Cerebral Palsy via the GAD1 Gene

Cerebral palsy (CP) is the most common pediatric motor impairment and is currently estimated to affect approximately 1-3.5 in 1,000 children by the age of 8 years, depending on their ethnic background (Centers for Disease Control and Prevention https://www.cdc.gov/ ; Clark and Hankins 2003. PubMed ID: 12634632). It is considered a spectrum of permanent non-progressive neurodevelopmental disorders that affect the patients’ movement and/or posture with varied etiology. Injury to the movement control brain areas during antepartum, intrapartum or postpartum developmental stages, can manifest a heterogeneous spectrum of CP disorders (Moreno-de-Luca et al. 2012. PubMed ID: 22261432; Shevell 2019. PubMed ID: 30568002).

Diagnosis of CP typically occurs by the second year of life. The early clinical signs appear during infancy and can include inadequate primitive reflexes, poor motor coordination and muscle control, weak muscle development and communication, poor fine motor coordination, poor feeding and delayed motor milestones (Australian Cerebral Palsy Registry Birth years 1995-2012, November 2018; Novak et al. 2017. PubMed ID: 28715518).

CP can be divided into four major types according to location of the central nervous system injury:

(i) Spastic: occurs due to an insult in the motor region of the cerebral cortex, this damage causes muscle stiffness and movement difficulty. Its’ signs and symptoms include tremors, hypertonia, scissor gait (due to stiff and increased lower muscles tone) and limb muscle weakness. Spastic CP is the most common type and accounts for 80% of cases.

(ii) Dyskinetic (athetoid): associated with damage to the basal ganglia, which leads to involuntary and uncontrolled movement. Its’ signs and symptoms include bradykinesia (slow movement) and writhing movements.

(iii) Ataxic: occurs due to cerebellum injury and therefore the individual has abnormality of coordination and balance impairment. As a result, wide-based gait is demonstrated in ataxic CP patients.

(iv) Mixed: a combination of more than one type of CP. The most common is spastic-dyskinetic.  (Surveillance of cerebral palsy in Europe eu-rd-platform.jrc.ec.europa.eu)

CP can also be grouped based on motor type and topography into pyramidal and extrapyramidal forms. Pyramidal CP is voluntary and therefore involves the cerebral cortex and can be divided to hemiplegia, paraplegia and quadriplegia. In hemiplegia, half of the body on one side is affected due to unilateral cerebral cortex damage; usually the upper limb is more affected than lower. In paraplegia (also known as diplegia), stiffness is mainly in the lower half of the body; the upper half can be partially affected. In quadriplegia (also known as tetraplegia or ‘whole-body involvement’), there is bilateral upper and lower limb involvement. It is the most severe, and the patient will have scissor stands with all limbs stiff.

In extrapyramidal CP, the whole body is affected. Extrapyramidal CP includes ataxic extrapyramidal, where the patient is displaying wide-based gait, and dyskinetic extrapyramidal, where the patient will have uncontrolled movement of the four limbs and dystonia (Birth Injury Help Center www.birthinjuryhelpcenter.org; van Eyk et al. 2018. PubMed ID: 29325622; Moreno-De-Luca et al. 2012. PubMed ID: 22261432; Novak et al. 2012. PubMed ID: 23045562; Patel et al. 2020. PubMed ID: 32206590; Rosenbaum et al. 2007. PubMed ID: 17370477; Surveillance of Cerebral Palsy in Europe 2000. PubMed ID: 11132255; Velde et al. 2019. PubMed ID: 31623303).

Although the CP hallmark feature is motor and posture impairment, it is accompanied by other disorders of the central nervous system, including intellectual disability (30–65%), seizures (30–50%), speech and language deficits (40%), visual impairments (40%), hearing loss (5–15%), psychological problems (2%),  autism spectrum disorders (9%),  amongst other difficulties (Ashwal et al. 2004. PubMed ID: 15037681; Van Eyk et al. 2018. PubMed ID: 29325622; Moreno-De-Luca et al. 2012. PubMed ID: 22261432; Novak et al. 2012. PubMed ID: 23045562).

