WiTNNess: An international natural history study of infantile-onset TNNT1 myopathy

OBJECTIVE

We created WiTNNess as a hybrid prospective/cross-sectional observational study to simulate a clinical trial for infantile-onset TNNT1 myopathy. Our aims were to identify populations for future trial enrollment, rehearse outcome assessments, specify endpoints, and refine trial logistics.

METHODS

Eligible participants had biallelic pathogenic variants of TNNT1 and infantile-onset proximal weakness without confounding conditions. The primary endpoint was ventilator-free survival. "Thriving" was a secondary endpoint defined as the ability to swallow and grow normally without non-oral feeding support. Endpoints of gross motor function included independent sitting and standing as defined by the Word Health Organization, a novel TNNT1 abbreviated motor score, and video mapping of limb movement. We recorded adverse events, concomitant medications, and indices of organ function to serve as comparators of safety in future trials.

RESULTS

Sixteen children were enrolled in the aggregate cohort (6 prospective, 10 cross-sectional; median census age 2.3 years, range 0.5-13.8). Median ventilator-free survival was 20.2 months and probability of death or permanent mechanical ventilation was 100% by age 60 months. All six children (100%) in the prospective arm failed to thrive by age 12 months. Only 2 of 16 (13%) children in the aggregate cohort sat independently and none stood alone. Novel exploratory motor assessments also proved informative. Laboratory and imaging data suggest that primary manifestations of TNNT1 deficiency are restricted to skeletal muscle.

INTERPRETATION

WiTNNess allowed us to streamline and economize the collection of historical control data without compromising scientific rigor, and thereby establish a sound operational framework for future clinical trials.

NPRL3 loss alters neuronal morphology, mTOR localization, cortical lamination and seizure threshold

Mutations in nitrogen permease regulator-like 3 (NPRL3), a component of the GATOR1 complex within the mTOR pathway, are associated with epilepsy and malformations of cortical development. Little is known about the effects of NPRL3 loss on neuronal mTOR signalling and morphology, or cerebral cortical development and seizure susceptibility. We report the clinical phenotypic spectrum of a founder NPRL3 pedigree (c.349delG, p.Glu117LysFS; n = 133) among Old Order Mennonites dating to 1727. Next, as a strategy to define the role of NPRL3 in cortical development, CRISPR/Cas9 Nprl3 knockout in Neuro2a cells in vitro and in foetal mouse brain in vivo was used to assess the effects of Nprl3 knockout on mTOR activation, subcellular mTOR localization, nutrient signalling, cell morphology and aggregation, cerebral cortical cytoarchitecture and network integrity. The NPRL3 pedigree exhibited an epilepsy penetrance of 28% and heterogeneous clinical phenotypes with a range of epilepsy semiologies, i.e. focal or generalized onset, brain imaging abnormalities, i.e. polymicrogyria, focal cortical dysplasia or normal imaging, and EEG findings, e.g. focal, multi-focal or generalized spikes, focal or generalized slowing. Whole exome analysis comparing a seizure-free group (n = 37) to those with epilepsy (n = 24) to search for gene modifiers for epilepsy did not identify a unique genetic modifier that explained the variability in seizure penetrance in this cohort. Nprl3 knockout in vitro caused mTOR pathway hyperactivation, cell soma enlargement and the formation of cellular aggregates seen in time-lapse videos that were prevented with the mTOR inhibitors rapamycin or torin1. In Nprl3 knockout cells, mTOR remained localized on the lysosome in a constitutively active conformation, as evidenced by phosphorylation of ribosomal S6 and 4E-BP1 proteins, even under nutrient starvation (amino acid-free) conditions, demonstrating that Nprl3 loss decouples mTOR activation from neuronal metabolic state. To model human malformations of cortical development associated with NPRL3 variants, we created a focal Nprl3 knockout in foetal mouse cortex by in utero electroporation and found altered cortical lamination and white matter heterotopic neurons, effects which were prevented with rapamycin treatment. EEG recordings showed network hyperexcitability and reduced seizure threshold to pentylenetetrazol treatment. NPRL3 variants are linked to a highly variable clinical phenotype which we propose results from mTOR-dependent effects on cell structure, cortical development and network organization.

Glutaric acidemia type 1: Treatment and outcome of 168 patients over three decades

Glutaric acidemia type 1 (GA1) is a disorder of cerebral organic acid metabolism resulting from biallelic mutations of GCDH. Without treatment, GA1 causes striatal degeneration in >80% of affected children before two years of age. We analyzed clinical, biochemical, and developmental outcomes for 168 genotypically diverse GA1 patients managed at a single center over 31 years, here separated into three treatment cohorts: children in Cohort I (n = 60; DOB 2006-2019) were identified by newborn screening (NBS) and treated prospectively using a standardized protocol that included a lysine-free, arginine-enriched metabolic formula, enteral l-carnitine (100 mg/kg•day), and emergency intravenous (IV) infusions of dextrose, saline, and l-carnitine during illnesses; children in Cohort II (n = 57; DOB 1989-2018) were identified by NBS and treated with natural protein restriction (1.0-1.3 g/kg•day) and emergency IV infusions; children in Cohort III (n = 51; DOB 1973-2016) did not receive NBS or special diet. The incidence of striatal degeneration in Cohorts I, II, and III was 7%, 47%, and 90%, respectively (p < .0001). No neurologic injuries occurred after 19 months of age. Among uninjured children followed prospectively from birth (Cohort I), measures of growth, nutritional sufficiency, motor development, and cognitive function were normal. Adherence to metabolic formula and l-carnitine supplementation in Cohort I declined to 12% and 32%, respectively, by age 7 years. Cessation of strict dietary therapy altered plasma amino acid and carnitine concentrations but resulted in no serious adverse outcomes. In conclusion, neonatal diagnosis of GA1 coupled to management with lysine-free, arginine-enriched metabolic formula and emergency IV infusions during the first two years of life is safe and effective, preventing more than 90% of striatal injuries while supporting normal growth and psychomotor development. The need for dietary interventions and emergency IV therapies beyond early childhood is uncertain.

