LRPPRC mutations cause early-onset multisystem mitochondrial disease outside of the French-Canadian population

The French-Canadian variant of COX-deficient Leigh syndrome (LSFC) is unique to Québec and caused by a founder mutation in the LRPPRC gene. Using whole exome sequencing, Oláhová et al. identify mutations in this gene associated with multisystem mitochondrial disease and early-onset neurodevelopmental problems in ten patients from different ethnic backgrounds.

Mitochondrial Complex IV [cytochrome c oxidase (COX)] deficiency is one of the most common respiratory chain defects in humans. The clinical phenotypes associated with COX deficiency include liver disease, cardiomyopathy and Leigh syndrome, a neurodegenerative disorder characterized by bilateral high signal lesions in the brainstem and basal ganglia. COX deficiency can result from mutations affecting many different mitochondrial proteins. The French-Canadian variant of COX-deficient Leigh syndrome is unique to the Saguenay-Lac-Saint-Jean region of Québec and is caused by a founder mutation in the LRPPRC gene. This encodes the leucine-rich pentatricopeptide repeat domain protein (LRPPRC), which is involved in post-transcriptional regulation of mitochondrial gene expression. Here, we present the clinical and molecular characterization of novel, recessive LRPPRC gene mutations, identified using whole exome and candidate gene sequencing. The 10 patients come from seven unrelated families of UK-Caucasian, UK-Pakistani, UK-Indian, Turkish and Iraqi origin. They resemble the French-Canadian Leigh syndrome patients in having intermittent severe lactic acidosis and early-onset neurodevelopmental problems with episodes of deterioration. In addition, many of our patients have had neonatal cardiomyopathy or congenital malformations, most commonly affecting the heart and the brain. All patients who were tested had isolated COX deficiency in skeletal muscle. Functional characterization of patients’ fibroblasts and skeletal muscle homogenates showed decreased levels of mutant LRPPRC protein and impaired Complex IV enzyme activity, associated with abnormal COX assembly and reduced steady-state levels of numerous oxidative phosphorylation subunits. We also identified a Complex I assembly defect in skeletal muscle, indicating different roles for LRPPRC in post-transcriptional regulation of mitochondrial mRNAs between tissues. Patient fibroblasts showed decreased steady-state levels of mitochondrial mRNAs, although the length of poly(A) tails of mitochondrial transcripts were unaffected. Our study identifies LRPPRC as an important disease-causing gene in an early-onset, multisystem and neurological mitochondrial disease, which should be considered as a cause of COX deficiency even in patients originating outside of the French-Canadian population.

Cultural aspects in the management of inborn errors of metabolism

European Health Care Systems have not yet accommodated both previous and current migration waves. Children from immigrant families, especially children with chronic conditions, are particularly affected from the shortcomings in medical care. One condition, phenylketonuria (PKU), is an inborn error of metabolism (IEM) which results in intellectual disability unless treated with a lifelong phenylalanine (Phe) restricted diet. In our PKU clinic, patients from families who previously had emmigrated from the geographic area of Turkey to Austria, exhibited worse blood Phe control and cognitive development than comparable patients from native Austrian families. Using structured and semi-structured interviews, questionnaires, and illness narratives, we identified language, psychosocial, economic, educational and cultural barriers as factors influencing adherence to treatment. Our findings led us to conclude that access to interpreter services, exploration of the socio-cultural background and of family ecology, as well as bi-directional communication and medical decision making according to the "best interest of the child" principle, may improve outcomes in patients requiring complex treatment and care.

Bacterial expression of mutant argininosuccinate lyase reveals imperfect correlation of in-vitro enzyme activity with clinical phenotype in argininosuccinic aciduria

BACKGROUND

The urea cycle defect argininosuccinate lyase (ASL) deficiency has a large spectrum of presentations from highly severe to asymptomatic. Enzyme activity assays in red blood cells or fibroblasts, although diagnostic of the deficiency, fail to discriminate between severe, mild or asymptomatic cases. Mutation/phenotype correlation studies are needed to characterize the effects of individual mutations on the activity of the enzyme.

METHODS

Bacterial in-vitro expression studies allowed the enzyme analysis of purified mutant ASL proteins p.I100T (c.299 T > C), p.V178M (c.532 G > A), p.E189G (c.566A > G), p.Q286R (c.857A > G), p.K315E (c.943A > G), p.R379C (c.1135 C > T) and p.R385C (c.1153 C > T) in comparison to the wildtype protein.

RESULTS

In the bacterial in-vitro expression system, ASL wild-type protein was successfully expressed. The known classical p.Q286R, the novel classical p.K315E and the known mutations p.I100T, p.E189G and p.R385C, which all have been linked to a mild phenotype, showed no significant residual activity. There was some enzyme activity detected with the p.V178M (5 % of wild-type) and p.R379C (10 % of wild-type) mutations in which K(m) values for argininosuccinic acid differed significantly from the wild-type ASL protein.

