Novel FMO3 mutations involved in trimethylaminuria disorder

Abstract

Trimethylaminuria is a human autosomal recessive disorder due to mutations in the gene of the enzyme flavin-containing monooxygenase 3 (FMO3). In this condition, odorous trimethylamine (TMA) cannot be converted into its non- odorous N-oxide. Consequently, TMA accumulates and is excreted through urine and sweat being responsible of the characteristic malodor of the patients. Clinical diagnosis is based on the quantification of TMA in urine samples and is confirmed by genetic analysis which additionally contributes to risk stratification. None of them are performed in Argentina, where diagnosis is based on the body odor. The aim of this case study was to correlate the clinical diagnosis with genetic variants of FMO3. The index patient is a female (12 y.o.) with fishy malodor since she was a toddler and with an exacerbation of the condition since puberty. Symptoms are only controlled by strict diet restriction. The patient and unaffected direct relatives were analyzed to build up and evaluate the genetic pedigree. gDNA was obtained from jaw swabs. We used specific primers designed for the coding exons (2 to 9) to amplify them by PCR followed by sequencing. The mutant enzyme activity was tested using heterologous expression. The index case beard 2 SNPs of the FMO3 gene: c.472G>A (E158K) and c.923A>G (E308G). She also carried two novel mutations resulting in amino acid substitutions: P73L and F140S. All of them were in heterozygosity. The mother carried the P73L/E158K/E308G allele and the father the F140S allele. Functional analyses showed a 50 and 90 % decrease of the catalytic capacity for the mother and the father alleles, respectively (n= 3). Based on the family pedigree, we identified a compound heterozygous patient for the two novel point mutations. Functional analysis demonstrated a drastic reduction in enzyme activity for each allele, which combined with the already known changes promote the severe condition exhibited by the patient.