Clinical and genetic characterization of a progressive RBL2-associated neurodevelopmental disorder
Authors: Aughey Gabriel N , Cali Elisa , Maroofian Reza , Zaki Maha S , Pagnamenta Alistair T , Ali Zafar , Abdulllah Uzma , Rahman Fatima , Menzies Lara , Shafique Anum , Suri Mohnish , Roze Emmanuel , Aguennouz Mohammed , Ghizlane Zouiri , Saadi Saadia Maryam , Fatima Ambrin , Cheema Huma Arshad , Anjum Muhammad Nadeem , Morel Godelieve , Robin Stephanie , McFarland Robert , Altunoglu Umut , Kraus Verena , Shoukier Moneef , Murphy David , Flemming Kristina , Yttervik Hilde , Rhouda Hajar , Lesca Gaetan , Chatron Nicolas , Rossi Massimiliano , Murtaza Bibi Nazia , Ur Rehman Mujaddad , Lord Jenny , Giacopuzzi Edoardo , Hayat Azam , Siraj Muhammad , Shervin Badv Reza , Seo Go Hun , Beetz Christian , Kayserili Hülya , Krioulie Yamna , Chung Wendy K , Naz Sadaf , Maqbool Shazia , Chandler Kate E , Kershaw Christopher J , Wright Thomas , Banka Siddharth , Gleeson Joseph G , Taylor Jenny C , Efthymiou Stephanie , Baig Shahid Mahmood , Severino Mariasavina , Jepson James E C , Houlden Henry 
Published: December 2024

Abstract
Retinoblastoma (RB) proteins are highly conserved transcriptional regulators that play important roles during development by regulating cell-cycle gene expression. RBL2 dysfunction has been linked to a severe neurodevelopmental disorder. However, to date, clinical features have been described in only six individuals carrying five biallelic predicted loss-of-function (pLOF) variants.
To define the phenotypic effects of RBL2 mutations in detail, we identified and clinically characterized a cohort of 35 patients from 20 families carrying pLOF variants in RBL2, including 15 new variants that substantially broaden the molecular spectrum. The clinical presentation of affected individuals is characterized by a range of neurological and developmental abnormalities. Global developmental delay and intellectual disability were observed uniformly, ranging from moderate to profound and involving lack of acquisition of key motor and speech milestones in most patients. Disrupted sleep was also evident in some patients. Frequent features included postnatal microcephaly, infantile hypotonia, aggressive behaviour, stereotypic movements, seizures and non-specific dysmorphic features. Neuroimaging features included cerebral atrophy, white matter volume loss, corpus callosum hypoplasia and cerebellar atrophy.
In parallel, we used the fruit fly, Drosophila melanogaster, to investigate how disruption of the conserved RBL2 orthologue Rbf impacts nervous system function and development. We found that Drosophila Rbf LOF mutants recapitulate several features of patients harbouring RBL2 variants, including developmental delay, alterations in head and brain morphology, locomotor defects and perturbed sleep. Surprisingly, in addition to its known role in controlling tissue growth during development, we found that continued Rbf expression is also required in fully differentiated post-mitotic neurons for normal locomotion in Drosophila, and that adult-stage neuronal re-expression of Rbf is sufficient to rescue Rbf mutant locomotor defects.
Taken together, our study provides a clinical and experimental basis to understand genotype–phenotype correlations in an RBL2-linked neurodevelopmental disorder and suggests that restoring RBL2 expression through gene therapy approaches might ameliorate some symptoms caused by RBL2 pLOF.