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Intellectual disability - microarray and sequencing v4.53 ALKBH8 Arina Puzriakova Tag Q3_22_rating was removed from gene: ALKBH8.
Intellectual disability - microarray and sequencing v4.53 ALKBH8 Arina Puzriakova commented on gene: ALKBH8: The rating of this gene has been updated to Green following NHS Genomic Medicine Service approval.
Intellectual disability - microarray and sequencing v4.52 ALKBH8 Arina Puzriakova Source NHS GMS was added to ALKBH8.
Source Expert Review Green was added to ALKBH8.
Rating Changed from Amber List (moderate evidence) to Green List (high evidence)
Intellectual disability - microarray and sequencing v3.1660 ALKBH8 Arina Puzriakova Publications for gene: ALKBH8 were set to 31130284; 31079898; 33544954; 34757492
Intellectual disability - microarray and sequencing v3.1659 ALKBH8 Arina Puzriakova edited their review of gene: ALKBH8: Changed publications to: 31079898, 33544954, 34757492, 35571055; Changed phenotypes to: Intellectual developmental disorder, autosomal recessive 71, OMIM:618504
Intellectual disability - microarray and sequencing v3.1659 ALKBH8 Arina Puzriakova Tag Q3_22_rating tag was added to gene: ALKBH8.
Intellectual disability - microarray and sequencing v3.1659 ALKBH8 Arina Puzriakova reviewed gene: ALKBH8: Rating: GREEN; Mode of pathogenicity: None; Publications: Intellectual developmental disorder, autosomal recessive 71, OMIM:618504; Phenotypes: 31079898, 33544954, 34757492, 35571055; Mode of inheritance: BIALLELIC, autosomal or pseudoautosomal
Intellectual disability - microarray and sequencing v3.1602 ALKBH8 Sarah Leigh Publications for gene: ALKBH8 were set to 31130284; 31079898
Intellectual disability - microarray and sequencing v3.1593 ALKBH8 Konstantinos Varvagiannis edited their review of gene: ALKBH8: Changed publications to: 31079898, 33544954, 34757492
Intellectual disability - microarray and sequencing v3.1593 ALKBH8 Konstantinos Varvagiannis edited their review of gene: ALKBH8: Added comment: Please consider upgrade to green rating.

2 additional relevant families reported in literature, as summarized below. While affected individuals from 3 (of the 4 total) families with the disorder were homozygous for truncating variants in the last exon (potentially corresponding to hypomorphic / incomplete LoF rather than null alleles), a more recent publication describes 2 sibs homozygous for a missense SNV with demonstrated loss-of-function in the context of normal protein levels.

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Saad et al (2020 - PMID: 33544954) report 2 sibs, born to consanguineous parents from Egypt homozygous for an ALKBH8 frameshift variant. Both exhibited global DD and ID (proband IQ of 51 / Stanford Binet test, sib: 42 using Weschler scale). There was no history of seizures. Family based exome sequencing of both sibs and parents revealed homozygosity for NM_001301010.1:c.1684delC [p.(Arg562Alafs*56))] within a region of AOH. As the authors note this variant also occurred in the last exon of the gene, likely escaping NMD and based on previous evidence from Monnies et al, hypothesize that truncating variants in the last exon represent hypomorphic alleles encoding for a partially functional protein, while protein truncating variants in earlier exons may be null alleles.

