Tuesday, July 3, 2018

GABBR2 mutations

Inspired by sibling (identical twin) patients

Yoo Y, Jung J, Lee YN, Lee Y, Cho H, Na E, Hong J, Kim E, Lee JS, Lee JS, Hong C, Park SY, Wie J, Miller K, Shur N, Clow C, Ebel RS, DeBrosse SD, Henderson LB, Willaert R, Castaldi C, Tikhonova I, Bilgüvar K, Mane S, Kim KJ, Hwang YS, Lee SG, So I, Lim BC, Choi HJ, Seong JY, Shin YB, Jung H, Chae JH, Choi M. GABBR2 mutations determine phenotype in rett syndrome and epileptic encephalopathy. Ann Neurol. 2017 Sep;82(3):466-478.


Rett syndrome (RTT) and epileptic encephalopathy (EE) are devastating neurodevelopmental disorders with distinct diagnostic criteria. However, highly heterogeneous and overlapping clinical features often allocate patients into the boundary of the two conditions, complicating accurate diagnosis and appropriate medical interventions. Therefore, we investigated the specific molecular mechanism that allows an understanding of the pathogenesis and relationship of these two conditions.

We screened novel genetic factors from 34 RTT-like patients without MECP2 mutations, which account for 90% of RTT cases, by whole-exome sequencing. The biological function of the discovered variants was assessed in cell culture and Xenopus tropicalis models.

We identified a recurring de novo variant in GABAB receptor R2 (GABBR2) that reduces the receptor function, whereas different GABBR2 variants in EE patients possess a more profound effect in reducing receptor activity and are more responsive to agonist rescue in an animal model.

GABBR2 is a genetic factor that determines RTT- or EE-like phenotype expression depending on the variant positions. GABBR2-mediated γ-aminobutyric acid signaling is a crucial factor in determining the severity and nature of neurodevelopmental phenotypes.

Vuillaume ML, Jeanne M, Xue L, Blesson S, Denommé-Pichon AS, Alirol S, Brulard C, Colin E, Isidor B, Gilbert-Dussardier B, Odent S, Parent P, Donnart A, Redon R, Bézieau S, Rondard P, Laumonnier F, Toutain A. A novel mutation in the transmembrane 6 domain of GABBR2 leads to a Rett-like phenotype. Ann Neurol. 2018 Feb;83(2):437-439.

We present a novel de novo heterozygous GABBR2 mutation, A707T, identified by whole exome sequencing, also located in TM6 of GABBR2 but associated with an RTT phenotype (Fig). The carrier, a 12‐year‐old girl, had profound intellectual disability, hand stereotypies, and sleep and breathing disturbances but no history of seizures. This mutation, predicted to be pathogenic by in silico analyses, lies in a region crucial for G‐protein–coupled receptor activation and positive allosteric modulation.5 To assess its impact on GABA signaling activity, we coexpressed the 2 GABAB subunits with the chimeric G‐protein Gαqi9 in HEK‐293 cells and measured the accumulation of inositol phosphate (IP‐1) induced by the ligand GABA. We showed that the signaling activity of our A707T mutant is weakly induced by the agonist compared to that of the wild type, and this without altering GABBR2 cell surface expression. The same results were observed with the A567T and I705N mutants. Moreover, the 4 mutants tested have a basal activity stronger than that of the wild type, which might explain why the mutated receptor cannot be stimulated efficiently by GABA. This basal activity is reversed by the competitive antagonist CGP54626, except for the mutant S695I, which is already fully active in the absence of GABA and does not respond to GABA in accordance with the data of Yoo et al. To conclude, our results show that GABBR2 mutations located within TM6 can also be associated with an RTT phenotype. The novel mutation described here, A707T, also exerts a deleterious effect on GABAB receptor activity. This deleterious effect could result from a constitutive activity of the mutated GABAB receptor, highlighting a novel putative pathogenic mechanism for GABBR2 variants.

Hamdan FF, Myers CT, Cossette P, Lemay P, Spiegelman D, Laporte AD, Nassif C, Diallo O, Monlong J, Cadieux-Dion M, Dobrzeniecka S, Meloche C, Retterer K, Cho MT, Rosenfeld JA, Bi W, Massicotte C, Miguet M, Brunga L, Regan BM, Mo K, Tam C, Schneider A, Hollingsworth G; Deciphering Developmental Disorders Study, FitzPatrick DR, Donaldson A, Canham N, Blair E, Kerr B, Fry AE, Thomas RH, Shelagh J, Hurst JA, Brittain H, Blyth M, Lebel RR, Gerkes EH, Davis-Keppen L, Stein Q, Chung WK, Dorison SJ, Benke PJ, Fassi E, Corsten-Janssen N, Kamsteeg EJ,
Mau-Them FT, Bruel AL, Verloes A, Õunap K, Wojcik MH, Albert DVF, Venkateswaran S, Ware T, Jones D, Liu YC, Mohammad SS, Bizargity P, Bacino CA, Leuzzi V, Martinelli S, Dallapiccola B, Tartaglia M, Blumkin L, Wierenga KJ, Purcarin G, O'Byrne JJ, Stockler S, Lehman A, Keren B, Nougues MC, Mignot C, Auvin S, Nava C, Hiatt SM, Bebin M, Shao Y, Scaglia F, Lalani SR, Frye RE, Jarjour IT, Jacques S, Boucher RM, Riou E, Srour M, Carmant L, Lortie A, Major P, Diadori P, Dubeau F, D'Anjou G, Bourque G, Berkovic SF, Sadleir LG, Campeau PM, Kibar Z, Lafrenière
RG, Girard SL, Mercimek-Mahmutoglu S, Boelman C, Rouleau GA, Scheffer IE, Mefford HC, Andrade DM, Rossignol E, Minassian BA, Michaud JL. High Rate of Recurrent De Novo Mutations in Developmental and Epileptic Encephalopathies. Am J Hum Genet. 2017 Nov 2;101(5):664-685.


Developmental and epileptic encephalopathy (DEE) is a group of conditions characterized by the co-occurrence of epilepsy and intellectual disability (ID), typically with developmental plateauing or regression associated with frequent epileptiform activity. The cause of DEE remains unknown in the majority of cases. We performed whole-genome sequencing (WGS) in 197 individuals with unexplained DEE and pharmaco-resistant seizures and in their unaffected parents. We focused our attention on de novo mutations (DNMs) and identified candidate genes containing such variants. We sought to identify additional subjects with DNMs in these genes by performing targeted sequencing in another series of individuals with DEE and by mining various sequencing datasets. We also performed meta-analyses to document enrichment of DNMs in candidate genes by leveraging our WGS dataset with those of several DEE and ID series. By combining these strategies, we were able to provide a causal link between DEE and the following genes: NTRK2, GABRB2, CLTC, DHDDS, NUS1, RAB11A, GABBR2, and SNAP25. Overall, we established a molecular diagnosis in 63/197 (32%) individuals in our WGS series. The main cause of DEE in these individuals was de novo point mutations (53/63 solved cases), followed by inherited mutations (6/63 solved cases) and de novo CNVs (4/63 solved cases). De novo missense variants explained a larger proportion of individuals in our series than in other series that were primarily ascertained because of ID. Moreover, these DNMs were more frequently recurrent than those identified in ID series. These observations indicate that the genetic landscape of DEE might be different from that of ID without epilepsy.

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