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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 (Bf) was measured in patient sera to characterize the binding of unconjugated bilirubin (BT) to albumin (A) and validate their molar concentration ratio (BT/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 BT/A at least 30% below intravascular BT binding capacity (i.e., BT/A = 1.0). Categorical clinical outcomes were ascertained by chart review, and a measure (Lf) was used to quantify liver fibrosis. Unbound bilirubin had a nonlinear relationship to BT (R2 = 0.71) and BT/A (R2 = 0.76), and Bf as a percentage of BT 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 BT ≥ 30 mg/dL and BT/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 BT and BT/A within safe limits throughout childhood, but BT increased 0.46 mg/dL per year to reach dangerous concentrations by 18 years of age. Liver transplantation (n = 17) normalized BT 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.

Mutations in nitrogen permease regulator-like 3 (NPRL3), a component of the GATOR1 complex within the mechanistic target of rapamycin (mTOR) pathway, are associated with epilepsy and malformations of cortical development. Little is known about the effects of NPRL3 loss on neuronal mTOR signaling 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 fetal mouse brain in vivo was used to assess effects of Nprl3 knockout on mTOR activation, subcellular mTOR localization, nutrient signaling, 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 KO cells, mTOR remained localized on the lysosome in a constitutively active conformation, as evidenced by phosphorylation of 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 KO in fetal 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 result from mTOR-dependent effects on cell structure, cortical development, and network organization.