Maddirevula et al (2021 - PMID: 34757492) describe the phenotype of 2 sibs, homozygous for a missense variant. Features included severe DD and ID, microcephaly, facial dysmorphism and epilepsy (the latter limited to the elder one). Exome with autozygome analysis identified homozygosity for a missense variant (NM_138775.2:c.1874G>A / p.Arg625His) with Sanger for confirmation / segregation studies.LC-MS/MS using tRNA isolated from LCLs from the affected individual, a carrier parent and controls revealed complete loss of ALKBH8-dependent tRNA posttranscriptional modifications, the results being suggestive of abrogation of the catalytic activities of both MT and Ox domains. The protein was detected at low levels in LCLs from control and patient samples, a finding that was also supported by immunoblot analysis suggesting that the observed loss-of-function is not mediated by loss of the protein.; Changed rating: GREEN; Changed publications to: 33544954, 34757492
Intellectual disability - microarray and sequencing v3.1513 ALKBH8 Arina Puzriakova Tag for-review was removed from gene: ALKBH8.
Intellectual disability - microarray and sequencing v3.1510 ALKBH8 Sarah Leigh commented on gene: ALKBH8
Intellectual disability - microarray and sequencing v3.1509 ALKBH8 Arina Puzriakova Source Expert Review Amber was added to ALKBH8.
Rating Changed from Green List (high evidence) to Amber List (moderate evidence)
Intellectual disability - microarray and sequencing v3.1139 ALKBH8 Arina Puzriakova Phenotypes for gene: ALKBH8 were changed from Global developmental delay; Seizures; Intellectual disability to Intellectual developmental disorder, autosomal recessive 71, OMIM:618504
Intellectual disability - microarray and sequencing v3.703 ALKBH8 Ivone Leong Classified gene: ALKBH8 as Green List (high evidence)
Intellectual disability - microarray and sequencing v3.703 ALKBH8 Ivone Leong Added comment: Comment on list classification: This gene has been promoted back to Green status to reflect the rating that was on the signed off version of this panel (version 3.2). This gene should be demoted to Amber at the next review.
Intellectual disability - microarray and sequencing v3.703 ALKBH8 Ivone Leong Gene: alkbh8 has been classified as Green List (High Evidence).
Intellectual disability - microarray and sequencing v3.702 ALKBH8 Ivone Leong Tag for-review tag was added to gene: ALKBH8.
Intellectual disability - microarray and sequencing v3.66 ALKBH8 Rebecca Foulger Classified gene: ALKBH8 as Amber List (moderate evidence)
Intellectual disability - microarray and sequencing v3.66 ALKBH8 Rebecca Foulger Added comment: Comment on list classification: Demoted from Green to Amber based on advice from the Genomics England Clinical Team. In email correspondence, Helen Brittain notes that this is a borderline gene in terms of evidence (two families, 6/7 individuals with seizures and not particularly extensive functional / supportive information). Zornitza's review on the Genetic Epilepsy Syndromes panel focuses on the differing ratings of ALKBH8 on the ID (Green) and Epilepsy (Amber) panels. Based on borderline evidence, I have demoted ALKBH8 to Amber on the ID panel to be consistent with the GLH consensus on the Epilepsy panel (R59 #402).
Intellectual disability - microarray and sequencing v3.66 ALKBH8 Rebecca Foulger Gene: alkbh8 has been classified as Amber List (Moderate Evidence).
Intellectual disability - microarray and sequencing v2.978 ALKBH8 Catherine Snow Source Expert Review Green was added to ALKBH8.
Source Expert Review was added to ALKBH8.
Added phenotypes Global developmental delay; Seizures; Intellectual disability for gene: ALKBH8
Publications for gene ALKBH8 were changed from 31079898 to 31130284; 31079898
Rating Changed from No List (delete) to Green List (high evidence)
Intellectual disability - microarray and sequencing v2.863 ALKBH8 Catherine Snow Publications for gene: ALKBH8 were set to
Intellectual disability - microarray and sequencing v2.853 ALKBH8 Konstantinos Varvagiannis gene: ALKBH8 was added
gene: ALKBH8 was added to Intellectual disability. Sources: Literature
Mode of inheritance for gene: ALKBH8 was set to BIALLELIC, autosomal or pseudoautosomal
Phenotypes for gene: ALKBH8 were set to Global developmental delay; Intellectual disability; Seizures
Penetrance for gene: ALKBH8 were set to Complete
Review for gene: ALKBH8 was set to AMBER
Added comment: Monies et al. (2019 - PMID: 31079898) report on 7 individuals from 2 different consanguineous Saoudi families, harboring homozygous truncating ALKBH8 pathogenic variants. The same individuals are included in another concurrent publication from the same group (Monies et al. 2019 - PMID: 31130284).

All presented with DD and ID (Fam1 : moderate in the proband, degree not commented on for his 3 sibs / Fam2 : mild in the proband, severe in all his 3 sibs). Epilepsy was reported for 6/7 individuals although the type has not been commented on (onset 9-12 months to 2 years). Variable other features were noted in few.

Affected subjects from the first family were homozygous for a stopgain variant (NM_001301010.1:c.1660C>T or p.Arg554Ter) while individuals from the second family were homozygous for a frameshift one (c.1794delC or p.Trp599Glyfs*19). The variants affected in both cases the last exon of ALKBH8 and RT-PCR confirmed that they escape NMD.

Alternative causes were ruled out, at least for the proband from the second family (chromosomal analysis, SNP-array, metabolic investigations).

Linkage analysis of both families confirmed linkage to the same autozygous interval of chr11q22.3 with a LOD score of 6.

Segregation analyses in both families, confirmed homozygosity for the truncating variants in affected members and heterozygosity in their parents (or several unaffected sibs, none of those studied was homozygous for the ref. allele).

In mouse or human cells, ALKBH8 has previously been shown to be involved in tRNA modifications of the wobble uridines of specific tRNAs (PMIDs cited: 20308323, 20583019, 21653555).

LC-MS/MS analyses of tRNA extracted from LCLs derived from affected individuals, unaffected relatives (UR) and independent controls (IC) revealed that wobble nucleotide modifications were completely absent (or dramatically decreased in the case of mcm5U) in affected individuals but readily detected in UR/IC. As specific modifications were absent, substantial amounts of precursors (eg. cm5U - the precursor of mcm5U) were detected in affected individuals but not in unaffected ones.

Absence of wobble modifications (eg. mchm5U) has equally been observed in Alkbh8 knockout mice. Alkbh8-deficient mice show similar increases in precursors. Alkbh8 KO mice are however phenotypically normal (the authors comment that eventual cognitive defects were not formally evaluated and might have been missed - PMIDs cited: 20123966, 21285950).

As a result, the studies carried out confirmed the loss-of-function effect and were in line with previous functional studies in animal models, although the pathogenesis of ID remains unclear.

The expression profile of ALKBH8 is also unclear (wide profile of expression suggested developmentally, the authors studied LCLs, other studies suggest that embryonic expression is broad but becomes progressively more restricted to specific neuronal cells).

Mutations in other genes involved in tRNA modification (eg. ADAT3, PUS3, PUS7) have been shown underlie disorders affecting the CNS, with ID as a feature.

ALKBH8 is not currently associated with any phenotype in OMIM / G2P.

As a result, this gene can be considered for inclusion in the ID/epilepsy panels as amber pending further evidence.
Sources: Literature