Early onset or syndromic epilepsy
Gene: CACNA2D1
Not associated with a phenotype in OMIM, Gen2Phen or MONDO. At least four variants have been reported in three unrelated cases, epilepsy was observed in two of these cases (PMID: 35293990).Created: 14 Apr 2022, 2:53 p.m. | Last Modified: 14 Apr 2022, 2:53 p.m.
Panel Version: 2.517
Consider inclusion in the current panel with amber rating.
Recent report of 2 unrelated individuals with DEE due to biallelic CACNA2D1 variants. Both referred to neurology/genetics for hypotonia/severe DD prior to onset of seizures.
One further individual with hypotonia and severe ID (seizures not discussed, age unknown).
Gene with established role, encoding α2δ-1 subunit of Cav channels. Studies for the variants support loss-of-function as the underlying effect.
Eventual contribution of monoallelic variants to NDD-phenotypes discussed (and put in question) in Ref [1] below.
There is currently no phenotype for CACNA2D1 in OMIM/G2P. In SysID this gene is listed among the candidates for ID, based on a previous report. CACNA2D1 is not currently included in the ID/epilepsy panels in PanelApp Australia.
See also relevant review by Dr. H. Lord.
Please consider also inclusion in other panels (e.g. microcephaly, corpus callosum, movement disorders, etc).
[1] ----
Dahimene et al (2022 - PMID: 35293990) describe the phenotype of 2 unrelated individuals with biallelic CACNA2D1 variants.
Overall, the phenotype corresponded to an early-onset DEE, characterized by abnormal muscle tone (axial hypotonia 2/2 with spasticity in extremities in 2/2), feeding difficulties (2/2), profound DD and ID (2/2), microcephaly (2/2 - approx. -2 SD in both), seizures (2/2 - 1st : onset 9m with absences and later generalized seizures, 2nd : onset 11m with hemi-clonic seizures and atypical absences). Other features included cortical visual impairment (2/2) and movement disorder (incl. choreiform movements 2/2, orofacial dyskinesia 2/2 and dystonic episodes 1/2). Brain MRI revealed corpus callosum anomalies (2/2) and cerebral atrophy (2/2). Both had echocardiography (abnormal in 1/2 - tiny PFO) and electrocardiography which was normal. Both exhibited insensibility to pain.
Presentation is relevant to the current panel as first symptoms in the first 3 months with severe hypotonia and poor head control (2/2) with evaluation in neurology/genetics preceding onset of seizures in both.
Trio ES was performed for both individuals and their (healthy) parents and revealed homozygosity for a fs variant in the first [NM_000722.3:c.818_822dup / p.(Ser275Asnfs*13)] and compound htz for a fs and a missense variant [c.13_23dup / p.(Leu9Alafs*5) and c.626G>A / p.(Gly209Asp)] in the second affected individual, respectively.
Eventual additional variants were not discussed.
Previous investigations are only provided for the 2nd and were all normal (karyotype, CMA, 15q methylation, epilepsy/neurometabolic gene panels).
Voltage-gated calcium channels are heteromultimers comprising different subunits incl. an alpha-1 (α1), α2δ (alpha-2/delta), beta (β) and gamma (γ). CACNA2D1 is one of the 4 genes (CACNA2D1-4) encoding the alpha-2/delta subunit. Its product is post-translationally processed into 2 peptides, an alpha-2 and a delta subunit, held by a disulfide bond.
Biallelic variants in CACNA2D2 - also encoding an alpha-2/delta subunit - cause cerebellar atrophy with seizures and variable developmental delay (# 618501).
Variant studies support loss-of-function effect for the studied variants, notably by NMD for the fs one, and severe impairment of the Cav2 channel function for the missense one :
- CACNA2D1 mRNA was reduced to 6-9% compared with control in fibroblasts from the 1st individual. mRNA levels for the 2nd subject were similar to control.
- Quantification of the protein in whole-cell lysates from fibroblasts revealed lower α2δ levels compared to control (10-12% and 31-38% applying to the 1st and 2nd individual).
- CACNA2D3 mRNA levels in fibroblasts from the 2nd patient were 2-7x higher compared to the 1st or controls suggesting a possible compensatory effect. CACNA2D2/4 mRNA levels were too low for quantification.
- Gly209 lies within the gabapentin and amino-acid binding pocket and this residue is invariable in CACNA2D1/CACNA2D2 in all vertebrates and paralogs.
- Transfection of tsA-201 cells with either WT or G209D HA-tagged α2δ revealed reduced cell surface expression for this missense variant (~80, for biotinylated form ~86%).
- In tsA-201 cells transfected with HA-tagged Cav2.2/β1b and either α2δ-1-WT, no α2δ-1 or α2δ-1-G209D, WT resulted in increased 13x currents with no increase applying to G209D (or in absence of α2δ). Plasma membrane expression of double (GFP/HA) tagged Cav2.2 was increased upon co-expression with WT α2δ-1 which was not the case for α2δ-1-G209D.
- In hippocampal neurons, double (GFP/HA)-tagged Cav2.2 could not be detected at the cell surface in the presence of α2δ-1-G209D (or no α2δ) in contrast with strong expression in presence of α2δ-1-WT. α2δ-1-G209D did not promote trafficking of Cav2.2 into hippocampal neurites, as indicated by reduced signals for both HA and GFP (for cell surface and total Cav2.2 respectively).
- Co-expression of double (GFP/HA) tagged Cav2.2 with β1b and either HA-α2δ-1-WT or HA-α2δ-1-G209D in tsA-201 cells, revealed reduced complex formation of G209D with Cav2.2 Co-immunoprecipitated HA-α2δ-1-G209D had higher molecular weight compared to HA-α2δ-1-WT which suggests that α2δ-1-G209D remains as the uncleaved immature form (probably in the ER).
