Project Detail
Molecular diagnosis of OTC deficienty: too many unsolved cases
Luísa Azevedo (PI), Alexandra Lopes, António Amorim, Mónica Lopes-Marques
Dates and Lifetime
From: 2009-02-01 To: 2011-09-30
Duration: 32 months

The urea cycle is responsible for the biosynthesis of urea using nitrogen atoms from amino acid catabolism, allowing the conversion of neurotoxic metabolites (ammonia derivates) into a non-toxic product, which is promptly excreted to avoid cell damage. The impairment of each one of the urea cycle enzymes causes a Urea Cycle Disorder (UCD). The general biochemical feature underlying all UCDs is the increment of plasmatic ammonia, a condition also known as hyperammonemia. Expectedly, taking into account the neurotoxic effect of ammonia, clinical signs often include behavioural and neurological abnormalities. In extreme, severe hyperammonemic coma lead to patient’s death. Ornithine Transcarbamylase Deficiency (OTCD; MIM 311250), the most common urea cycle diseases, result from reduced OTC expression/activity. The heterogeneity in enzyme expression/activity (and consequently in phenotype presentation) is a consequence of the wide mutational spectrum revealed by the OTC gene. Although being an X-linked Mendelian trait, de novo mutations are common and estimated to affect as much as 3/4 of female abnormal chromosomes. Technical advances in molecular strategies have allowed the identification of the molecular defect underlying OTCD condition. Even so, the causative mutation is not found in about 30% of the patients (unsolved cases). The reasons for such ineffective result may lie in the presence of mutations within intronic and/or regulatory regions, or to large deletions encompassing one or more exons.

The confirmation of the deficiency by molecular studies is of major relevance as only when the causative mutation is known can be tested in at-risk patients’ relatives and be applied in prenatal diagnosis. Moreover, early diagnosis will have a direct impact on patient psychiatric status and obvious family burden.
This application is clearly focussed at the improvement of molecular strategies that can allow a definitive and individual diagnosis for the unsolved OTCD cases. For that, we intent to: (a) identify relevant regulatory regions within the OTC gene that may be responsible for reduced gene expression or alternative splicing; (b) use semi-quantitative techniques in the detection of large deletions within the gene. As a complement, we will optimize OTC mRNA analyses from non-specific tissues (e.g. peripheral blood), an effort that will represent an eligible non-invasive alternative to the hepatic biopsy.
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