Intellectual disability - microarray and sequencing
Gene: ADD1
The rating of this gene has been updated to Green and the mode of inheritance set to 'BOTH monoallelic and biallelic, autosomal or pseudoautosomal' following NHS Genomic Medicine Service approval.Created: 30 Jan 2023, 5:50 p.m. | Last Modified: 30 Jan 2023, 5:50 p.m.
Panel Version: 4.53
New gene added by Konstantinos Varvagiannis. There is sufficient evidence to promote to Green at the next GMS panel update.
The ADD1 gene is associated with alternative phenotype in OMIM. Rated as Green on PanelApp Australia.
Qi et al (2022 - PMID: 34906466) describe the phenotype of 3 unrelated individuals with monoallelic de novo ADD1 pathogenic variants as well as of a fourth homozygous for a missense SNV (segregation data supplied).Sufficient families combined with mouse model to classify as Green.Created: 2 Aug 2022, 2:21 p.m. | Last Modified: 2 Aug 2022, 2:21 p.m.
Panel Version: 3.1644
A recent study suggests an ADD1-related phenotype (3 subjects with monoallelic de novo variants/1 with biallelic variants) with DD/ID and ventriculomegaly or corpus callosum dysgenesis and possibly seizures among the features.
There is currently no associated phenotype in other databases (OMIM, G2P, SysID, PanelApp Australia).
Consider inclusion in the current panel with amber / green rating (3 subjects/variants/families, role of the gene and mouse models recapitulating ventriculomegaly/CC abnormalities, relevant expression, variant studies demonstrating abn. protein levels and/or disruption of adducin heterodimer formation || monoallelic vs bi-allelic variants).
Please consider inclusion in other possibly relevant gene panels (e.g. for corpus callosum / ventriculomegaly) [ Not added ].
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Qi et al (2022 - PMID: 34906466) describe the phenotype of 3 unrelated individuals with monoallelic de novo ADD1 pathogenic variants as well as of a fourth homozygous for a missense SNV.
Overall, the authors propose a common phenotype consisting of morphological brain abnormalities (incl. ventriculomegaly and corpus callosum dysgenesis) and neurological symptoms such as DD and/or ID and attention deficit.
All individuals were investigated with singleton/trio ES.
De novo variants - phenotype:
One individual investigated for hypotonia, DD & ID, partial ACC, well controlled seizures (on ketogenic diet) and proportional short stature harbored a de novo stopgain variant (NM_014189.3:c.1418G>A / p.Trp473*) absent from gnomAD.
Another affected subject with hypotonia, FTT/feeding difficulties, mild motor delays complete ACC, a seizure (2y11m), staring spells without EEG correlate, and fatigue (with low coenz. Q10, and complex I & IV deficiency in muscle biopsy) had a de novo fs variant (NM_001119:c.2029_2039del / p.Glu680Argfs*7 - gnomAD:0) and a VUS in a gene not associated with phenotype to date.
A 3rd subject investigated for seizures (onset:1y), speech delay, mild ID, ADHD, without MRI abnormalities harbored a de novo missense SNV (NM_001119:c.670C>T / p.His224Tyr - gnomAD:0) and with cmp htz for 2 missense SPTBN2 SNV not fitting the phenotype (no ataxia).
Biallelic variants - phenotype:
One individual with ID, and ACC, abnormal sulcation, enlarged lateral and 3rd ventricles, abnormal of white matter and hypoplastic vermis upon MRI was reported to harbor in homozygosity a missense SNV (NM_001119:c.169A>T / p.Arg57Trp). There was an additional variant in a gene without associated phenotype to date and not expressed in brain.
Role of the encoded protein:
ADD1 encodes adducin 1/alpha (similar to ADD2, ADD3 encoding other adducins). As the authors note, adducins are cytoskeleton proteins critical for osmotic rigidity and cell shape. In neurons they have been reported to form membrane associated periodic ring-like structures with actin and β-spectrin. Deletion of Add1 in mice results in increased MPS ring diameter and axonal degeneration (several refs provided).
ADD1/2/3 form heterodimers which in turn form heterotetramers. ADD1 is expressed in most tissues.
Mouse model:
Previous mouse models have demonstrated that Add1 null mice have also undetectable ADD2/3 (suggesting a role for stabilization of the latter) and exhibit growth delay, anemia and develop lethal hydrocephalus and ventriculomegaly with 50% penetrance (cited PMIDs: 27068466, 18723693). Here the authors demonstrated that surviving mice had ventriculomegaly and thinning of corpus callosum thus recapitulating the respective human phenotypes. Htz mice also presented thinner CC, though not to a statistically significant extent.
ADD1 expression and isoforms:
- Performing mRNA studies and W.Blot in (developing - GW15-17) human or mouse brain (E12.5-P40) the authors demonstrated dynamic expression of ADD1 with differentially expressed isoforms, notably alternative splicing of ex10 and ex15 with NM_176801 (extended ex10, inclusion of ex15) corresponding to a neuronal isoform and NM_001119 (shorter ex10, exclusion of ex15) corresponding to a neural progenitor cell (NPC) isoform.
- Variants here reported appear to affect both isoforms with the exception of NM_001119:c.2029_2039del / p.Glu680Argfs*7 affecting only the longer NPC one.
- PTBP1 is an RNA binding protein expressed in NPCs known to suppress neuronal exon insertion. The authors demonstrated in mouse Neuro2A cells, through shRNA targeting of Ptbp1, that the latter suppresses the neuronal Add1 isoform.
Variant studies demonstrated that effect of variants was mediated by decreased protein levels and/or disruption of adducin complex formation (ADD1-ADD2 dimer formation known to be mediated by N- and C- terminal ADD1 domains):
- Expression of Arg57Trp (found in hmz in one individual) NPC and neuronal isoforms in Neuro2a cells showed that while protein levels were not significantly affected, there were (also) truncated protein products for both isoforms suggesting that aberrant splicing or protein translation/cleavage may apply.
- The authors generated HEK293FT cells for the truncating variants demonstrating decreased protein levels (using N-/C- terminal antibodies).
- Reduced (HA-tagged)-ADD1-(V5-tagged)-ADD2 protein interaction was shown to apply for the Arg57Trp and Arg473* in HEK293FT cells. Similarly in Neuro2a cells, reduced ADD1-ADD2 interaction was shown for His224Tyr.
Sources: LiteratureCreated: 10 May 2022, 10:27 a.m.
Mode of inheritance
BOTH monoallelic and biallelic, autosomal or pseudoautosomal
Phenotypes
Global developmental delay; Intellectual disability; Seizures; Ventriculomegaly; Abnormality of the corpus callosum
Publications
Tag Q3_22_rating was removed from gene: ADD1.
Source NHS GMS was added to ADD1. Source Expert Review Green was added to ADD1. Rating Changed from Amber List (moderate evidence) to Green List (high evidence)
Gene: add1 has been classified as Amber List (Moderate Evidence).
Gene: add1 has been classified as Amber List (Moderate Evidence).
Tag Q3_22_rating tag was added to gene: ADD1.
gene: ADD1 was added gene: ADD1 was added to Intellectual disability. Sources: Literature Mode of inheritance for gene: ADD1 was set to BOTH monoallelic and biallelic, autosomal or pseudoautosomal Publications for gene: ADD1 were set to 34906466 Phenotypes for gene: ADD1 were set to Global developmental delay; Intellectual disability; Seizures; Ventriculomegaly; Abnormality of the corpus callosum Penetrance for gene: ADD1 were set to unknown Review for gene: ADD1 was set to AMBER