**5. Tiny small RNA or microRNA**

**•** Twitching and cramping of muscles, especially those in the hands and feet

**•** Paralysis (adapted from http://www.hopkinsmedicine.org/neurology\_neurosurgery/

ALS occurs between the ages of 40 years and 70 years, but the disease can occur at a younger age also. It affects throughout the world without any ethnic, racial or socioeconomic bounda‐ ries. ALS is responsible for almost five of every 100,000 deaths in people aged 20 or older. The frequent age for ALS is after 60 years age. The incidence of ALS is five times higher than Huntington's disease and almost equal to multiple sclerosis. Fifty percent of affected patients live at least three or more years after diagnosis, 20 percent live 5 years or more and up to 10 percent will survive more than 10 years (adapted from http://www.hopkinsmedicine.org/ neurology\_neurosurgery/centers\_clinics/als/conditions/als\_amyotrophic\_lateral\_sclero‐

Making a proper diagnosis in ALS is complicated because symptoms can vary in each patient. Based on the symptom, ALS can be classified in five broad ranges based on the disease symbol:

Classical ALS – characterized by the deterioration of upper and lower motor neurons [nerve cells]. This symptom generally affects more than two-thirds of patients with the disease.

centers\_clinics/als/conditions/als\_amyotrophic\_lateral\_sclerosis.html)

**•** Loss of motor control in the hands and arms

**•** Uncontrollable periods of laughing or crying

As the disease progresses, symptoms may comprise:

**•** Slurred or thick speech and difficulty in projecting the voice

**•** Impaired use of the arms and legs

**•** Weakness and fatigue

138 Update on Amyotrophic Lateral Sclerosis

**•** Tripping and falling

**•** Dropping things

**•** Shortness of breath

**•** Difficulty in breathing

**•** Difficulty in swallowing

**3. Symptom statistics**

**4. Medical classification of ALS**

sis.html).

It seems the world of non-coding RNAs is expanding like the universe, with the progress of science. After the discovery of miRNA in 1993, our knowledge about miRNA is increasing exponentially. *Lin-4* was the first miRNA to be discovered, in 1993, by the joint efforts of Victor Ambros's and Gary Ruvkun's laboratories [13]. After that, *let-7* is a heterochronic gene of *Caenorhabditis elegans* and was the second miRNA to be discovered, in 2000, 7 years after the finding of the first miRNA [14]. MicroRNAs [mi-RNAs] are 18–25 nt RNAs produced from a cellular RNA with a stem-loop structure. These evolutionarily conserved, naturally abundant, small, regulatory non-coding RNAs can inhibit gene expression at the post-transcriptional level in a sequence-specific manner [8]. It can be (i) intronic and (ii) intergenic [15].

All miRNAs undergo 5′ capping and 3′ polyadenylation, which are common events in case of mRNA processing also [16]. Intronic miRNAs are produced under host gene promoter [15]. Intergenic miRNAs have own promoter [15]. In the nucleus, genes encoding miRNA are generally transcribed by RNA polymerase II (Pol II) into large primary miRNA transcripts (pri-miRNA) (sized >1 kb) [15–17]. The pri-miRNA is maximally 2.2 kb long [15, 17]. Processing of pri-miRNA:pri-miRNA > pre-miRNA > miRNA [15]. miRNAs bind to the 3′ UTR region of target mRNAs [8]. Once the miRNA binds to a completely complementary region of target mRNA, mRNA gets degraded [18]. miRNA-mediated regulation does not require to have a perfect match with its target-binding region [19]. Only seven-base sequence between second and eighth nucleotides from the 5′ end is called "seed region (sequence)" and a complete match of these sequence is required for degradation of target mRNA[s] [20]. It is believed that the strength of the inhibition varies depending on the sequences [20]. A single miRNA may directly affect the expression of hundreds of proteins at one time and several miRNAs can also target the same mRNA and result in enhanced translational inhibition. It causes post-transcriptional gene control [8].

## **6. Link between miRNA and ALS**

In last few decades, neuroscience has made a remarkable progress. This has led to accumula‐ tion of humongous information including neuronal signalling and neuronal circuits. Neuro‐ science itself is complex and RNA interference (RNAi) molecules have made it much more intricate. The finer details are being searched to cope up with the neurodegenerative disorders. In the postgenomic era, we are still searching mysteries related to non-coding RNA. These small RNA molecules are the controller of our genome.

The microRNA molecules are acting as nodes and links of gene expression. These experts work in various biological functions. Using complementarity against cognate mRNA, this 21–22 nt stretch plays havoc. A miRNA can target several mRNAs, and in combination with other regulatory molecules, it can alter a cell's expression and activity. Neurology includes net‐ working between the cells of nervous system and other cells of the organism. Direct and indirect targets of one miRNA make a cell's system much more complex. ALS is a motor neuron disorder and it has been proved in various reports that ALS can be caused due to dysregulation of miRNAs and thus misexpression of proteins in the cells (**Figure 2**). This chapter deals with the miRNAs which are culprits behind ALS [21].

**Figure 2.** In ALS, there are mutations in multiple genes like TDP-43, FUS–TLS and SOD1 and these results in activation of stress response involving phosphorylation of eIF2α and stress granule formation. In ALS, reduced pre-miRNA proc‐ essing and decreased levels of different miRNAs are caused by remodelling of Dicer complex in cytoplasm. Thus, the translation of target mRNAs is relieved, resulting in degeneration of motor neurons (adapted from Bicker S, Schratt G. MicroRNAs in ALS: small pieces to the puzzle. EMBO J. 2015;34[3]:2601–3).
