MED27, SLC6A7, and MPPE1 variants in a complex neurodevelopmental disorder with severe dystonia

Reid, Kimberley M.; Farabella, Irene; Kurian, Manju A.

doi:http://dx.doi.org/10.1002/mds.29147

MED27, SLC6A7, and MPPE1 variants in a complex neurodevelopmental disorder with severe dystonia

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Reid KM, Spaull R, Salian S, Barwick K, Meyer E, Zhen J et al. MED27, SLC6A7, and MPPE1 variants in a complex neurodevelopmental disorder with severe dystonia. Mov Disord. 2022 Oct;37(10):2139-46. DOI: 10.1002/mds.29147

Abstract

Background: Despite advances in next generation sequencing technologies, the identification of variants of uncertain significance (VUS) can often hinder definitive diagnosis in patients with complex neurodevelopmental disorders. Objective: The objective of this study was to identify and characterize the underlying cause of disease in a family with two children with severe developmental delay associated with generalized dystonia and episodic status dystonicus, chorea, epilepsy, and cataracts. Methods: Candidate genes identified by autozygosity mapping and whole-exome sequencing were characterized using cellular and vertebrate model systems. Results: Homozygous variants were found in three candidate genes: MED27, SLC6A7, and MPPE1. Although the patients had features of MED27-related disorder, the SLC6A7 and MPPE1 variants were functionally investigated. SLC6A7 variant in vitro overexpression caused decreased proline transport as a result of reduced cell-surface expression, and zebrafish knockdown of slc6a7 exhibited developmental delay and fragile motor neuron morphology that could not be rescued by L-proline transporter-G396S RNA. Lastly, patient fibroblasts displayed reduced cell-surface expression of glycophosphatidylinositol-anchored proteins linked to MPPE1 dysfunction. Conclusions: We report a family harboring a homozygous MED27 variant with additional loss-of-function SLC6A7 and MPPE1 gene variants, which potentially contribute to a blended phenotype caused by multilocus pathogenic variants. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.