Identification of disease-relevant modulators of the SHH pathway in the developing brain.

Nora Mecklenburg,Jessica Görne,Bettina Purfürst,Annette Hammes,Hannes Gonschior,Matthias Richter,Izabela Kowalczyk,Tamrat M Mamo,Alena Laier,Franziska Witte,Martin Lehmann,Norbert Hübner,Anje Sporbert

doi:10.1242/dev.199307

Abstract

Pathogenic gene variants in humans that affect the sonic hedgehog (SHH) pathway lead to severe brain malformations with variable penetrance due to unknown modifier genes. To identify such modifiers, we established novel congenic mouse models. LRP2-deficient C57BL/6N mice suffer from heart outflow tract defects and holoprosencephaly caused by impaired SHH activity. These defects are fully rescued on a FVB/N background, indicating a strong influence of modifier genes. Applying comparative transcriptomics, we identified Pttg1 and Ulk4 as candidate modifiers upregulated in the rescue strain. Functional analyses showed that ULK4 and PTTG1, both microtubule-associated proteins, are positive regulators of SHH signaling, rendering the pathway more resilient to disturbances. In addition, we characterized ULK4 and PTTG1 as previously unidentified components of primary cilia in the neuroepithelium. The identification of genes that powerfully modulate the penetrance of genetic disturbances affecting the brain and heart is likely relevant to understanding the variability in human congenital disorders.

Highlights

(HPE) is the most common structural defect of human forebrain development
Identification of PTTG1 as a sonic hedgehog (SHH) pathway modifier We focused on highly regulated genes that had never hitherto been associated with the primary cilium and canonical SHH pathway, and went back to the unbiased set of differentially expressed genes (DEGs)
We suggest that ULK4 and PTTG1, which are highly expressed in the FVB/N and F1 rescue backgrounds, are not necessary, but instead are enhancing components of the SHH machinery

Summary

Introduction

(HPE) is the most common structural defect of human forebrain development. Even within pedigrees carrying the same SHH mutation, HPE phenotypes vary among relatives and can range from alobar HPE, to facial abnormalities typical of HPE, to asymptomatic appearance of the carrier Such intrafamilial variability of HPE phenotypes could be due to both environmental and genetic factors (Heussler et al, 2002; Hong and Krauss, 2018; Ming and Muenke, 2002; Muenke and Beachy, 2000; Muenke and Cohen, 2000; Roessler et al, 1996)

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