Tau-tubulin kinase 1 (TTBK1) is a neuron-enriched kinase implicated in τ phosphorylation and neurodegeneration. Human phenotypes associated with constitutional TTBK1 variants remain undefined. Two siblings with a severe neurodevelopmental phenotype were assessed using quartet exome sequencing, segregation analysis and standardised clinical and neuroimaging evaluations. Both children exhibited profound global developmental delay, non-ambulation, axial hypotonia with lower-limb spasticity and proportionate postnatal growth failure with microcephaly. Epilepsy was present in the older sibling. Brain MRI showed a thin brainstem and corpus callosum, periventricular T2 hyperintensities and mild cerebellar atrophy in the older sibling and external hydrocephalus in the younger. Exome sequencing identified a homozygous frameshift variant in TTBK1 (NM_032538.3:c.1899del; p.Thr634Argfs*39) that segregated with the disease. The variant was absent from population databases and predicted to cause loss-of-function. According to the American College of Medical Genetics and Genomics criteria, it fulfils PVS1 and PM2_P. We report the two siblings with a neurodevelopmental disorder due to a biallelic TTBK1 loss-of-function variant, establishing TTBK1 as critical for human neurodevelopment. Together with preclinical data, these findings underscore its role in motor and cognitive circuits. Additional cases and functional studies will be essential to delineate the clinical spectrum and mechanistic basis of TTBK1 deficiency. FOXG1 syndrome is characterized by moderate-to-profound developmental delay and intellectual disability, postnatal growth deficiency, congenital or postnatal microcephaly, hyperkinetic/dyskinetic movement disorder, hypotonia, neurobehavioral/psychiatric manifestations (motor stereotypies, impairment of social interaction, abnormal sleep patterns, unexplained episodes of crying, restlessness, and bruxism), feeding difficulties with poor weight gain, strabismus, seizures, spasticity, gastroesophageal reflux, and aspiration. Some individuals have cortical visual impairment, kyphosis, scoliosis, and/or abnormal breathing. Characteristic neuroimaging findings include corpus callosum anomalies (especially a marked, filiform thinning of the rostrum of the corpus callosum), a simplified gyral pattern, and hyperplasia of the fornices. The diagnosis of FOXG1 syndrome is established in a proband with clinical and/or characteristic neuroimaging findings and a heterozygous pathogenic variant in FOXG1 identified by molecular genetic testing. Treatment of manifestations: Developmental and educational support; consideration of anti-dyskinetic pharmacotherapy; treatment for seizures by an experienced neurologist; treatment of spasticity per orthopedist; physical medicine and rehabilitation, physical therapy, and occupational therapy to help avoid contractures and falls; anti-spasmodic pharmacotherapy; feeding therapy with gastrostomy tube placement as needed; standard treatment of gastroesophageal reflux; treatment for refractive errors and strabismus per ophthalmologist; standard treatments for scoliosis; social work and family support. Surveillance: At each visit, monitor developmental progress, educational needs, seizures, changes in tone, movement disorders, growth, nutritional status, and safety of oral intake; behavioral assessment for irritability and sleep issues; assess for evidence of gastroesophageal reflux, aspiration, and/or respiratory insufficiency; physical medicine, occupational therapy, physical therapy assessment for mobility and self-help skills; monitor for strabismus and need for low vision services per treating ophthalmologist; assess family needs. FOXG1 syndrome is an autosomal dominant disorder typically caused by a de novo pathogenic variant. Risk to future pregnancies is presumed to be low as the proband most likely has a de novo FOXG1 pathogenic variant. There is, however, a recurrence risk to sibs based on the possibility of parental germline mosaicism. Given this risk, prenatal and preimplantation genetic testing may be considered.

