Asthma is a disease characterized by chronic inflammation of the airway, thought to result from inappropriate activation of the Th2 immune response. Recent work has shown that chitinase proteins are strongly upregulated during Th2 inflammation in the lungs of mice. Studies in mouse models have implicated chitinase activity in both promotion of Th2 inflammation and protection from the Th2-priming effects of chitin in the lungs.
Herein we detail investigation of chitinases in human asthma. Namely, we explored the possibility that variants affecting the enzymatic activity of the two active chitinase proteins, acidic mammalian chitinase (AMCase) and chitotriosidase (CHIT1), may alter risk for human asthma. Secondly, we have determined expression and activity of these enzymes in lungs of healthy and asthmatic subjects.
We identified a coding haplotype in the AMCase gene strongly associated with protection from asthma among African American asthma case-control groups. This association was also observed in family-based analysis of a group of Latino American asthmatic trios. Through recombinant protein expression and biochemical characterization we have found the protective AMCase isoform to be considerably more active at pH values corresponding to the lung, stomach, and lysosomes. In contrast, no association was observed with a common null genetic variant in the CHIT1 gene in either African Americans, or Latinos.
We found CHIT1 to be responsible for the majority of chitinase activity in the human lung based on pH profile of enzymatic activity and genetic analysis of a CHIT1 null variant. Median chitinase activity in BAL tended to be lower than normal in asthma. AMCase protein was detectable in the lung, but mRNA transcripts expressed from lung tissues was consistent with an isoform lacking enzymatic activity. AMCase protein expression was observed to be lower in the BAL of asthmatics.
This work is the first evidence that functional genetic variants in chitinase genes are associated with risk for asthma, and are consistent with possible Th2-priming effects of environmental chitin. Additionally, we find differential expression of the two active chitinases in the human lung and reveal differences in chitinase protein expression and enzymatic activity in lung disease.