Allele-specific mitochondrial stress induced by Multiple Mitochondrial Dysfunctions Syndrome 1 pathogenic mutations modeled in Caenorhabditis elegans

Abstract

Multiple Mitochondrial Dysfunctions Syndrome 1 (MMDS1) is a rare, autosomal recessive disorder caused by mutations in the NFU1 gene. NFU1 is responsible for delivery of iron-sulfur clusters (ISCs) to recipient proteins which require these metallic cofactors for their function. Pathogenic variants of NFU1 lead to dysfunction of its target proteins within mitochondria. To date, 20 NFU1 variants have been reported and the unique contributions of each variant to MMDS1 pathogenesis is unknown. Given that over half of MMDS1 individuals are compound heterozygous for different NFU1 variants, it is valuable to investigate individual variants in an isogenic background. In order to understand the shared and unique phenotypes of NFU1 variants, we used CRISPR/Cas9 gene editing to recreate exact patient variants of NFU1 in the orthologous gene, nfu-1 (formerly lpd-8), in C. elegans. Five mutant C. elegans alleles focused on the presumptive iron-sulfur cluster interaction domain were generated and analyzed for mitochondrial phenotypes including respiratory dysfunction and oxidative stress. Phenotypes were variable between the mutant nfu-1 alleles and generally presented as an allelic series indicating that not all variants have lost complete function. Furthermore, reactive iron within mitochondria was evident in some, but not all, nfu-1 mutants indicating that iron dyshomeostasis may contribute to disease pathogenesis in some MMDS1 individuals.

Patient-specific variants of NFU1/NFU-1 disrupt cholinergic signaling in a model of multiple mitochondrial dysfunctions syndrome 1

“…Our prior results ( Kropp et al, 2021a ) and those presented here indicate that patient-specific variants of nfu-1 , both of which are reduction- or loss-of-function alleles, result in neuromuscular dysfunction, with each nfu-1 variant resulting in distinct neuromuscular defects ( Fig. 7 ).…”

“…We previously demonstrated that patient-specific MMDS1 variants in the C. elegans gene nfu-1 display mitochondrial stress, including morphological and functional impairments, in multiple tissues ( Kropp et al, 2021a ). Because mitochondrial stress can result in movement defects due to dysfunction in the neuromuscular system, we assessed the motility of nfu-1 variants.…”

“…The mechanistic differences between these variants are very likely due to the specific differences in how these variants coordinated ISCs. We and others have previously shown that the Gly147Arg variant of NFU-1 fails to dimerize as effectively around ISCs, thereby impairing delivery to target proteins and causing excessive oxidative stress ( Kropp et al, 2021a ; Wesley et al, 2017 a ; Wesley et al, 2017b ). Gly166Cys, owing to the additional cysteine residue, fails to release ISCs, thereby also impairing delivery to target proteins ( Kropp et al, 2021a ; Wachnowsky et al, 2017 ).…”

“…We previously established a Caenorhabditis elegans model of MMDS1 by recreating exact patient-specific NFU1 variants in the C. elegans ortholog NFU-1. We studied five variants affecting the ISC interaction domain of NFU1/NFU-1 and demonstrated that they caused dysregulation of cellular iron as well as oxidative stress ( Kropp et al, 2021a ). In agreement with biochemical studies from another research group ( Wachnowsky et al, 2017 ; Wesley et al, 2017a , b ), our data supported a role for altered NFU1/NFU-1 dimerization as the cause of allele-specific phenotypes of NFU-1 variants.…”

Exploring therapeutic potential of mitophagy modulators using Drosophila models of Parkinson’s disease

“…Interestingly, another siderophore 2′2-bipyridyl (BP) has been reported to enhance mitochondrial degradation through a PINK1, PDR-1, and DCT-1 dependent mechanism by triggering a hypoxia-like response in C. elegans ( Schiavi et al, 2015 ). Additionally, BP treatment has also been shown to promote longevity through the mitophagy induction mechanism in C. elegans ( Kropp et al, 2021 ). Several studies have demonstrated mitophagy-inducing properties of these iron chelators, further investigations are required to understand their underlying mechanisms of actions and therapeutic potential.…”