Crigler-Najjar Syndrome Type 1: Pathophysiology, Natural History, and Therapeutic Frontier

BACKGROUND AND AIMS

We describe the pathophysiology, treatment, and outcome of Crigler-Najjar type 1 syndrome (CN1) in 28 UGT1A1 c.222C>A homozygotes followed for 520 aggregate patient-years.

APPROACH AND RESULTS

Unbound ("free") bilirubin (B_f ) was measured in patient sera to characterize the binding of unconjugated bilirubin (B_T ) to albumin (A) and validate their molar concentration ratio (B_T /A) as an index of neurological risk. Two custom phototherapy systems were constructed from affordable materials to provide high irradiance in the outpatient setting; light dose was titrated to keep B_T /A at least 30% below intravascular B_T binding capacity (i.e., B_T /A = 1.0). Categorical clinical outcomes were ascertained by chart review, and a measure (L_f ) was used to quantify liver fibrosis. Unbound bilirubin had a nonlinear relationship to B_T (R²  = 0.71) and B_T /A (R²  = 0.76), and B_f as a percentage of B_T correlated inversely to the bilirubin-albumin equilibrium association binding constant (R²  = 0.69), which varied 10-fold among individuals. In newborns with CN1, unconjugated bilirubin increased 4.3 ± 1.1 mg/dL per day. Four (14%) neonates developed kernicterus between days 14 and 45 postnatal days of life; peak B_T  ≥ 30 mg/dL and B_T /A ≥ 1.0 mol:mol were equally predictive of perinatal brain injury (sensitivity 100%, specificity 93.3%, positive predictive value 88.0%), and starting phototherapy after age 13 days increased this risk 3.5-fold. Consistent phototherapy with 33-103 µW/cm² •nm for 9.2 ± 1.1 hours/day kept B_T and B_T /A within safe limits throughout childhood, but B_T increased 0.46 mg/dL per year to reach dangerous concentrations by 18 years of age. Liver transplantation (n = 17) normalized B_T and eliminated phototherapy dependence. Liver explants showed fibrosis ranging from mild to severe.

CONCLUSION

Seven decades after its discovery, CN1 remains a morbid and potentially fatal disorder.

Branched-chain α-ketoacid dehydrogenase deficiency (maple syrup urine disease): Treatment, biomarkers, and outcomes

Over the past three decades, we studied 184 individuals with 174 different molecular variants of branched-chain α-ketoacid dehydrogenase activity, and here delineate essential clinical and biochemical aspects of the maple syrup urine disease (MSUD) phenotype. We collected data about treatment, survival, hospitalization, metabolic control, and liver transplantation from patients with classic (i.e., severe; n = 176), intermediate (n = 6) and intermittent (n = 2) forms of MSUD. A total of 13,589 amino acid profiles were used to analyze leucine tolerance, amino acid homeostasis, estimated cerebral amino acid uptake, quantitative responses to anabolic therapy, and metabolic control after liver transplantation. Standard instruments were used to measure neuropsychiatric outcomes. Despite advances in clinical care, classic MSUD remains a morbid and potentially fatal disorder. Stringent dietary therapy maintains metabolic variables within acceptable limits but is challenging to implement, fails to restore appropriate concentration relationships among circulating amino acids, and does not fully prevent cognitive and psychiatric disabilities. Liver transplantation eliminates the need for a prescription diet and safeguards patients from life-threatening metabolic crises, but is associated with predictable morbidities and does not reverse pre-existing neurological sequelae. There is a critical unmet need for safe and effective disease-modifying therapies for MSUD which can be implemented early in life. The biochemistry and physiology of MSUD and its response to liver transplantation afford key insights into the design of new therapies based on gene replacement or editing.

Recessive GM3 synthase deficiency: Natural history, biochemistry, and therapeutic frontier

GM3 synthase, encoded by ST3GAL5, initiates synthesis of all downstream cerebral gangliosides. Here, we present biochemical, functional, and natural history data from 50 individuals homozygous for a pathogenic ST3GAL5 c.862C>T founder allele (median age 8.1, range 0.7-30.5 years). GM3 and its derivatives were undetectable in plasma. Weight and head circumference were normal at birth and mean Apgar scores were 7.7 ± 2.0 (1 min) and 8.9 ± 0.5 (5 min). Somatic growth failure, progressive microcephaly, global developmental delay, visual inattentiveness, and dyskinetic movements developed within a few months of life. Infantile-onset epileptic encephalopathy was characterized by a slow, disorganized, high-voltage background, poor state transitions, absent posterior rhythm, and spike trains from multiple independent cortical foci; >90% of electrographic seizures were clinically silent. Hearing loss affected cochlea and central auditory pathways and 76% of children tested failed the newborn hearing screen. Development stagnated early in life; only 13 (26%) patients sat independently (median age 30 months), three (6%) learned to crawl, and none achieved reciprocal communication. Incessant irritability, often accompanied by insomnia, began during infancy and contributed to high parental stress. Despite catastrophic neurological dysfunction, neuroimaging showed only subtle or no destructive changes into late childhood and hospitalizations were surprisingly rare (0.2 per patient per year). Median survival was 23.5 years. Our observations corroborate findings from transgenic mice which indicate that gangliosides might have a limited role in embryonic neurodevelopment but become vital for postnatal brain growth and function. These results have critical implications for the design and implementation of ganglioside restitution therapies.