Genotype-Phenotype Landscape of NALCN and UNC80-Related Disorders

BACKGROUND AND OBJECTIVES: The NALCN channelosome regulates the resting membrane potential through sodium leak currents, influencing cellular excitability. Genetic variants in NALCN and UNC80, a subunit of the NALCN channelosome, cause ultra-rare and severe neurodevelopmental disorders. Autosomal dominant congenital contractures of the limbs and face, hypotonia, and developmental delay (CLIFAHDD) syndrome is associated with gain-of-function (GOF) variants in NALCN. Infantile hypotonia with psychomotor retardation and characteristic facies (IHPRF) 1 syndrome is associated with biallelic variants in NALCN and IHPRF 2 syndrome with biallelic variants in UNC80, both resulting in a loss-of-function (LOF). This study aims to expand the phenotypes associated with these syndromes, exploring potential genotype-phenotype associations. METHODS: This is a cross-sectional study including patients with pathogenic or likely pathogenic variants in NALCN and UNC80. Phenotypes were evaluated through a structured interview, questionnaires, and review of medical records. Associations between variants, clinical features, and syndromes were analyzed. RESULTS: Fifty-one patients were included (34 with CLIFAHDD, 9 with IHPRF 1, 8 with IHPRF 2; 3 months-27 years; 37.3% female). All exhibited neurodevelopmental delay, more severe in patients with LOF variants (p = 0.019). Neurodevelopmental regression was observed in 29.4% of patients with CLIFAHDD syndrome, associated with the onset of ataxia (70%). Patients with CLIFAHDD had more severe respiratory symptoms at birth (11.7% orotracheal intubation). Distal arthrogryposis (76.5%), episodic ataxia (41.2% of ambulatory patients), and paroxysmal dystonia (11.7%) were exclusively diagnosed in patients with CLIFAHDD. Patients with LOF variants presented more frequently with failure to thrive (88.2%, p = 0.001), central sleep apnea (CSA, 64.7%, p < 0.001), and epilepsy (70.6%, p < 0.001). Epilepsy was associated with more severe cognitive delays (p = 0.016) and was refractory in 58.8% of patients. Earlier seizure onset was associated with refractory epilepsy (p = 0.014). Patients with CLIFAHDD and premature death, epilepsy, or paroxysmal dystonia carried variants within NALCN pore domains. DISCUSSION: This study provides an in-depth clinical characterization of NALCN-related and UNC80-related disorders. Distal arthrogryposis, episodic ataxia, and paroxysmal dystonia were diagnosed in patients with CLIFAHDD while failure to thrive, CSA, and epilepsy were associated with LOF variants. We suggest potential genotype-phenotype associations, formulating hypotheses for validation in future studies with larger cohorts.