Early onset or syndromic epilepsy

Gene: AFF3

As a result of watchlist tag audit the watchlist tag was removed from AFF3- this is now a green gene. Preprint information is now available Voisin et al, 2019 https://www.biorxiv.org/content/10.1101/693937v1

Created: 13 Jan 2020, 1:21 p.m. | Last Modified: 13 Jan 2020, 1:25 p.m.

Panel Version: 2.0

Green List (high evidence)

Voisin et al, 2019 (preprint) - New AD disorder associated with de novo missense variants in the degron of AFF3 - a 9 AA seq important for its degredation. 10 unrelated aff individuals - de novo missense variants in the ALF of AFF3. The 4 identified variants are not in gnomAD - A258S (probands 1 & 2), A258T (probands 3-8), A258V (proband 9 & V260G (proband 10). In addition an 11 th individual had a 500kb del and an overlapping phenotype. This deletion removes exons 4-13 and is proposed to act as a dom neg. 10/11 had epilepsy. Knock out and knock in mouse studies done.

Created: 5 Sep 2019, 2:22 p.m. | Last Modified: 5 Sep 2019, 2:22 p.m.

Panel Version: 1.261

Mode of inheritance
MONOALLELIC, autosomal or pseudoautosomal, imprinted status unknown

I don't know

3 DM variants on HGMDPro, but not associated with epilepsy phenotype. No apparent association with epilepsy/seizures on OMIM; however, this may change in the future with the publications listed in the reviews below.

Created: 23 Aug 2019, 10:14 a.m. | Last Modified: 23 Aug 2019, 10:14 a.m.

Panel Version: 1.256

I don't know

As discussed with members of the GMS Neurology Specialist Test Group on the Webex call 22nd November 2019 for Clinical Indication R59 Early onset or syndromic epilepsy: Agreed that there is enough evidence to rate this gene Green: Sufficient evidence in pre-print Voisin et al., 2019 article. Promoted AFF3 from Amber to Green.

Created: 25 Nov 2019, 8:14 p.m. | Last Modified: 25 Nov 2019, 8:14 p.m.

Panel Version: 1.425

Review and rating collated by Helen Lord (Oxford University Hospitals NHS Foundation Trust, 2019_08_30) on behalf of West Midlands, Oxford and Wessex GLH for GMS Neurology specialist test group. This gene was added to the Genetic epilepsy syndromes panel after the initial panel was reviewed by West Midlands, Oxford and Wessex GLH: this gene was therefore reviewed following the group Webex call on 2019_08_08 for Clinical Indication R59 Early onset or syndromic epilepsy.

Created: 5 Sep 2019, 2:26 p.m. | Last Modified: 5 Sep 2019, 2:26 p.m.

Panel Version: 1.262

As noted by Konstantinos Varvagiannis, Voisin et al., 2019 (not yet in PubMed) describe de novo missense variants in the degron of AFF3 (a region required for its degradation) in 10 unrelated individuals with symptoms including seizures. 4 different missense variants were identified (p.A258S, p.A258T, p.A258V and p.V260G). Although there are sufficient cases with a relevant phenotype, I have rated as Amber pending publication of the 2019 article: as OMIM note in their correspondance on AFF3, information changes from the initial bioRxiv upload to peer-reviewed publication. Therefore added 'watchlist' tag (in addition to missense tag), and will re-curate when the paper is published.

Created: 8 Aug 2019, 7:48 a.m. | Last Modified: 8 Aug 2019, 7:48 a.m.

Panel Version: 1.195

Green List (high evidence)

Voisin et al. (2019 - https://doi.org/10.1101/693937) report on 10 individuals with de novo missense AFF3 variants affecting a 9-amino-acid sequence (degron) important for the protein's degradation and summarize the phenotype of an additional individual previously described by Steichen-Gersdorf et al. (2008 - PMID: 18616733) with a 500 kb deletion affecting only AFF3 (LAF4) and removing also this sequence.

The phenotype of missense variants consisted of kidney anomalies, mesomelic dysplasia, seizures, hypertrichosis, intellectual disability and pulmonary problems and was overlapping with that of the deletion. [10 of 11 subjects exhibited severe developmental epileptic encephalopathy].

9 probands harbored missense variants affecting the codon 258 while one individual had a variant affecting codon 260 [c.772G>T or p.Ala258Ser (x2), c.772G>A or p.Ala258Thr (x6), c.773C>T or p.Ala258Val (x1) and c.779T>G or p.(Val260Gly) (x1) - NM_001025108.1 / NP_001020279.1]. The deletion removed exons 4-13.

AFF1-4 are ALF transcription factor paralogs, components of the transcriptional super elongation complex regulating expression of genes involved in neurogenesis and development.

Using HEK293T cells expressing FLAG-tagged AFF3 (and AFF4) wt or mutants, accumulation of mutated forms was shown upon immunoblot.

Aff3+/- and/or -/- mice exhibit skeletal defects. These were more pronounced in homozygous mice which demonstrated also some elements in favor of kidney dysfunction and/or metabolic deregulation and possible neurological dysfunction (signs of impaired hearing and diminished grip strength). Homozygous mice had CNS anomalies (enlarged lateral ventricles and decreased corpus callosum size) similar to some affected individuals, although these were not observed in another Aff3-/- model. Knock-in mice modeling the microdeletion and the Ala258Thr variant displayed lower mesomelic limb deformities and early lethality respectively [cited PMIDs : 21677750, 25660031, knock-in model was part of the present study].

Accumulation of the protein in zebrafish (by overexpression of the human wt AFF3 mRNA), led to morphological defects.

Reanalysis of transcriptome data from previously generated HEK293T cell lines knocked down for AFF2, AFF3 and AFF4 by shRNAs (study) suggested that these transcription factors are not redundant.

Finally, CHOPS syndrome (#616368) due to mutations of AFF4 also leading to increased protein stability presents a partially overlapping phenotype (incl. cognitive impairment) to that of AFF3.
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In G2P, AFF3 is associated with Skeletal dysplasia with severe neurological disease (disease confidence : probable / ID and seizures among the assigned phenotypes). There is no associated phenotype in OMIM.
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As a result this gene can be considered for inclusion in the epilepsy panel as green (relevant phenotype and severity, sufficient cases, evidence for accumulation similar to AFF4, animal models, etc) or amber (pending publication of the article).
Sources: Literature

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Shimizu et al. (8/2019 - PMID: 31388108) describe an additional individual with de novo AFF3 missense variant. The phenotype overlaps with that summarized by Voisin et al. incl. mesomelic dysplasia with additional skeletal anomalies, bilateral kidney hypoplasia and severe DD at the age of 2.5 years. Seizures and pulmonary problems were not observed. Although a different RefSeq is used the variant is among those also reported by Voisin et al. [NM_002285.2:c.697G>A (p.Ala233Thr) corresponding to NM_001025108.1:c.772G>A (p.Ala258Thr)].

Created: 7 Aug 2019, 5:33 p.m. | Last Modified: 10 Dec 2019, 1:03 p.m.

Panel Version: 2.0

Mode of inheritance
MONOALLELIC, autosomal or pseudoautosomal, imprinted status unknown

Phenotypes
Intellectual disability; Seizures; Abnormality of skeletal morphology; Abnormality of the urinary system; Hypertrichosis; Abnormality of the respiratory system

Publications

• https://doi.org/10.1101/693937
• 18616733

Mode of pathogenicity
Loss-of-function variants (as defined in pop up message) DO NOT cause this phenotype - please provide details in the comments