015
	V	0.307	0.194	1.86	1.4×10⁻⁷	2.06	2.8×10⁻⁷
13	F	0.071	0.141	0.48	1.7×10⁻⁵	0.66	0.038
	G	0.040	0.045	0.87	0.60
	H	0.307	0.187	1.96	1.5×10⁻⁸	2.11	5.5×10⁻⁸
	R	0.104	0.159	0.63	0.0024
	S	0.351	0.325	1.12	0.28	1.38	0.014
	Y	0.127	0.142	0.88	0.38
33	H	0.307	0.187	1.96	1.5×10⁻⁸	1.96	1.527×10⁻⁸
33	N	0.693	0.813	0.51	1.5×10⁻⁸
37	F	0.147	0.161	0.89	0.39
	L	0.022	0.017	1.27	0.49
	N	0.256	0.219	1.09	0.46
	S	0.164	0.282	0.51	8.0×10⁻⁸	0.59	0.00012
	Y	0.402	0.276	1.63	3.2×10⁻⁶	1.37	0.0055

Forward stepwise regression was used to determine whether there are independent effects of more than one amino acid residue at the same position. Only amino acids that were retained by the stepwise regression model are shown in the last two columns. Odds ratio (OR) of less than 1.0 indicates a protective effect, whereas OR of more than 1.0 indicates a susceptibility effect. Note the high level of linkage disequilibrium at this locus; Additional file 1 shows the amino acid residues within the three hypervariable regions of HLA-DRB1