Whole-body L-leucine oxidation in patients with variant form of maple syrup urine disease

Treatment Outcomes for Maple Syrup Urine Disease Detected by Newborn Screening

OBJECTIVE

Maple syrup urine disease (MSUD), a life-threatening metabolic disorder, is included in newborn screening (NBS) programs worldwide. The study aims to evaluate the impact of NBS on the long-term outcome of MSUD patients.

METHODS

We performed a prospective, national, multicenter, observational study.

RESULTS

In the studied NBS cohort (N = 33; 22 classic MSUD [cMSUD], 11 variant MSUD [vMSUD]; median age at last visit 10.4 years), 32 (97%) patients survived, 58% of them had normal cognitive functions (median IQ 87). Initial peak leucine increased linearly with age in cMSUD (median: 1712 µmol/L), but not in vMSUD. Global IQ correlated inversely with the initial peak leucine concentration (P = .04; β = -0.0081) and the frequency of decompensations (P = .02; β = -9.133). A cluster analysis identified 2 subgroups differing in their long-term metabolic control (median leucine concentration: 162 vs 278 µmol/L; P < .001). In cMSUD, lower leucine concentrations were associated with a higher IQ (95.5 vs 80; P = .008). Liver transplantation (median age 5.8 years) was not associated with better cognitive outcome. NBS is highly sensitive for cMSUD, but vMSUD might be missed (N = 2 missed by NBS).

CONCLUSIONS

NBS and the early start of treatment improve survival and long-term outcome in individuals with cMSUD. Disease severity is an important modifier of outcome; however, the time to NBS report and the quality of long-term metabolic control had an independent impact on cognitive outcome, highlighting the importance of an early diagnosis and the quality of treatment.

Challenges in Diagnosing Intermediate Maple Syrup Urine Disease by Newborn Screening and Functional Validation of Genomic Results Imperative for Reproductive Family Planning

Maple syrup urine disease is caused by a deficiency of branched-chain alpha-ketoacid dehydrogenase, responsible for degradation of leucine, isoleucine, and valine. Biallelic pathogenic variants in BCKDHA, BCKDHB, or DBT genes result in enzyme deficiency. We report the case of a female infant who presented with mild gross motor delay at 4 months, and seizures with hypoglycaemia at 5 months. Newborn screening returned total leucine/isoleucine at the 99.5th centile of the population; however, as second-tier testing reported minimal alloisoleucine, the results were considered inconsistent with MSUD. Plasma amino acid and urine organic acid analyses at 5 months were, however, consistent with a diagnosis of MSUD. A brain MRI showed bilateral symmetrical T2 hyperintense signal abnormalities involving white matter, globus pallidus, thalamus, brainstem, and dentate nuclei with restricted diffusion. A repeat MRI 10 months post-dietary-intervention showed the resolution of these changes and progression in myelination. Her clinical phenotype, including protein tolerance, correlated with intermediate MSUD. Molecular analysis of all three genes identified two variants of uncertain significance, c.434-15_434-4del and c.365A>G (p. Tyr122Cys) in the DBT gene. The rate of leucine decarboxylation in fibroblasts was reduced, but not to the extent observed in classical MSUD patients, supporting an intermediate form of MSUD. Previously reported mRNA splicing studies supported a deleterious effect of the c.434-15_434-4del variant. This functional evidence and confirmation that the variants were in trans, permitted their reclassification as pathogenic and likely pathogenic, respectively, facilitating subsequent prenatal testing. This report highlights the challenges in identifying intermediate MSUD by newborn screening, reinforcing the importance of functional studies to confirm variant pathogenicity in this era of molecular diagnostics.