Mouse model (several Refs in text):
Mild cardiac phenotype and reduced ventricular myocyte Ca current density was observed in hmz ko mice. Similarly to the insensibility to pain human phenotype, mice had delayed neuropathic pain-related responses. Overexpression of a2δ-1 resulted in epileptiform EEG and behavioral arrest, overall supporting a critical role of α2δ-1 for mouse brain.
The authors underscore that the parents of both patients (htz carriers) were healthy and review previous literature for association of monoallelic variants with epilepsy, ID and arrhythmogenic disorders (in suppl.) [Refs not here reviewed].
As for the NDD phenotype, CACNA2D1 is within a previously defined small region of overlap for 7q21.11 microdeletions associated with ID+/-epilepsy. The same study did not reveal de novo SNVs in any of the 3 contained genes within this SRO (HGF, CACNA2D1, PCLO) in 4293 patients with NDD [cited PMID: 28240412]. A frameshift variant (c.2625del) was identified in a 13-yo girl with infantile spasms and normal intelligence [cited PMID: 25877686]. A 1-bp insertion (c.659-2_659-1insT / not studied at the mRNA level) was identified in another 14-yo female with ID and epilepsy [cited PMID: 34356170]. The authors state that the phenotype (/differences) of these individuals as well as presence of pLoF CACNA2D1 variants in gnomAD [still pLI of 1] put in question pathogenicity of monoallelic variants for these phenotypes.
The role of heterozygous missense variants described in relation to arrhythmogenic disorders is also discussed extensively (some downgraded to LB/VUS, others having a relatively high MAF and presence of 1-2 homozygotes in gnomAD).
[2] ----
In an article cited by SysID for CACNA2D1 (2017 - PMID: 28097321), Reuter et al studied with WES and autozygosity mapping individuals with NDD belonging to consanguineous families.
As in eTables1/3, a male - single affected individual born to consanguineous parents from Turkey (MR150) - was investigated by singleton ES.
This individual was homozygous for a missense CACNA2D1 SNV [NM_000722.2:c.1514C>T;p.(Thr505Ile)].
Prior investigations are unavailable (although individuals with previously known P/LP CNVs were excluded).
The phenotype - briefly reported - included hypotonia, severe ID, stereotypic behaviors, inguinal hernia and omphalocele. Presence of seizures was not commented on. The age of this individual was not reported.Created: 27 Mar 2022, 9:54 a.m. | Last Modified: 27 Mar 2022, 9:54 a.m.
Panel Version: 2.503
Mode of inheritance
BIALLELIC, autosomal or pseudoautosomal
Phenotypes
Abnormal muscle tone; Feeding difficulties; Global developmental delay; Intellectual disability; Seizures; Microcephaly; Abnormality of the corpus callosum; Cerebral atrophy; Abnormality of movement; Cortical visual impairment; Pain insensitivity
Publications
Dahimne et al, 2022, 35293990 - two unrelated patients with biallelic CACNA2D1 variants - trio exome sequencing - identified by gene matcher.
Patient 1: homozygous fs variant c.818_821dup p.(Ser275fs) - healthy parents het carriers.
Patient 2: compound het for a fs c.13_23dup p.(Leu9fs) and a missense variant c.626G>A p.(Gly209Asp) - parents healthy carriers of each of the variants.
Both individuals show a highly consistent phenotype, corresponding to DEE - microcephalic, severe hypotonia, absent speech, spasticity, choreiform movements, orofacial dyskinesia and cortical visual impairement. Brain imaging showed corpus callosum hypoplasia and progressive volume loss in both. Both patients also showed insensitivity to pain.
Looking at mRNA level in patient fibroblasts in both patients showed it was reduced to 6-9% compared to control fibroblasts. Also data suggest the fs variants result in LOF.
Gly209 - this Gly is important for maintaining a 3-strand-beta-sheet-stability and simultaneously providing a critial turn in the structure and is absoloutely invariant in both CACNA2D1 and CACNA2D2 orthologs in all vertebrates and paralogs and predecessors from low invertebrates.
Functional work on the missense change shows it disrupts plasma membrane a2d-1 expression by 86.2% compared to wt. It abolishes the ability of a2d-1 to promote Cav2.2 currents, it does not promote Cav2.2 cell surface expression or trafficking in hippocampal neurons and it shows reduced complex formation with Cav2.2 and limited proteolytic cleavage. Suggests the HA-a2b-1 with the G209D variant remains largely as the uncleaved imaature form, indicating that it probably remains in the endoplasmic reticulum - suggests LOF effect.
Sources: NHS GMSCreated: 23 Mar 2022, 11:34 a.m. | Last Modified: 23 Mar 2022, 11:36 a.m.
Panel Version: 2.500
Mode of inheritance
BIALLELIC, autosomal or pseudoautosomal
Phenotypes
Early onset developmental epilepsy
Publications
Gene: cacna2d1 has been classified as Amber List (Moderate Evidence).
Publications for gene: CACNA2D1 were set to 35293990
gene: CACNA2D1 was added gene: CACNA2D1 was added to Genetic epilepsy syndromes. Sources: NHS GMS Mode of inheritance for gene: CACNA2D1 was set to BIALLELIC, autosomal or pseudoautosomal Publications for gene: CACNA2D1 were set to 35293990 Phenotypes for gene: CACNA2D1 were set to Early onset developmental epilepsy Review for gene: CACNA2D1 was set to AMBER