Biochemical and molecular studies in mild flavin monooxygenase 3 deficiency.

Abstract

Flavin-containing monooxygenases (FMOs) are membrane-associated microsomal mixed-function oxidases that are required for the oxidation and detoxification of many nitrogen-, sulphur- and phosphorus-containing substances and are present in nearly all secretory cell types (Dolphin et al 1997; Treacy et al 1998). At least five different isoforms (1 to 5) have been identified in mammalian tissues (Lawton et al 1994). In humans, FMO3 is the major catalytic isoform expressed in the liver (Lomri et al 1993), although it is also present in the kidney and probably in other tissues. Substrates include endogenous amines (e.g. adrenaline), tyramine, nicotine, drugs (e.g. tricyclic antidepressants, ranitidine), as well as trimethylamine (TMA), a volatile malodourous substance that is present in fish or may be produced by the action of gut bacteria on choline, lecithin or carnitine (Lin et al 1996; Ziegler 1993). It has been shown that a deficiency of FMO3 is the cause of the recessive disorder trimethylaminuria (TMA-uria) or fish-odour syndrome' (Dolphin et al 1997; Treacy et al 1998). Individuals affected by this disease have a reduced capacity to oxidize free TMA to its odourless, nonvolatile metabolite TMA N-oxide (TMAO) and display a strong, fish-like odour due to the excretion of large amounts of TMA in urine, sweat and breath (Ayesh et al 1993). The diagnosis is made biochemically on the basis of elevated urinary free TMA; under physiological conditions, >95% of the total TMA (= free TMA + TMAO) is excreted as TMAO. In classical fish-odour syndrome, the TMAO/total TMA ratio is strongly decreased. The FMO3 gene is located on chromosome 1q23-25 and contains one noncoding and eight coding exons. A small number of severe, disease-causing mutations and several polymorphic variants in the gene have been reported (Akerman et al 1999; Dolphin et al 1997; Treacy et al 1998). We have recently found that a frequent variant allele carrying two amino acid polymorphisms, c.472G > A and c.923A > G [E308G; E158K], is associated with reduced N-oxidation capacity after oral trimethylamine challenge (Zschocke et al 1999). Patients previously reported to suffer from 'transient' trimethylaminuria (Mayatepek and Kohlmuller 1998) were compound heterozygous for the variant allele and a severe FMO3 mutation, corresponding to mild enzyme deficiency. Here we present the results of molecular analyses and biochemical studies under physiological conditions in patients with mild FMO3 deficiency and report the first symptomatic patient who is homozygous for the variant allele.