Molecular elucidation of loss-of-function HACE1 mutations

Neurodevelopmental disorders (NDDs) are a group of complex and heterogeneous central nervous system disorders that manifest early in life and affect approximately 3% of children worldwide. NDDs disrupt the tightly coordinated complex chain of events that lead to abnormal brain development resulting in impaired cognition, communication, adaptive behavior, and psychomotor skills. NDDs typically include intellectual disability (ID), autism spectrum disorder, attention deficit hyperactivity disorder, schizophrenia, bipolar disorder, and epilepsy. Although NDDs are highly heritable, multiple genes/biological pathways are known to be involved, and a single genetic cause is rare. NDDs share molecular pathways involving neuronal migration, differentiation, and synaptic plasticity. A thorough understanding of these pathway mechanisms will help identify common therapeutic candidates. The relative inaccessibility of patients neural tissues and the unavailability of disease-specific rodent models that closely recapitulate the multifactorial complexity of NDDs have hampered a detailed understanding of the disease`s pathophysiology. The current project will address the abovementioned issues by employing human patient-derived stem cell models and novel gene-editing techniques. We will focus on the molecular characterization of a very rare, newly reported autosomal recessive disorder known as Spastic Paraplegia with Psychomotor Retardation with or without Seizures (SPPRS). It typically shows an infantile-onset, starting with hypotonia at birth, followed by severely impaired global development, ID, and notable motor disability. Variants in the HACE1 gene, which encodes for the HECT domain, and ankyrin repeat-containing E3 ubiquitin-protein ligase are known to be causative of SPPRS. As a ubiquitin (ub) protein ligase, HACE1 catalyzes ub molecules on its target proteins and signals proteasomal degradation. We will perform a detailed analysis of the neurodevelopmental pathophysiology of 4 SPPRS patients (compound heterozygous variant p.R748*, homozygous variants p.R332*, and p.E119*) that were previously reported. Firstly, we will study the molecular mechanisms underlying the HACE1 mutation in patient-derived neuronal cells and further aim to correct the mutations causing the HACE1-deficiency using gene editing techniques. Combined with patient-specific studies, these precision medicine approaches will shed light on a novel rare disease. The methods described here can be applied to other monogenic disorders, even if patient material is unavailable, and can serve as a blueprint for molecular dissection of other related diseases.