Bi-allelic loss-of-function variants in genes that encode subunits of the adaptor protein complex 4 (AP-4) lead to prototypical yet poorly understood forms of childhood-onset and complex hereditary spastic paraplegia: SPG47 (AP4B1), SPG50 (AP4M1), SPG51 (AP4E1) and SPG52 (AP4S1). Here, we report a detailed cross-sectional analysis of clinical, imaging and molecular data of 156 patients from 101 families. Enrolled patients were of diverse ethnic backgrounds and covered a wide age range (1.0-49.3 years). While the mean age at symptom onset was 0.8 ± 0.6 years [standard deviation (SD), range 0.2-5.0], the mean age at diagnosis was 10.2 ± 8.5 years (SD, range 0.1-46.3). We define a set of core features: early-onset developmental delay with delayed motor milestones and significant speech delay (50% non-verbal); intellectual disability in the moderate to severe range; mild hypotonia in infancy followed by spastic diplegia (mean age: 8.4 ± 5.1 years, SD) and later tetraplegia (mean age: 16.1 ± 9.8 years, SD); postnatal microcephaly (83%); foot deformities (69%); and epilepsy (66%) that is intractable in a subset. At last follow-up, 36% ambulated with assistance (mean age: 8.9 ± 6.4 years, SD) and 54% were wheelchair-dependent (mean age: 13.4 ± 9.8 years, SD). Episodes of stereotypic laughing, possibly consistent with a pseudobulbar affect, were found in 56% of patients. Key features on neuroimaging include a thin corpus callosum (90%), ventriculomegaly (65%) often with colpocephaly, and periventricular white-matter signal abnormalities (68%). Iron deposition and polymicrogyria were found in a subset of patients. AP4B1-associated SPG47 and AP4M1-associated SPG50 accounted for the majority of cases. About two-thirds of patients were born to consanguineous parents, and 82% carried homozygous variants. Over 70 unique variants were present, the majority of which are frameshift or nonsense mutations. To track disease progression across the age spectrum, we defined the relationship between disease severity as measured by several rating scales and disease duration. We found that the presence of epilepsy, which manifested before the age of 3 years in the majority of patients, was associated with worse motor outcomes. Exploring genotype-phenotype correlations, we found that disease severity and major phenotypes were equally distributed among the four subtypes, establishing that SPG47, SPG50, SPG51 and SPG52 share a common phenotype, an 'AP-4 deficiency syndrome'. By delineating the core clinical, imaging, and molecular features of AP-4-associated hereditary spastic paraplegia across the age spectrum our results will facilitate early diagnosis, enable counselling and anticipatory guidance of affected families and help define endpoints for future interventional trials.

Micro and Martsolf syndromes are rare clinically and genetically overlapping disorders caused by mutations in RAB3GAP1, RAB3GAP2, RAB18 and TBC1D20 genes. We describe 34 new patients, 27 with Micro and seven with Martsolf. Patients presented with the characteristic clinical manifestations of the two syndromes, including postnatal microcephaly, congenital cataracts, microphthalmia, optic atrophy, spasticity and intellectual disability. Brain imaging showed in the majority of cases polymicrogyria, thin corpus callosum, cortical atrophy, and white matter dysmyelination. Unusual additional findings were pectus excavatum (four patients), pectus carinatum (three patients), congenital heart disease (three patients) and bilateral calcification in basal ganglia (one patient). Mutational analysis of RAB3GAP1 and RAB3GAP2 revealed 21 mutations, including 14 novel variants. RAB3GAP1 mutations were identified in 22 patients with Micro, including a deletion of the entire gene in one patient. On the other hand, RAB3GAP2 mutations were identified in two patients with Micro and all Martsolf patients. Moreover, exome sequencing unraveled a TBC1D20 mutation in an additional family with Micro syndrome. Our results expand the phenotypic and mutational spectrum associated with Micro and Martsolf syndromes. Due to the overlapped severities and genetic basis of both syndromes, we suggest to be comprehended as one entity "Micro/Martsolf spectrum" or "RAB18 deficiency."