**3. Diseases arising from mutations**

Numerous computational tools have been developed for the interpretation, analysis, and prioritization of variations and their effects [20]. Many DNA/protein variations and disease-causing mutation databases are now available for references. For instance, the locus specific variation database (LSVD) is present at Leiden Open Variation Database (LOVD) system for all human genes [21]. Although some of the databases seem to contain similar information, however, the LSDBs are listed at the Human Genome Variation Society (HGVS) Website (http://www.hgvs.org/locusspecific-mutation-databases), the LOVD site (http://grenada.lumc.nl/LSDB\_list/ lsdbs), the GEN2PHEN server (http://www.gen2phen.org/data/lsdbs) [22], and at the Web Analysis of the Variome (http://bioinformatics.ua.pt/WAVe/) [20, 23, 24].

Besides the above databases, there are many others that were most recently covered ([20, 25], and the references therein). Moreover, recent advances in genome-wide association studies, next-generation sequencing technologies coupled with genetic linkage analysis have enhanced output in the analysis of mutationcausing diseases. Many of these methods are useful for detecting single-nucleotide polymorphisms (SNPs), which are found to be common in aberrant gene functioning. However, it may also be noted, the majority of structural variations (SVs) that occur in the human genome are yet to be fully characterized by single short-read platforms [26]. Suffice, for many genetic diseases, association studies have relied most heavily upon short read, high throughput sequencing technologies [27, 28].

Some `genetic variations with the consequence encoded proteins are known to manifest into disease phenotypes with the deleterious outcome to the patient. Within these are hemoglobinopathy including sickle cell disease (SCD), which are caused by a single germ-line mutation substituting (A to T) in the codon for amino acid 6. The change converts a glutamic acid codon (GAG) to a valine codon (GTG) [29, 30].

#### **3.1 Single mutation as a lead cause of amyotrophic lateral sclerosis (ALS)**

Most recently, due to the advances mentioned above, it led to the finding that a mutation in the C9orf72 gene (chromosome 9 open reading frame 72 genes) is the primary genetic cause of amyotrophic lateral sclerosis (ALS). These losses of function, induced by the mutation of the C9orf72 gene are thought to affect communication between motor neurons and muscles in people with ALS [31]. Further, this mutation is thought in part to be responsible for 40–50% of hereditary cases of ALS, and 5–10% of cases without family history. This mutation consists of an expansion of a sequence of hexanucleotide (GGGGCC) DNA bases, going from a few copies (less than 20 in a healthy person) to more than 1000 copies [30]. Until now, it still remains unclear how this GGGGCC base repeat expansions cause neurodegeneration in ALS. Although, mechanistically, the C9orf72 protein function in a complex with the WDR41 and SMCR proteins (guanine exchange factors (GEF)) for Rab8 and Rab39 [31].

*Recent Progress in Drug Repurposing Using Protein Variants and Amino Acids in Disease… DOI: http://dx.doi.org/10.5772/intechopen.102571*

In a more recent study, the gene C9orf72 role on the protein TDP-43 (transactive response DNA binding protein-43) was revealed. The TDP-43 protein plays an important role in ALS. It is thought that the C9orf72 gene may affect the protein TDP-43's location within the cell. "In approximately 97% of ALS patients, it is being observed that the TDP-43 protein is depleted from the nucleus, forming aggregates in the cytoplasm rather than being in the nucleus, as is the case in healthy people [26, 32, 33].

The average incidence rate of ALS worldwide is about one in 50,000 people per year and the average age of onset of the disease is about 60 years, with men at a slightly higher risk compared to women. FDA-approved treatments for ALS are only modestly effective and the disease still results in complete paralysis and death within the first 5 years after diagnosis [31, 32].

#### **3.2 Troponin variation in cardiomyopathy**

The calcium-mediated interaction between actin and myosin is controlled by cardiac regulatory proteins, cardiac troponin T (cTnT) and troponin I (cTnI). The cardiac forms of these regulatory proteins theoretically have the potential of being unique to the myocardium [34], as they are coded for by specific genes.

Cardiac troponins are detected in the serum by the use of monoclonal antibodies to epitopes of cTnI and cTnT. These antibodies are highly specific for cardiac troponin and have negligible cross-reactivity with skeletal muscle troponins. Indeed, cTnI has not been identified outside the myocardium [34]. Cardiac troponin T is expressed to a small extent in skeletal muscle; however, the current cTnT assay does not identify skeletal troponins [35].