The factors that disrupt the fetus’s or infant’s developing brain and lead to CP include, genetic variants, preterm delivery, birth asphyxia, intrauterine infection and inflammation, intrauterine growth restriction, fetal stroke, multiple pregnancies, placental pathology, maternal thyroid disease, infant brain infection and infant head trauma (Van Eyk et al. 2018. PubMed ID: 29325622; MacLennan et al. 2015. PubMed ID: 26003063). Discoveries of the genetic contribution to CP has been growing over the last several years (Emrick and Dicarlo. 2020. PubMed ID: 31760988; Fahey et al. 2017. PubMed ID: 28042670). For instance, one study included a population of 681 patients in west Sweden, and estimated that 40% of etiologically undiagnosed CP cases are genetically caused (Costeff. 2004. PubMed ID: 15469428; Moreno-De-Luca et al. 2012. PubMed ID: 22261432). The mode of inheritance in most cases is autosomal recessive, but rare autosomal dominant and X-linked forms have been described (Van Eyk et al. 2018. PubMed ID: 29325622).

Benefits of genetic testing for CP:

- Assist in clinical diagnosis of CP, especially in challenging cases such as those with inconclusive and negative neuroimaging (MRI and CT scans) findings (Takezawa et al. 2018. PubMed ID: 29761117).

- Impact the prognosis and management of CP cases carrying genetic variants with known phenotypic consequence (Fahey et al. 2017. PubMed ID: 28042670; Moreno-De-Luca et al. 2012. PubMed ID: 22261432). 

- Identify or rule out conditions other than CP, and influence treatment options, where an alternative differential diagnosis for a disease of shared clinical features is determined (Nygaard et al. 1991. PubMed ID: 1899474).

-Reveal whether CP is due to an inherited or de novo variant, thus supporting informed reproductive decisions of family planning and proper management of ongoing pregnancies (Emrick and Dicarlo. 2020. PubMed ID: 31760988).  

GAD1 was the first gene to be linked to hereditary CP (Lynex et al. 2004. PubMed ID: 15571623). A rare missense variant (c.35C>G; p.Ser12Cys) was identified in a family with an autosomal recessive mode of inheritance. The studied family was a consanguineous Pakistani family with four affected siblings with spastic quadriplegic CP. The siblings were reported to have ataxia of the upper limbs and severe intellectual disability or developmental delay (Lynex et al. 2004. PubMed ID: 15571623).

A multicenter cohort study in Saudi Arabia identified a novel homozygous missense variant (c.1402T>G; p.Trp468Gly) in a proband suffering from spastic quadriplegic CP, with the following additional clinical phenotype: dysmorphic features, hypotonia, severe developmental delay and myoclonic seizures (Alfares et al. 2017. PubMed ID: 28454995).

GAD1 encodes the glutamate decarboxylase 1 enzyme, which catalyzes the production of gamma aminobutyric acid (GABA) neurotransmitter from glutamic acid (Bu et al. 1992. PubMed ID: 1549570). GAD1 is a tissue specific gene, with enriched expression in the brain (The Human Protein Atlas www.proteinatlas.org). Disease related variants in this gene are extremely rare with less than 10 reported families, all demonstrating autosomal recessive inheritance (Calvo et al. 2010. PubMed ID: 20818383; Carraro et al. 2019. PubMed ID: 31144778; Giacopuzzi et al. 2017. PubMed ID: 28787007; Lynex et al. 2004. PubMed ID: 15571623; Magri et al. 2018. PubMed ID: 30341396).

Pathogenic variants in GAD1 have also been reported in other neurological disease cohort studies, including a single patient with schizophrenia, a family with a history of an unspecified neurodevelopmental disorder, and two patients with isolated mitochondrial complex I deficiency (Calvo et al. 2010. PubMed ID: 20818383; Carraro et al. 2019. PubMed ID: 31144778; Giacopuzzi et al. 2017. PubMed ID: 28787007; Magri et al. 2018. PubMed ID: 30341396). GAD1 is relatively intolerant to both missense and loss of function variants (gnomAD).

In a Gad1 conditional knockout mouse model, GABA synthesis selectively reduced in the direct output pathway from the basal ganglia striatum, due to the partial loss of Gad1. The mice demonstrated some behavioral phenotypes, including clasping behavior and decreased motor coordination (Heusner et al. 2008. PubMed ID: 18615733). GAD1 has been cited as a nonessential gene for growth of human tissue culture cells (Online Gene Essentiality, ogee.medgenius.info).

To date, pathogenic variants in GAD1 have been reported in just a few patients diagnosed with CP. Therefore, although the clinical sensitivity of this test is unknown, it is expected to be low given the clinical and genetic heterogeneity of CP.

This test is performed using Next-Gen sequencing with additional Sanger sequencing as necessary.

This test provides full coverage of all coding exons of the GAD1 gene plus 10 bases flanking noncoding DNA in all available transcripts in addition to non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as ≥20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).