Human mild traumatic brain injury decreases circulating branched-chain amino acids and their metabolite levels

The pathophysiology of traumatic brain injury (TBI) is complex and not well understood. Because pathophysiology has ramifications for injury progression and outcome, we sought to identify metabolic cascades that are altered after acute human mild and severe TBI. Because catabolism of branched-chain amino acids (BCAAs; i.e., valine, isoleucine, and leucine) leads to glucose and energy metabolism, and neurotransmitter synthesis and availability, we investigated BCAA metabolites in plasma samples collected within 24 h of injury from mild TBI (Glasgow Coma Scale [GCS] score >12), severe TBI (GCS ≤8), orthopedic injury, and healthy volunteers. We report decreased levels of all three BCAAs in patients with mild TBI relative to healthy volunteers, while these BCAAs levels in patients with severe TBI were further reduced compared with all groups. Orthopedic patients exhibited reductions in BCAA comparable to those in patients with mild TBI. The decrease in patients with mild and severe TBI persisted for derivatives of BCAA catabolic intermediates. Only plasma levels of methylglutarylcarnitine, a derivative of a leucine metabolite, were increased in patients with severe TBI compared with all other groups. Notably, logistic regression combination of three BCAA metabolites whose levels were changed by 24 h post-injury provided prognostic value (area under the curve=0.92) in identifying patients with severe TBI in whom elevated intracranial pressure (≥25 mm Hg) developed. These changes suggest alteration of BCAA metabolism after TBI may contribute to decreased energy production and neurotransmitter synthesis and may contribute to TBI pathophysiology. Supplementation of BCAAs and/or their metabolites may reduce TBI pathology and improve outcome.

Asynchronous supply of indispensable amino acids reduces protein deposition in milk-fed calves

A balanced supply of indispensable amino acids (AA) is required for efficient protein synthesis. Different absorption kinetics (e.g., free vs. protein-bound AA) may, however, create asynchrony in postabsorptive availability of individual AA, thereby reducing the efficiency of protein deposition. We studied the effects of AA asynchrony on protein metabolism in growing, milk-fed calves. In 2 experiments, each with a change-over design including 8 calves, a milk replacer deficient in Lys and Thr was used. In Expt. 1, L-Lys and L-Thr were parenterally supplemented, either in synchrony (SYN), asynchrony (ASYN), or partial asynchrony (PART) with dietary AA. In Expt. 2, l-Lys and l-Thr were orally supplemented, either in SYN or ASYN with dietary AA. In Expt. 1, digested protein was used less efficiently for growth for ASYN (31.0%) than for SYN (37.7%), with PART being intermediate (36.0%). Indicator AA oxidation tended (P = 0.06) to be higher for ASYN. In Expt. 2, the efficiency of protein utilization was lower for ASYN (34.9%) than for SYN (46.6%). Calves spared AA from oxidation when the limiting AA were provided in excess after a short period (<24 h) of deprivation. Restoring AA balance by parenteral supplementation resulted in a 19% lower efficiency of digestible protein utilization than by oral supplementation, likely caused by splanchnic oxidation of imbalanced AA in excess to Thr. In conclusion, asynchronous availability of individual indispensable AA reduces the efficiency by which digested protein is retained in milk-fed calves. Furthermore, an AA imbalance in the splanchnic tissues may result in disproportionate AA oxidation.

Domino liver transplantation: how far can we push the paradigm?

Domino liver transplantation (DLT) has emerged as a strategy for increasing the number of liver grafts available: morphologically normal livers from donors with metabolic diseases can be used for select recipients with hepatocellular carcinoma (usually outside the Milan criteria). Familial amyloidotic polyneuropathy (FAP) is the most common indication for DLT. When FAP patients are involved in DLT, the indications and outcomes are clear and good, although de novo FAP development within various periods of time has been described in DLT recipients of FAP livers. With the increasing need for organs, livers explanted from patients with rare metabolic diseases, such as primary hyperoxaluria (PH), acute intermittent porphyria (AIP), maple syrup urine disease (MSUD), and homozygous familial hypercholesterolemia (HFHC), are being used for DLT. However, insufficient data about the use of livers from patients with these rare metabolic diseases are available. In this review, we focus on the latter disorders. PH is not a good indication for DLT because recipients of PH livers develop hyperoxaluria and early acute renal failure. AIP also seems to be a debatable indication for DLT because of the rapid development of neurotoxicity in AIP liver recipients. However, the outcomes of DLT with HFHC and MSUD liver grafts (which include the risk of the de novo development of these genetic diseases) are promising. For rare metabolic liver diseases to be established as indications for DLT, more reports and studies are needed.

