Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the TRAPPC9 gene.

Autosomal recessive spinocerebellar ataxia-17 (SCAR17) is a neurologic disorder characterized by onset of gait ataxia and cerebellar signs in early childhood. Patients also have variably impaired intellectual development (summary by Evers et al., 2016).

GRIN2B-related neurodevelopmental disorder is characterized by mild to profound developmental delay / intellectual disability (DD/ID) in all affected individuals. Muscle tone abnormalities (spasticity and/or hypotonia, occasionally associated with feeding difficulties), as well as epilepsy and autism spectrum disorder (ASD) / behavioral issues, are common. Other infantile- or childhood-onset findings include microcephaly; dystonic, dyskinetic, or choreiform movement disorder; and/or cortical visual impairment. Brain MRI reveals a malformation of cortical development in a minority of affected individuals. To date, fewer than 100 individuals with GRIN2B-related neurodevelopmental disorder have been reported.

CATIFA syndrome is characterized by global developmental delay and impaired intellectual development ranging from mild to severe, with most patients exhibiting attention-deficit hyperactivity disorder (ADHD). Patients show an elongated face with long philtrum and small ears. Ocular anomalies include congenital cataracts, strabismus, and amblyopia, which may be associated with reduced vision; other anomalies include cleft lip and/or palate and misaligned teeth with extensive caries (Unlu et al., 2020).

Lissencephaly-10 (LIS10) is a neurologic disorder characterized by variably delayed development with mildly to moderately impaired intellectual development and language delay, as well as seizures, which are often intractable. There is a spectrum of severity, with some patients having normal early development and only borderline to mild cognitive impairment. Brain imaging shows features consistent with neuronal migration defects, including posterior-predominant lissencephaly, pachygyria, agyria, and subcortical band heterotopia (summary by Tsai et al., 2020). For a general description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

Silver-Russell Syndrome (SRS) is typically characterized by asymmetric gestational growth restriction resulting in affected individuals being born small for gestational age, with relative macrocephaly at birth (head circumference =1.5 SD above birth weight and/or length), prominent forehead usually with frontal bossing, and frequently body asymmetry. This is followed by postnatal growth failure, and in some cases progressive limb length discrepancy and feeding difficulties. Additional clinical features include triangular facies, fifth-finger clinodactyly, and micrognathia with narrow chin. Except for the limb length asymmetry, the growth failure is proportionate and head growth normal. The average adult height in untreated individuals is ~3.1±1.4 SD below the mean. The Netchine-Harbison Clinical Scoring System (NH-CSS) is a sensitive diagnostic scoring system. Clinical diagnosis can be established in an individual who meets at least four of the NH-CSS clinical criteria – prominent forehead/frontal bossing and relative macrocephaly at birth plus two additional findings – and in whom other disorders have been ruled out.