# ER Morphology in the Pathogenesis of Hereditary
## 3 ER Shaping Proteins
### Atlastin-1 (SPG3a)
The sequence of reactions leading to dimerization can occur between atlastin molecules that reside in the same or different membranes (cis- or trans-interactions, respectively), but membrane fusion can only occur during trans-interactions. A large number of mutations rather uniformly distributed across all domains of the ATL1 protein have been identified as the cause of SPG3A disease. These mutations include small deletions, small insertions, splice site mutations, and whole exon deletions. However, the large majority is represented by missense variants. A review of the literature indicates that there is no clear genotype-phenotype correlation in HSP-SPG3A patients [48]. Since SPG3A is a dominant disease, it might be expected that all disease variants would have a crippling impact on the function of ATL1. However, the membrane fusion activity of ATL1 cannot be tested directly since human atlastins do not possess the ability to catalyze the fusion of synthetic membranes in vitro. Furthermore, experiments that address ATL1 fusion in vivo or in cell culture have not produced satisfactory functional answers for reasons that clearly include genetic redundancy due to the existence in vertebrates of three highly homologous atlastin genes that most probably serve overlapping functions. To overcome this limitation, the fusogenic activity of Atlastin-1/SPG3A pathological mutations has been assessed by reproducing point mutations in the Drosophila protein and testing the mutant fly atlastin vitro. However, only a few of the more than 70 pathogenic mutations identified to date have been analyzed in this manner [46], in part since mutations that do not occur in conserved aminoacids cannot be replicated. Another strategy to evaluate the impact of ATL1 pathological variants has employed analysis in the soluble phase to test their ability to hydrolyze GTP and dimerize in a nucleotide-dependent manner as a proxy for functional performance but, for many of the variants examined, the impairment of GTP hydrolysis and dimer formation turned out to be rather modest [46; 47]. Consequently, neither the studies on _Drosophila_ atlastin nor those probing the biochemical properties of ATL1 were able to uncover a clear correlation between SPG3A disease-causing mutations and protein activity. This lack of understanding of the pathological process is further underscored by the results of investigations relying on the properties of _Drosophila_ atlastin in cell culture, in vitro and in vivo. Exogenous _Drosophila_ atlastin was shown to be able to functionally replace human atlastins in HeLa cells depleted of the endogenous proteins [49], but simultaneous assessment of conserved pathological mutations in the _Drosophila_ orthologique on both ER morphogenesis using this replacement assay and membrane fusion catalysis in vitro did not provide a better understanding of the genotype-phenotype correlation for SPG3A [50]. Likewise, the analysis of a few atlastin CRISPRed knock-in pathological mutant fly lines indicated that no obvious correlation can be identified between the behavior of pathological mutations in vivo and their activity [50; 51; 52]. The work by [PERSON] et al. has helped to partially elucidate the pathogenic mechanism of atlastin mutations, although the results should not be generalized because only four mutants were analyzed. While the prevailing theory posits that most atlastin mutations act through a dominant-negative mechanism [53; 54], this work showed that in vivo in flies both overexpression of these four pathological variants and heterozygosity of the same variants in CRISPRed individuals, do not elicit the loss-of-function effects expected for dominant-negative mutations, suggesting instead that the variants tested share a straightforward loss-of-function mechanism [52]. How is ER membrane fusion linked to HSP? This remains one crucial, unanswered question. From the biochemical standpoint, atlastin is a transmembrane enzyme that uses GTP hydrolysis to fuel the fusion of ER membranes.