## 5 Epigenetic Regulation of ALS
### Studies Using Drosophila Models
#### 5.2.1 Noncoding RNAs  
Neuron-specific expression of human TDP-43 in _Drosophila_ induces neuronal degeneration, shorter lifespan, and locomotive defects, reproducing the major symptoms of ALS/FTD. Eye imaginal disc-specific expression of human TDP-43 induces the rough eye phenotype accompanied by retinal degeneration [93; 94]. Genetic modifier screening using these transgenic fly lines identified 12 out of 2933 fly lines with an exacerbated TDP-43-induced phenotype and 23 fly lines exhibiting suppression of the phenotype [95]. In this genetic screen, knockdown of the _suppressor of triplolethal (Su(Tpl))_ gene, also known as the _elongation factor for RNA polymerase II (Ell)_, effectively suppressed the TDP-43-induced morphological defects of the compound eye and internal retina [95]. Furthermore, upregulation of _Su(Tpl)_ enhanced the TDP-43-induced defects [95]. The protein encoded by _Su(Tpl)_ is a component of two complexes: little elongation complex (LEC) and super elongation complex (SEC) [96; 97; 98]. In _Drosophila_, LEC contains Su(Tpl), Eaf, Ice1, and Ice2, regulating the initiation and elongation of RNA polymerase II (Pol II)-transcribed genes encoding small nuclear RNAs (snRNAs) [98; 99]. SEC is composed of Su(Tpl), Eaf, Ear, Lilli, and P-TEFb [98; 100]. In the ALS model flies expressing TDP-43, several noncoding RNAs were upregulated and knockdown of _Su(Tpl)_ restored the levels of the increased noncoding RNAs back to normal without affecting TDP-43 expression [95]. Among these RNAs, U12 snRNA and a stress-induced _hsr\(\omega\)_ lncRNA were found to be responsible for TDP-43-induced defects [95]. Of note, _Sat III_ lncRNA, the possible human homologue of _hsr\(\omega\)_, is upregulated in a human cellular disease model and FTD patient tissues. The _Drosophila Su(Tpl)_ gene is orthologous to the human _elongation factor for RNA polymerase II (ELL)_ gene and _ELL2_ gene. Physical interaction between TDP-43 and human ELL2 was also demonstrated by co-immunoprecipitation assay using human cells [95]. Thus, Su(Tpl)-containing complexes LEC and SEC were suggested to play important roles in TDP-43-associated toxicity. Pan-neuron-specific knockdown of _caz_ induced a short life span and locomotive defects, accompanied by short synapse length at the NMJ compared with control flies. The eye imaginal disc-specific knockdown of _caz_ induced the rough eye phenotype. Pan-neuron- and motor-neuron-specific knockdown of _hsr\(\omega\)_ lncRNA similarly induced locomotive defects and abnormal NMJ structure [101]. In the _hsr\(\omega\)_ knockdown flies, _caz_ mRNA levels were reduced and abnormal cytoplasmic localization of the caz protein was also induced. In addition to _hsr\(\omega\)_, knockdown of _CR18854_ lncRNA suppressed the locomotive defects and short synapse branch length phenotype at the NMJ induced by the _caz_ knockdown, whereas knockdown of other lncRNAs, such as _CR32207_ and _CR43988_, had no effect (Table 1) [102]. Considering these observations in _Drosophila_, some human lncRNAs may similarly function in the pathogenesis of ALS/FTD associated with _FUS_. The wild-type human FUS (_hFUSUt_) expressed in _Drosophila_ is mainly soluble and neurotoxic, inducing the rough eye phenotype with the loss of pigmentation, short life span, locomotive defects, decreased numbers of synaptic bouton and active zone at the NMJ, degeneration of axons, and enlarged motoneurons [19; 103; 104; 105; 106; 107]. Knockdown of _hsr\(\omega\)_ lncRNA in the _hFUSUt_-expressing flies reduced _hFUSUt_ mRNA levels and induced the formation of insoluble inclusions containing the nontoxic hFUSwt protein and LAMP1 in the cytoplasm [108]. These mechanisms may be effective to titrate soluble hFUSwt to suppress its neurotoxicity. Thus, lncRNA-dependent pathways play important roles in controlling aggregation-prone heterogeneous ribonucleoproteins (hnRNPs).