BTNL2 gene variant and sarcoidosis

Sarcoidosis is an inflammatory granulomatous disorder that primarily affects the lungs and lymph nodes. Other organs such as the brain, eyes, heart, and skin can also be affected. The disease is characterised by non‐caseating granulomas and an exaggerated cellular immune response caused by increased inflammatory activity.¹ The course of the disease is acute and mild in approximately 20% of all patients. In most patients a chronic stage develops which can lead to lung fibrosis. Although the exact pathogenesis of sarcoidosis remains unclear, familial clustering of the disease and the increased risk of relatives to develop sarcoidosis suggest that there might be a genetic predisposition to develop the disease.²

A significant association was recently reported in Germany between sarcoidosis and a frequent single nucleotide polymorphism (SNP) in the BTNL2 gene, rs2076530.³BTNL2 is a member of the immunoglobulin gene family and is related to CD80 and CD86 co‐stimulatory receptors, although its exact function is unknown.⁴BTNL2 is located on chromosome 6p21.3 in close proximity to the HLA gene cluster. rs2076530 is located at position 1 of the donor splice site in intron 5 and the associated A allele causes the usage of an alternative donor site leading to a 4 bp deletion at the mRNA level, frameshift, and premature truncation at the protein level.³ The rs2076530 SNP alone was also associated with sarcoidosis in a case‐control study of white American subjects.⁴ No replication of the BTNL2 rs2076530 susceptibility to sarcoidosis has yet been studied in an independent German case‐control study. We therefore performed a case‐control association study in 210 patients with sarcoidosis and 202 controls. Written informed consent was given by each participant and the study was approved by the ethics committee of Bonn University School of Medicine.

The diagnosis of sarcoidosis was based on evidence of non‐caseating epitheloid cell granuloma in biopsy specimens and chest radiographic abnormalities. Different stages were defined as previously described.⁵ A chronic course was defined as disease over at least 2 years or at least two episodes in a lifetime. Acute sarcoidosis was defined as one episode of acute sarcoidosis which had totally resolved at the date of the examination. None of the individuals in the control group (healthy white German subjects of mean age 38.32 (15.53) years) had a history of lung disease or showed any symptoms of lung or other disease by chest radiography or laboratory blood tests. Genotyping of rs207653 was performed using the Taqman technique with a commercially available assay (Applied Biosystems, Germany). SPSS Version 12 was used for statistical analysis. The genotype distributions in the cohort were in accordance with the Hardy‐Weinberg equilibrium.

The A allele frequency of rs2076530 was significantly increased in sarcoidosis patients compared with controls (A = 0.6929, G = 0.3071 in cases; A = 0.6188, G = 0.3812 in controls). It was significantly associated with an increased risk of sarcoidosis in co‐dominant and dominant models (OR 2.31 (95% CI 1.27 to 4.23); p<0.006, table 1), but not in a recessive model (p = 0.276). The calculated population attributable risk (PAR) for AA homozygotes and AG heterozygotes was 34.6%. Our results were in accordance with the reported association between BTNL2 and sarcoidosis and replicated the finding that A allele carriers of rs2076530 have a more than twofold increased risk of developing sarcoidosis compared with GG homozygotes in the German population.

Table 1 Statistical analysis of the case‐control study.

	Co‐dominant				Dominant (AA/AG v GG)				Recessive (AA v AG/GG)
	AA	AG	GG	p value	AA/AG	GG	OR (95% CI)	p value	AA	GG/AG	OR (95% CI)	p value
Controls	84 (41%)	82 (41%)	36 (18%)		166	36			84	116
Cases	99 (47%)	93 (44%)	18 (9%)	0.021	192	18	2.31 (1.27 to 4.23)	0.006	99	111	1.25 (0.85 to 1.85)	0.276
Acute	30 (42%)	32 (45%)	9 (13%)	0.576	62	9	1.49 (0.68 to 3.28)	0.358	30	41	0.99 (0.57 to 1.71)	0.97
Chronic	59 (52%)	47 (41%)	8 (7%)	0.021	106	8	2.87 (1.29 to 6.42)	0.007	59	55	0.67 (0.43 to 1.07)	0.095

Significant associations are shown in bold.

We also examined whether this increased risk is present in both chronic and acute forms of sarcoidosis. Interestingly, we found that the chronic form—but not the acute form—was significantly associated with the A allele in co‐dominant and dominant models (OR 2.87 (95% CI 1.29 to 6.42), p<0.0069; table 1) with a PAR for AA homozygotes and AG heterozygotes of 50%.

This study underlines the importance of the association of BTNL2 rs2076530 variant with the susceptibility to develop sarcoidosis in a German population. Furthermore, our data suggest that susceptibility is preferentially towards the chronic form of the disease.