CONCLUSION

The bacterially expressed enzymes proved that the mutations found in patients and studied here indeed are detrimental. However, as in the case of red cell ASL activity assays, some mutations found in genetically homozygous patients with mild presentations resulted in virtual loss of enzyme activity in the bacterial system, suggesting a more protective environment for the mutant enzyme in the liver than in the heterologous expression system and/or in the highly dilute assays utilized here.

Long-term outcome of patients with argininosuccinate lyase deficiency diagnosed by newborn screening in Austria

Twenty-three patients with late onset argininosuccinate lyase deficiency (ASLD) were identified during a 27-year period of newborn screening in Austria (1:95,600, 95% CI=1:68,036-1:162,531). One additional patient was identified outside the newborn screening with neonatal hyperammonemia. Long-term outcome data were available in 17 patients (median age 13 years) ascertained by newborn screening. Patients were treated with protein restricted diet and oral arginine supplementation during infancy and childhood. IQ was average/above average in 11 (65%), low average in 5 (29%), and in the mild intellectual disability range in 1 (6%) patients. Four patients had an abnormal EEG without evidence of clinical seizures and three had abnormal liver function tests and/or evidence of hepatic steatosis. Plasma citrulline levels were elevated in four patients. Plasma ammonia levels were within normal range prior and after a protein load in all patients. Seven different mutations were identified in the 16 alleles investigated. Four mutations were novel (p.E189G, p.R168C, p.R126P, and p.D423H). All mutations were associated with low argininosuccinate lyase activities (0-15%) in red blood cells. Newborn screening might be beneficial in the prevention of chronic neurologic and intellectual sequelae in late onset ASLD, but a proportion of benign variants might have contributed to the overall favorable outcome as well.

Screening for X-linked creatine transporter (SLC6A8) deficiency via simultaneous determination of urinary creatine to creatinine ratio by tandem mass-spectrometry

High urinary creatine to creatinine ratio (U-CrCrtR) is a potential diagnostic marker of X-linked creatine transporter (SLC6A8) deficiency. We developed a tandem mass-spectrometry method to simultaneously determine urinary creatine and creatinine in 975 individuals (0-18 years). U-CrCrtR increased up to 8 years and decreased thereafter. U-CrCrtR was 2.29 and 2.12 (99th percentile: 1.87) in two males with subsequently confirmed SLC6A8 mutations. The frequency of SLC6A8 deficiency was 2.3% in 157 males at risk.

Oral beta-hydroxybutyrate supplementation in two patients with hyperinsulinemic hypoglycemia: monitoring of beta-hydroxybutyrate levels in blood and cerebrospinal fluid, and in the brain by in vivo magnetic resonance spectroscopy

In persistent hyperinsulinemic hypoglycemia of infancy, ketone body concentrations are abnormally low at times of hypoglycemia, depriving the brain of its most important alternative fuel. The neuroprotective effect of endogenous ketone bodies is evidenced by animal and human studies, but knowledge about exogenous supply is limited. Assuming that exogenous ketone body compounds as a dietetic food might replace this alternative energy source for the brain, we have monitored the fate of orally supplemented DL sodium beta-hydroxybutyrate (beta-OHB) in two 6-mo-old infants with persistent hyperinsulinemic hypoglycemia for 5 and 7 mo, while on frequent tube-feedings and treatment with octreotide. Near total (95%) pancreatectomy had been ineffective in one patient and was refused in the other. In blood, concentrations of beta-OHB increased to levels comparable to a 16- to 24-h fast while on DL sodium beta-OHB 880 to 1000 mg/kg per day. In cerebrospinal fluid, concentrations of beta-OHB increased to levels comparable to a 24- to 40-h fast, after single dosages of 4 and 8 g, respectively. High ratios of beta-OHB to acetoacetate indicated exogenous origin of beta-OHB. An increase of intracerebral concentrations of beta-OHB could be demonstrated by repetitive single-voxel proton magnetic resonance spectroscopy by a clear doublet at 1.25 ppm. Oral DL sodium beta-OHB was tolerated without side effects. This first report on oral supplementation of DL sodium beta-OHB in two patients with persistent hyperinsulinemic hypoglycemia demonstrates effective uptake across the blood-brain barrier and could provide the basis for further evaluation of the neuroprotective effect of beta-OHB in conditions with hypoketotic hypoglycemia.

Budd-Chiari syndrome associated with coagulation abnormalities in a child with carbohydrate deficient glycoprotein syndrome type Ix

A 6-year-old male patient presented with Budd-Chiari syndrome and glycoprotein abnormalities associated with carbohydrate deficient glycoprotein syndrome type I with yet unidentified molecular defect (type Ix). Budd-Chiari syndrome most likely developed after hepatic venous thrombosis caused by coagulation abnormalities resulting from hypoglycosylation and functional impairment of anticoagulant proteins.