Phenylbutyrate therapy for maple syrup urine disease

Therapy with sodium phenylacetate/benzoate or sodium phenylbutyrate in urea cycle disorder patients has been associated with a selective reduction in branched-chain amino acids (BCAA) in spite of adequate dietary protein intake. Based on this clinical observation, we investigated the potential of phenylbutyrate treatment to lower BCAA and their corresponding α-keto acids (BCKA) in patients with classic and variant late-onset forms of maple syrup urine disease (MSUD). We also performed in vitro and in vivo experiments to elucidate the mechanism for this effect. We found that BCAA and BCKA are both significantly reduced following phenylbutyrate therapy in control subjects and in patients with late-onset, intermediate MSUD. In vitro treatment with phenylbutyrate of control fibroblasts and lymphoblasts resulted in an increase in the residual enzyme activity, while treatment of MSUD cells resulted in the variable response which did not simply predict the biochemical response in the patients. In vivo phenylbutyrate increases the proportion of active hepatic enzyme and unphosphorylated form over the inactive phosphorylated form of the E1α subunit of the branched-chain α-keto acid dehydrogenase complex (BCKDC). Using recombinant enzymes, we show that phenylbutyrate prevents phosphorylation of E1α by inhibition of the BCKDC kinase to activate BCKDC overall activity, providing a molecular explanation for the effect of phenylbutyrate in a subset of MSUD patients. Phenylbutyrate treatment may be a valuable treatment for reducing the plasma levels of neurotoxic BCAA and their corresponding BCKA in a subset of MSUD patients and studies of its long-term efficacy are indicated.

Variant maple syrup urine disease (MSUD)--the entire spectrum

BACKGROUND

In the rare inborn autosomal recessive disorder maple syrup urine disease (MSUD) the accumulation of the branched-chain amino acids (BCAAs) and their metabolic products results in acute and chronic brain dysfunction. About 20% of the patients suffer from non-classic variant forms of MSUD of different clinical severity.

AIM

Up to now variant cases have mostly been published as individual case reports; the aim of this study was to give a comparative description of 16 individuals (aged 6-30 years) with different forms of variant MSUD.

METHODS

Laboratory data, information on clinical course and treatment as well as aspects of developmental, intellectual and social outcome were obtained retrospectively. Data from in vitro and in vivo methods measuring the degree of enzyme deficiency were included.

RESULTS

In addition to a mild phenotype, which fits well into the so-called intermittent variant, and a more severe phenotype with a wider range from a mild variant to an almost classic form, which fits well into the so-called intermediate variant, we assume the existence of an asymptomatic, non-disease variant of MSUD. These clinical phenotypes are not unambiguously differentiable on the basis of biochemical parameters.

CONCLUSION

A continuum of clinical severity from asymptomatic to very severe (border to classic) exists in variant MSUD. Apart from newborns with classic MSUD, also those with variant forms benefit from early diagnosis and start of adequate treatment.

Duration of extracorporeal therapy in acute maple syrup urine disease: a kinetic model

Maple syrup urine disease (MSUD, MIM 248600) can be complicated by metabolic crises necessitating extracorporeal removal therapy (ECRT). Since leucine levels are usually not immediately available during therapy, an accurate kinetic model of leucine plasma levels during removal would be useful to establish the duration of ECRT. Such a kinetic model is available for neonates undergoing continuous ECRT (CECRT) with a leucine clearance>or=35 ml min-1 1.73 m-2. The current study tests the validity of this model in older children. Plasma leucine levels were obtained from eleven ECRT sessions [seven CECRT and four intermittent hemodialysis (HDi) sessions] in seven children aged 1-14 years. No hemodynamic instability or neurological complications were observed during treatment. HDi provided a higher leucine clearance and required shorter sessions than CECRT (5.4+/-0.6 vs. 17.1+/-6.0 h). All patients regained precrisis neurological status except for one patient who had severe neurological damage (severe cerebral edema) at the time of dialysis and subsequently died despite efficient leucine removal. A leucine clearance>or=50 ml min-1 1.73 m-2 is required to obtain a kinetic model similar to that reported in neonates, both with CECRT and HDi. This model should be helpful in predicting the duration of therapy needed to attain desired leucine levels.

