Genetic analysis in nine unrelated Italian patients affected by OTC deficiency: detection of novel mutations in the OTC gene☆
Introduction
Ornithine transcarbamylase deficiency (OTCD; MIM# 311250) is the most common urea cycle disorder, caused by a defect of the mithocondrial enzyme ornithine transcarbamylase (OTC, EC 2.1.3.3), which is expressed specifically in the liver and small intestine [1]; it is inherited as an X-linked trait.
The OTC gene is subject to heterogeneous and frequent new mutations, which may lead to partial or complete enzymatic deficiency. Clinical presentation is highly variable, usually more severe in newborn male patients. In hemizygous male patients the onset of the disease can also occur later in infancy, during childhood or even in adult life with varying degrees of severity, which is principally correlated to different mutations and enzyme activity. In heterozygous female manifesting carriers the clinical phenotype depends on X-inactivation [2] and it ranges from asymptomatic to severe hyperammonemia.
The structural gene encoding for the enzyme OTC has been mapped to the short arm of the X chromosome at band Xp21.1 [3]. It spans a region of 73 kb and contains 10 exons and 9 introns of highly variable size [4]. The OTC mRNA contains 1062 nt and it encodes a 40 kDa protein precursor of 354 amino acids which includes the amino terminus leader peptide of 32 amino acids [5].
Up to now about 230 mutations in the OTC gene have been reported (http://63.75.201.100/otc/) [6], most of which are “private” mutations [7]. Large deletions of one exon or more are seen in approximately 7% of patients, small deletions or insertions are seen in about 9%, and the remaining mutations are single base substitutions; approximately 15% of mutations affect RNA splicing sites [8].
In this study genetic analysis by direct sequencing of OTC cDNA, genomic DNA, and enzymatic restriction analysis on the patients' genomic DNA and total RNA isolated from peripheral blood lymphocytes led to the identification of three new mutations and six previously reported mutations in the OTC gene of nine unrelated Italian patients and their families. Interestingly, eight of the nine patients were females. These patients came from different hospitals, they were not selected and they represented a casual sampling.
Section snippets
Patients
One OTCD male patient (1.1) and eight OTCD manifesting carriers (2.1, 3.1, 4.1, 5.1, 6.1, 7.1, 8.1, 9.1), segregating in nine unrelated families, were studied. The male patient with the mild form and onset at three years showed classical OTCD clinical symptoms (vomiting, confusion, lethargy) and characteristic laboratory findings (high ammonemia, high glutaminemia, low citrullinemia, and high urinary orotic acid excretion). The manifesting carriers presented clinical heterogeneity varying from
Results and discussion
The OTC gene is subject to heterogeneous and frequent novel genetic lesions, most of which are “private” mutations, that may lead to partial or complete OTC enzymatic deficiency. The high new mutation rate and the wide spectrum of mutations mean that direct mutation analysis is essential for the identification of the causative genetic lesion in affected families.
Genetic analysis in one OTCD male patient and eight female manifesting carriers from nine unrelated Italian families led to the
Acknowledgements
This work was partially supported by grants from the Cassa di Risparmio di Firenze, Association AMMEC and Azienda Ospedaliera Meyer.
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Cited by (15)
A female carrier of ornithine carbamoyltransferase deficiency masquerading as attention deficit-hyperactivity disorder
2014, Brain and DevelopmentCitation Excerpt :In cases in which there is a strong suspicion of the disease, negative ammonia or amino acid level results alone should not result in disease exclusion. Provocative tests, such as the allopurinol test or protein loading, have been used in the past to detect carriers, however, recently mutation analysis is used more frequently and considered a reliable and applicable tool [9]. The OTC gene is located on Xp21.1 and encodes the ornithine transcarbamylase.
A comment to the paper Rheumatoid atlantoaxial instability treated by anterior transarticular C1-C2 fixaton. Case report by Zapalstrok;owicz K. et al
2013, Neurologia i Neurochirurgia PolskaCitation Excerpt :Despite this, an allopurinol loading test showed no elevated orotic acid level and resulted in no identification of the disease-carrier status of the woman. Our observation of the normal orotic acid level in the allopurinol loading test in the patient's mother, along with the identified OTC gene mutation, supported the probability of undefined or false negative results reported previously for heterozygous females [13–17]. Direct mutation analysis of the OTC gene is therefore important for providing accurate carrier detection and prenatal diagnosis in OTC deficiency families.
Magnetic resonance spectroscopy and molecular studies in ornithine transcarbamylase deficiency novel mutation c.802A>G in exon 8 (p.Met268Val)
2013, Neurologia i Neurochirurgia PolskaCitation Excerpt :Despite this, an allopurinol loading test showed no elevated orotic acid level and resulted in no identification of the disease-carrier status of the woman. Our observation of the normal orotic acid level in the allopurinol loading test in the patient's mother, along with the identified OTC gene mutation, supported the probability of undefined or false negative results reported previously for heterozygous females [13–17]. Direct mutation analysis of the OTC gene is therefore important for providing accurate carrier detection and prenatal diagnosis in OTC deficiency families.
Hypocitrullinemia in expanded newborn screening by LC-MS/MS is not a reliable marker for ornithine transcarbamylase deficiency
2009, Journal of Pharmaceutical and Biomedical AnalysisOrnithine transcarbamylase deficiency: A urea cycle defect
2003, European Journal of Paediatric Neurology
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The authors S. Bisanzi and A. Morrone contributed to this work equally.