Brain transcriptome analysis reveals subtle effects on mitochondrial function and iron homeostasis of mutations in the SORL1 gene implicated in early onset familial Alzheimers disease

BackgroundTo prevent or delay the onset of Alzheimers disease (AD), we must understand its molecular basis. The great majority of AD cases arise sporadically with a late onset after 65 years of age (LOAD). However, rare familial cases of AD can occur due to dominant mutations in a small number of genes that cause an early onset prior to 65 years of age (EOfAD). As EOfAD and LOAD share similar pathologies and disease progression, analysis of EOfAD genetic models may give insight into both subtypes of AD. Sortilin-related receptor 1 (SORL1) is genetically associated with both EOfAD and LOAD and provides a unique opportunity to investigate the relationships between both forms of AD. Currently, the role of SORL1 mutations in AD pathogenesis is unclear. MethodsTo understand the molecular consequences of SORL1 mutation, we performed targeted mutagenesis of the orthologous gene in zebrafish. We generated an EOfAD-like mutation, V1482Afs, and a putatively null mutation, to investigate whether EOfAD-like mutations in sorl1 display haploinsufficiency by acting through loss-of-function mechanisms. We performed mRNA-sequencing on whole brains comparing normal (wild type) fish with their siblings heterozygous for EOfAD-like or complete loss-of-function mutations in sorl1 or transheterozygous for these mutations. Differential gene expression and gene set enrichment analyses identified, respectively, changes in young adult zebrafish brain transcriptomes, and putative effects on neural subcellular functions. ResultsWe identified subtle effects on expression of genes involved in energy production, mRNA translation and mTORC1 signalling in both the EOfAD-like and null mutant brains, implying that these effects are due to sorl1 haploinsufficiency. Surprisingly, we also observed changes to expression of genes occurring only in the EOfAD-mutation carrier brains, suggesting gain-of-function effects. Transheterozygosity for the EOfAD-like and null mutations (i.e. lacking wild type sorl1), caused apparent effects on iron homeostasis and other transcriptome changes distinct from the single-mutation heterozygous fish. ConclusionsOur results provide insight into the possible early brain molecular effects of an EOfAD mutation in human SORL1. Differential effects of heterozygosity and complete loss of normal SORL1 expression are revealed.