A promoter variant in the OTC gene associated with late and variable age of onset hyperammonemia

Perspectives on long-term medical management of urea cycle disorders: insights from a survey of UK healthcare professionals

BACKGROUND

Urea cycle disorders (UCDs) are rare inborn errors of metabolism which impact the body's ability to detoxify ammonia produced during protein metabolism. In the UK, there is a nationally adopted guideline for the emergency management of hyperammonaemia in UCD patients, however there is no guideline for long‑term management, and treatment decisions are left to the discretion of individual healthcare professionals (HCPs).

RESULTS

Twenty-three HCPs, comprising 13 (57%) metabolic consultants, two (9%) specialist nurses, four (17%) pharmacists, and four (17%) dietitians, participated in interviews to document their attitudes and beliefs regarding the long‑term management of UCD patients, including their current practices, treatment goals, and clinical ambitions. The highest priority for 14/23 (61%) of HCPs was to minimise the risk of hyperammonaemia, however the ammonia level that HCPs advised they aimed for varied significantly, with some targeting above the upper limit of normal. Glycerol phenylbutyrate was the highest ranked ammonia scavenger treatment amongst HCPs for safety, tolerability, duration of scavenging action and reducing patient burden, and HCPs suggested that it would be the first-line treatment in an updated guideline. All prescribing HCPs agreed they would prefer their patients receive a licenced product rather than an unlicensed one for reasons including more reliable supply, greater insurance/legitimacy, and the reassurance of regulatory scrutiny and approval. However, analysis of NHS England's dispensing data between July 2023 and June 2024 indicated annual spend on nitrogen scavengers of £6.7 million with unlicensed specials accounting for £3 million (45%) of the total. Differences between HCPs in the awareness of clinically relevant characteristics of ammonia scavengers, including their sodium and propylene glycol content, were observed.

CONCLUSIONS

To standardise the treatment of UCDs within and between metabolic centres in the UK, there is merit in developing a UK-specific treatment guideline.

A novel de novo missense OTC mutation in an Iranian girl: a case report

OBJECTIVES

Ornithine transcarbamylase deficiency (OTCD) is the most common inborn error of the urea cycle, caused by mutations in the OTC gene located on the X chromosome. OTCD presents in early and late-onset forms, with variable severity. Despite the high genetic heterogeneity, genotype-phenotype correlations help in prognosis and treatment planning. This study presents a novel missense mutation in an Iranian girl with OTCD, occurring de novo, contributing to the understanding of the disease's genetic landscape.

CASE PRESENTATION

A 2-year-old girl from a consanguineous marriage presented with nausea, recurrent vomiting, and seizure. Elevated plasma ammonia, liver enzyme tests, and hepatomegaly suggested metabolic disorders. Following whole exome test, a novel heterozygous missense mutation in exon 7 of the OTC gene (c.674C>T) was identified in the patient. Despite maternal and paternal testing, no mutation was detected.

CONCLUSIONS

Identifying new mutations in populations helps mitigate the high mortality rates associated with OTCD hyperammonemic episodes and provides the best course of treatment, especially considering the diverse phenotypic variations.

Datamining approaches for examining the low prevalence of N-acetylglutamate synthase deficiency and understanding transcriptional regulation of urea cycle genes

Ammonia, which is toxic to the brain, is converted into non-toxic urea, through a pathway of six enzymatically catalyzed steps known as the urea cycle. In this pathway, N-acetylglutamate synthase (NAGS, EC 2.3.1.1) catalyzes the formation of N-acetylglutamate (NAG) from glutamate and acetyl coenzyme A. NAGS deficiency (NAGSD) is the rarest of the urea cycle disorders, yet is unique in that ureagenesis can be restored with the drug N-carbamylglutamate (NCG). We investigated whether the rarity of NAGSD could be due to low sequence variation in the NAGS genomic region, high NAGS tolerance for amino acid replacements, and alternative sources of NAG and NCG in the body. We also evaluated whether the small genomic footprint of the NAGS catalytic domain might play a role. The small number of patients diagnosed with NAGSD could result from the absence of specific disease biomarkers and/or short NAGS catalytic domain. We screened for sequence variants in NAGS regulatory regions in patients suspected of having NAGSD and found a novel NAGS regulatory element in the first intron of the NAGS gene. We applied the same datamining approach to identify regulatory elements in the remaining urea cycle genes. In addition to the known promoters and enhancers of each gene, we identified several novel regulatory elements in their upstream regions and first introns. The identification of cis-regulatory elements of urea cycle genes and their associated transcription factors holds promise for uncovering shared mechanisms governing urea cycle gene expression and potentially leading to new treatments for urea cycle disorders.

Father-to-daughter transmission in late-onset OTC deficiency: an underestimated mechanism of inheritance of an X-linked disease

BACKGROUND

Ornithine Transcarbamylase Deficiency (OTCD) is an X-linked urea cycle disorder characterized by acute hyperammonemic episodes. Hemizygous males are usually affected by a severe/fatal neonatal-onset form or, less frequently, by a late-onset form with milder disease course, depending on the residual enzymatic activity. Hyperammonemia can occur any time during life and patients could remain non- or mis-diagnosed due to unspecific symptoms. In heterozygous females, clinical presentation varies based on the extent of X chromosome inactivation. Maternal transmission in X-linked disease is the rule, but in late-onset OTCD, due to the milder phenotype of affected males, paternal transmission to the females is possible. So far, father-to-daughter transmission of OTCD has been reported only in 4 Japanese families.

RESULTS

We identified in 2 Caucasian families, paternal transmission of late-onset OTCD with severe/fatal outcome in affected males and 1 heterozygous female. Furthermore, we have reassessed the pedigrees of other published reports in 7 additional families with evidence of father-to-daughter inheritance of OTCD, identifying and listing the family members for which this transmission occurred.

CONCLUSIONS

Our study highlights how the diagnosis and pedigree analysis of late-onset OTCD may represent a real challenge for clinicians. Therefore, the occurrence of paternal transmission in OTCD should not be underestimated, due to the relevant implications for disease inheritance and risk of recurrence.

Intronic variants in inborn errors of metabolism: Beyond the exome

Non-coding regions are areas of the genome that do not directly encode protein and were initially thought to be of little biological relevance. However, subsequent identification of pathogenic variants in these regions indicates there are exceptions to this assertion. With the increasing availability of next generation sequencing, variants in non-coding regions are often considered when no causative exonic changes have been identified. There is still a lack of understanding of normal human variation in non-coding areas. As a result, potentially pathogenic non-coding variants are initially classified as variants of uncertain significance or are even overlooked during genomic analysis. In most cases where the phenotype is non-specific, clinical suspicion is not sufficient to warrant further exploration of these changes, partly due to the magnitude of non-coding variants identified. In contrast, inborn errors of metabolism (IEMs) are one group of genetic disorders where there is often high phenotypic specificity. The clinical and biochemical features seen often result in a narrow list of diagnostic possibilities. In this context, there have been numerous cases in which suspicion of a particular IEM led to the discovery of a variant in a non-coding region. We present four patients with IEMs where the molecular aetiology was identified within non-coding regions. Confirmation of the molecular diagnosis is often aided by the clinical and biochemical specificity associated with IEMs. Whilst the clinical severity associated with a non-coding variant can be difficult to predict, obtaining a molecular diagnosis is crucial as it ends diagnostic odysseys and assists in management.