Domino liver transplantation in maple syrup urine disease

Liver transplantation has been reported in a few cases of maple syrup urine disease (MSUD), but is controversial. Many patients with approved indications for liver transplantation die before grafts are available. A 25-yr-old man with MSUD underwent liver transplantation, and his liver was used as a domino graft for a 53-yr-old man with hepatocellular carcinoma who had low priority on the liver transplant waiting list and was unlikely to survive until routine organ procurement. Both transplants were performed as "piggy back" procedures, reconstructing the domino graft with caval segments from the cadaveric donor. Neither required veno-venous bypass. Whole body leucine oxidation was estimated by 13CO2 in breath after oral boluses of L-[1-13C]-leucine, before and after transplantation in both patients and a control subject. The surgical outcome was successful. The patient with MSUD had marked decreases in plasma branched-chain amino acids (BCAAs) and alloisoleucine (from 255 +/- 66 to 16 +/- 7 micromol/L), despite advancement of dietary protein from 6 to >40 gm/day. The domino recipient maintained near-normal levels of plasma amino acids with no detectable alloisoleucine on unrestricted diet. Leucine oxidation increased in the patient with MSUD (from 2.2 to 5.6% recovered in 4 hours) and decreased in the recipient (from 9.7 to 6.2%). Neither patient demonstrated any apparent symptoms of MSUD over more than 7 months. In conclusion, liver transplantation substantially corrects whole body BCAA metabolism in MSUD and greatly attenuates the disease. Livers from patients with MSUD may be considered as domino grafts for patients who might otherwise not survive until transplantation.

Atypical phenotype in a boy with a maple syrup urine disease

Maple syrup urine disease (MSUD) is a metabolic disorder due to a block in the decarboxylation step in the catabolic pathways of the branched-chain amino acids (BCAAs). We describe an atypical presentation in an infant male. The patient presented with psychomotor retardation, profound hypotonia and elevated plasma levels of BCAAs, but no elevation of alloisoleucine. Cranial magnetic resonance imaging showed prominent diffuse CSF spaces, delayed myelin maturation and symmetrical signal abnormality within the globi pallidi, midbrain, dorsal pons and medulla. The cerebellar white matter was specifically spared. A mitochondrial disorder was suggested. After correction of feeding problems with G-tube feeds, his high BCAAs persisted and, on fourth analysis, alloisoleucine was seen. Subsequent fibroblast enzyme and mutation analysis confirmed MSUD due to E(1)-alpha subunit deficiency. After starting dietary treatment, there was no significant improvement in his hypotonia or his psychomotor development. However, the high signal within the globi pallidi had resolved. MSUD may have diverse clinical presentations, and should be considered in children who present with chronic psychomotor delay but no acute encephalopathic episodes. BCAA levels may not be very high, alloisoleucine may not always be detected in MSUD even with severe enzyme deficiency, and imaging may be misleading if seen in the chronic phase only.

Kinetic modeling of plasma leucine levels during continuous venovenous extracorporeal removal therapy in neonates with maple syrup urine disease

A kinetic modeling of leucine plasma concentration changes is proposed to describe the plasma leucine reduction rate during continuous extracorporeal removal therapy (CECRT) in neonates with maple syrup urine disease. Data were obtained from seven neonates using a bicompartmental model for the best fitted curve of plasma leucine decrease during CECRT. During the first 3 h, leucine plasma levels decreased according to an exponential curve: [Leu](t) = [Leu](i) x 0.95 x 10(-0.09t) where [Leu](t) is the leucine plasma level (mumol/L) at time t (h) during CECRT and [Leu](I) is the initial plasma level. From h 4 to the end of CECRT, a second exponential curve was observed: [Leu](t) = [Leu](i) x 0.74 x 10(-0.05t). Plasma leucine levels obtained from three other neonates were similar to those predicted by the model. The apparent distribution volumes for leucine that correspond to the two exponential equations obtained were calculated from the leucine mass removal collected in the spent dialysate and ultrafiltrate. The distribution volume was 34 +/- 3% of body weight during the first 3 h of CECRT and 72 +/- 7% from h 4 to the end of CECRT. These figures are similar to known values for the extracellular water compartment and for total body water in the newborn. The findings suggest that leucine handling during CECRT is similar to that of nonprotein-bound small-molecular-weight solutes such as urea.