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Adenosine deaminase deficiency: genotype-phenotype correlations based on expressed activity of 29 mutant alleles
F X Arredondo-Vega1, I Santisteban, S Daniels
1Department of Medicine, Duke University Medical Center, Durham, NC, USA.
Insights
Adenosine deaminase (ADA) deficiency causes immune deficiency. This study links specific ADA gene mutations to disease severity and substrate buildup, offering a new way to classify patients based on genotype.
Area of Science:
- Genetics
- Immunology
- Biochemistry
Background:
- Adenosine deaminase (ADA) deficiency leads to immunodeficiency and lymphopenia due to toxic substrate accumulation.
- Clinical presentations range from severe combined immunodeficiency disease (SCID) in infants to delayed or late-onset forms in older individuals, with some healthy individuals exhibiting partial ADA deficiency.
Purpose of the Study:
- To establish a quantitative framework for correlating adenosine deaminase (ADA) genotypes with clinical phenotypes.
- To analyze the relationship between specific ADA mutations, enzyme activity, and patient outcomes.
Main Methods:
- Quantified the expression and activity of 29 ADA sequence-altering alleles using a genetically modified Escherichia coli strain.
- Correlated expressed ADA activity with clinical phenotypes and erythrocyte deoxyadenosine nucleotide (dAXP) levels in 52 patients with diverse ADA genotypes.
- Developed a ranking system of 13 genotype categories based on potential ADA activity.
Main Results:
- Disease-associated ADA alleles expressed significantly lower enzyme activity (0.001%-0.6%) compared to alleles from individuals with partial deficiency (5%-28%).
- 31 SCID patients were predominantly classified into 3 genotype categories expressing less than or equal to 0.05% of wild-type ADA activity.
- A strong inverse correlation was observed between red blood cell dAXP levels and the total ADA activity expressed by a patient's alleles.
Conclusions:
- The study provides a quantitative genotype-phenotype correlation system for ADA deficiency.
- This framework aids in classifying patients based on their genetic makeup and predicted enzyme function.
- The findings support using genotype-based categorization to understand disease severity and guide clinical management in ADA deficiency.
Abstract:
Adenosine deaminase (ADA) deficiency causes lymphopenia and immunodeficiency due to toxic effects of its substrates. Most patients are infants with severe combined immunodeficiency disease (SCID), but others are diagnosed later in childhood (delayed onset) or as adults (late onset); healthy individuals with "partial" ADA deficiency have been identified. More than 50 ADA mutations are known; most patients are heteroallelic, and most alleles are rare. To analyze the relationship of genotype to phenotype, we quantitated the expression of 29 amino acid sequence-altering alleles in the ADA-deleted Escherichia coli strain SO3834. Expressed ADA activity of wild-type and mutant alleles ranged over five orders of magnitude. The 26 disease-associated alleles expressed 0.001%-0.6% of wild-type activity, versus 5%-28% for 3 alleles from "partials." We related these data to the clinical phenotypes and erythrocyte deoxyadenosine nucleotide (dAXP) levels of 52 patients (49 immunodeficient and 3 with partial deficiency) who had 43 genotypes derived from 42 different mutations, including 28 of the expressed alleles. We reduced this complexity to 13 "genotype categories," ranked according to the potential of their constituent alleles to provide ADA activity. Of 31 SCID patients, 28 fell into 3 genotype categories that could express <=0.05% of wild-type ADA activity. Only 2 of 21 patients with delayed, late-onset, or partial phenotypes had one of these "severe" genotypes. Among 37 patients for whom pretreatment metabolic data were available, we found a strong inverse correlation between red-cell dAXP level and total ADA activity expressed by each patient's alleles in SO3834. Our system provides a quantitative framework and ranking system for relating genotype to phenotype.