Two new studies exemplify utility of Drosophila for human genetic disease-related studies

 (1)

Am J Hum Genet. 2022 Apr 7;109(4):571-586. doi: 10.1016/j.ajhg.2022.01.020. Epub
2022 Mar 2.

Loss-of-function variants in TIAM1 are associated with developmental delay,
intellectual disability, and seizures.

Lu et al.

TIAM Rac1-associated GEF 1 (TIAM1) regulates RAC1 signaling pathways that affect the control of neuronal morphogenesis and neurite outgrowth by modulating the actin cytoskeletal network. To date, TIAM1 has not been associated with a Mendelian disorder. Here, we describe five individuals with bi-allelic TIAM1 missense variants who have developmental delay, intellectual disability, speech delay, and seizures. Bioinformatic analyses demonstrate that these variants are rare and likely pathogenic. We found that the Drosophila ortholog of TIAM1, still life (sif), is expressed in larval and adult central nervous system (CNS) and is mainly expressed in a subset of neurons, but not in glia. Loss of sif reduces the survival rate, and the surviving adults exhibit climbing defects,are prone to severe seizures, and have a short lifespan. The TIAM1 reference (Ref) cDNA partially rescues the sif loss-of-function (LoF) phenotypes. We also assessed the function associated with three TIAM1 variants carried by two of the probands and compared them to the TIAM1 Ref cDNA function in vivo. TIAM1 p.Arg23Cys has reduced rescue ability when compared to TIAM1 Ref, suggesting that it is a partial LoF variant. In ectopic expression studies, both wild-type sif and TIAM1 Ref are toxic, whereas the three variants (p.Leu862Phe, p.Arg23Cys, and p.Gly328Val) show reduced toxicity, suggesting that they are partial LoF variants. In summary, we provide evidence that sif is important for appropriate neural function and that TIAM1 variants observed in the probands are disruptive, thus implicating loss of TIAM1 in neurological phenotypes in humans.

Copyright © 2022 American Society of Human Genetics. Published by Elsevier Inc.
All rights reserved.

DOI: 10.1016/j.ajhg.2022.01.020
PMID: 35240055 

Conflict of interest statement: Declaration of interests M.J.G.S. is a salaried employee of GeneDx Inc.

(2)

Am J Hum Genet. 2022 Apr 7;109(4):601-617. doi: 10.1016/j.ajhg.2022.03.002.

Germline variants in tumor suppressor FBXW7 lead to impaired ubiquitination and
a neurodevelopmental syndrome.

Stephenson et al.

Neurodevelopmental disorders are highly heterogenous conditions resulting from abnormalities of brain architecture and/or function. FBXW7 (F-box and WD-repeat-domain-containing 7), a recognized developmental regulator and tumor suppressor, has been shown to regulate cell-cycle progression and cell growth and survival by targeting substrates including CYCLIN E1/2 and NOTCH for degradation via the ubiquitin proteasome system. We used a genotype-first approach and global data-sharing platforms to identify 35 individuals harboring de novo and inherited FBXW7 germline monoallelic chromosomal deletions and nonsense, frameshift, splice-site, and missense variants associated with a neurodevelopmental syndrome. The FBXW7 neurodevelopmental syndrome is distinguished by global developmental delay, borderline to severe intellectual disability, hypotonia, and gastrointestinal issues. Brain imaging detailed variable underlying structural abnormalities affecting the cerebellum, corpus collosum, and white matter. A crystal-structure model of FBXW7 predicted that missense variants were clustered at the substrate-binding surface of the WD40 domain and that these might reduce FBXW7 substrate binding affinity. Expression of recombinant FBXW7 missense variants in cultured cells demonstrated impaired CYCLIN E1 and CYCLIN E2 turnover. Pan-neuronal knockdown of the Drosophila ortholog, archipelago, impaired learning and neuronal function. Collectively, the data presented herein provide compelling evidence of an F-Box protein-related, phenotypically variable neurodevelopmental disorder associated with monoallelic variants in FBXW7.

Copyright © 2022 American Society of Human Genetics. Published by Elsevier Inc.
All rights reserved.

DOI: 10.1016/j.ajhg.2022.03.002
PMID: 35395208

Conflict of interest statement: Declaration of interests I.E.S. has served on scientific advisory boards for UCB, Eisai, GlaxoSmithKline, BioMarin, Nutricia, Rogcon, Chiesi, Encoded Therapeutics, Xenon Pharmaceuticals, and Knopp Biosciences; has received speaker honoraria from GlaxoSmithKline, UCB, BioMarin, Biocodex, and Eisai; has received funding for travel from UCB, Biocodex, GlaxoSmithKline, Biomarin and Eisai; has served as an investigator for Zogenix, Zynerba, Ultragenyx, GW Pharma, UCB, Eisai, Anavex Life Sciences, Ovid Therapeutics, Epygenyx, Encoded Therapeutics and Marinus; and has consulted for Zynerba Pharmaceuticals, Atheneum Partners, Ovid Therapeutics, Care Beyond Diagnosis, Epilepsy Consortium and UCB. She may accrue future revenue on pending patent WO2009/086591; her patent for SCN1A testing is held by Bionomics and is licensed to various diagnostic companies; and she has a patent for a molecular diagnostic/therapeutic target for benign familial infantile epilepsy (BFIE) (PRRT2), WO/2013/059884. She receives and/or has received research support from the National Health and Medical Research Council of Australia, Medical Research Future Fund, Health Research Council of New Zealand, CURE, Australian Epilepsy Research Fund, and the National Institute of Neurological Disorders and Stroke of the National Institutes of Health. J.P. is co-chief scientific officer for Global Gene Corp. All other authors declare no competing interests.