Are Polymorphisms the Answer in Hypersensitivity Pneumonitis?

Mark Schuyler, M.D.

University of New Mexico School of Medicine, Albuquerque, New Mexico

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Hypersensitivity pneumonitis (HP) can illuminate an issue of importance in our understanding of many illnesses, that is, individual susceptibility to disease. Patients who present with HP are a minority of those exposed and do not have apparent increased exposure to the offending agent, compared with exposed, but not ill individuals. Although one environmental factor (i.e., patients with HP smoke less frequently than exposed subjects without disease) has been identified (1), it is clear that genetic differences between individuals must be very important in determining the outcome of exposure (sensitization ± clinical illness). Because HP is clearly associated with exposure to particular antigens, we can focus the investigation on the genetic factors that determine the result of exposure.

Our understanding of disease pathogenesis is dependent on our current understanding of physiology and on the technology available to answer questions. Recent advances in molecular biology techniques allow us to measure differences between individuals that may be important in determining susceptibility.

The initial interaction of T lymphocytes with antigen and antigen-presenting cells requires the participation of MHC class II molecules. It is very reasonable that one explanation as to why only some subjects become ill after exposure to the relevant antigen(s) is that different MHC class II molecules have different affinities for a particular antigen, which results in sensitization to the pertinent antigen and clinical illness. This has been demonstrated in another granulomatous lung disease in that susceptibility to berylliosis is associated with a certain MHC molecule (HLA-DP1 glutamate 69) (2). Initial investigations using serological techniques did not consistently identify MHC haplotypes that conferred increased susceptibility to HP (3).

Recent advances in technology have allowed rapid and accurate identification of polymorphisms in various genes that are associated with pulmonary illnesses. Initial investigations have focused on genes for enzymes or cytokines/chemokines thought to be important in disease pathogenesis (4). Because polymorphisms of different genes can be mixed and matched, it is possible that certain combinations of polymorphisms confer increased or decreased susceptibility to a particular illness.

Selman's group has taken the next step, that is, to attempt to identify the association of certain polymorphisms and the expression of disease using polymerase chain reaction (PCR)- based MHC class II typing and promoter analysis (pp. 1528- 1533) (5). They focused on polymorphisms of the 5' promoter region of the tumor necrosis factor- (TNF-) gene on chromosome 6 in pigeon breeder's disease (PBD), a very common example of HP. TNF- is a reasonable choice of investigation in HP, as both animal models (6) and human studies (7) have demonstrated that TNF- is an important mediator in HP.

This group compared 44 patients with PBD with 2 control groups (a general population sample and exposed but not ill subjects), and found that an increase of the frequency of HLA-DRB1*1305 and HLA-DRQB1*0501 and a decrease of HLA-DQB1*0402 alleles were associated with increased susceptibility to disease. There was also an increase of certain DRB1-BQB1 haplotypes in subjects with HP compared with both control groups, compatible with the allelic data. The authors examined linkage disequilibrium in a small family-based study in the HP group and found an increase of the appropriate haplotypes in patients with HP.

They also found an increase of the allele frequency of the TNF-2³⁰⁸ promoter in patients with HP compared with the control groups. The PBD patients with the TNF-2³⁰⁸ promoter allele were slightly younger (34 versus 44 yr), had a lower duration of exposure to avian antigens (15 versus 80 mo), and increased bronchoalveolar lavage (BAL) lymphocytosis (88% versus 69%) compared with those subjects with PBD without this allele. Plasma and BAL fluid (BALF) levels of TNF- were increased in the patients with PBD, but TNF- levels in plasma and BALF were not increased in those subjects with the TNF-2³⁰⁸ allele.

The advantages of this study are that all subjects with PBD had lung biopsies performed and thus definitely suffered from HP. The investigators also examined the appropriate control groups in a presumably homogeneous population group. The disadvantages of this study are that exposure to pigeon antigens was not directly measured, so that it is possible that differences of pigeon antigen exposure or perhaps concomitent exposures were not determined. The frequency of the TNF-2³⁰⁸ promoter allele was quite low (9%) in the group with PBD, implying that other factors must be important in the majority of patients with PBD. Also, the lack of increase of TNF- in serum and BALF in the subjects with the TNF-2³⁰⁸ allele is somewhat disappointing and weakens the association between this allele and functional consequences. However, it is certainly possible that TNF- is increased in other body compartments and/or at different times during development of HP in subjects with the TNF-2³⁰⁸ allele. Therefore, Selman's study represents an important first step in this exciting new approach to understanding HP.

The next step will be to apply microarray technology to samples from patients with HP, in a manner similar to that recently reported in breast cancer (8). This technology measures the expression of mRNA from many (up to several thousand) genes simultaneously. Expression patterns from different subgroups (for example, breast cancer associated with different genes) were compared. The equivalent approach in HP would be to compare gene expression in patients with HP with expression in control groups. Another approach is to compare mRNA from samples before and after an intervention, such as antigen exposure in HP. Sophisticated informatic support is required to analyze the results from this kind of assay. Microarray technology has the advantage of not requiring exact knowledge of the pathophysiology of a disease, but the disadvantage of an observational study that cannot directly distinguish important from unimportant differences of mRNA and subsequent protein expression. Therefore, the results of microarray analyses must be placed in the context of our understanding of the pathophysiology of particular illnesses.

Application of these concepts and techniques may lead to genetic fingerprinting, which might be able to predict susceptibility to both pulmonary diseases with known exposure (i.e., HP), as well as those with unknown exposure, such as sarcoidosis.

	References

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