The majority of cTnI and cTnT form part of the contractile apparatus within the myocardial cell with lower concentrations found in the cytoplasm [35]. Whenever there is myocardial ischemia resulting in myocardial necrosis, the cTn will be released from the cytosolic pool into the bloodstream within a few hours of the injury. This is typically followed by a more prolonged and sustained elevation of cTn due to degradation of the contractile apparatus, which may also be a reflection of the size of the infarct [35].

However, the release kinetics of cTn after the myocardial injury can differ between individuals and is also dependent on myocardial blood flow. It can also differ between cTnI and cTnT which are thought to have monophasic and biphasic concentration-time profiles respectively, and with the increase in cTnT tending to last for longer than that of cTnI [34].

After the onset of an acute coronary event, cardiac troponins may not be detected in the serum for up to 4 hours and should be repeated 12 hours after the first test, if the troponin concentration is not raised in an individual presenting with chest pain.

In the identification of cardiac muscle damage, the measurement of serum cTnI and cTnT are superior in terms of sensitivity and specificity to cardiac muscle enzyme measurements [36]. Elevated cardiac troponin concentrations are now an acceptable standard biochemical marker for the diagnosis of myocardial infarction [37].

In order to enhance the comparison of results for cTnT, from one laboratory to another, troponin T is measured using a single assay, and a cutoff value of 0.1 μg/liter is indicative of myocardial damage [38]. However, there are several cTnI assays with different sensitivities and cutoff values. According to the European Society of Cardiology and American College of Cardiology consensus criteria, serum cTnI values that indicate myocyte necrosis/myocardial damage range from 0.1 to 2 μg/liter [38].

In the management of patients with acute chest pain, the measurement of cardiac troponins as markers of myocardial damage has produced two important beneficial effects on clinical practice [39]. The first beneficial effect is that more patients with chest pain who would not have been diagnosed as having myocardial damage with conventional muscle enzyme assays are being diagnosed with myocardial infarction, even in the absence of ST-segment elevation. The second beneficial effect is that mortality is reduced because many of these patients are at high risk of full-thickness myocardial infarction or even death within 6 month period [40, 41].

The *Universal Definition of Myocardial Infarction* requires at least one cTn concentration above the 99th percentile value of a normal reference population for the diagnosis of myocardial injury [38]. However, there have been some concerns regarding the use of a 99th percentile threshold value for hs-cTn because of its limitations [42]. Firstly, the 99th percentile varies with assay [43]. Secondly, the 99th percentile varies with reference population selection (age, gender, ethnicity, and definition of healthy status), reference population size, and the statistical method used to calculate it [44, 45]. Some studies have shown that elevations of hs-cTn can be seen in older adults, which may be independent of pathological conditions [46, 47]. Thirdly, detectable chronic elevations in cTn above the 99th percentile are commonly seen in conditions such as chronic renal or cardiac failure [48, 49]. In addition, the improved analytical sensitivity of these assays has resulted in the detection of elevated cTn in numerous cardiac and non-cardiac conditions that cause myocardial cell necrosis, such as myocarditis, arrhythmia, cardiac procedures, pulmonary embolism, and sepsis [34, 41]. Due to these challenges, international guidelines have sought to promote consistency by proposing recommendations for determining 99th percentiles [50, 51]. It would therefore seem that the 99th percentile should not be the only metric for diagnosing acute myocardial injury.

Cardiac troponins may also be elevated in many other conditions associated with secondary ischaemic injury [44], such as large pulmonary emboli, coronary spasm, cardiac arrhythmias [52], hypertrophic cardiomyopathy [52], idiopathic dilated cardiomyopathy [53, 54]. It can also be elevated in conditions that cause myocardial injuries, such as cardiac trauma, chemotherapy [55], myopericarditis [55, 56], septicemia [57].

Some studies also found that cTn was detectable in nearly all children, where concentrations increased with increasing age and left ventricular mass, thus supporting the notion that cTn release is not always pathological [58].

In addition, it has recently been demonstrated that cTn may exhibit diurnal variations [59, 60]. One study noted that cTnT concentrations exhibited a decreasing trend between morning and afternoon (0830 hours and 1430 hours) for healthy individuals and individuals requiring hemodialysis [59]. For cTnI concentrations, a decreasing trend during these hours was also noted in individuals requiring hemodialysis, however, the pattern was not apparent in healthy individuals [59]. Furthermore, another study in men with type 2 diabetes found that cTnT decreased during the day and then increased during the night, with peak concentrations in the morning at 0830 hours [58]. This was further confirmed in another study of healthy individuals, where cTnT exhibited diurnal variation but cTnI did not have such variation [60]. In other words, cTn can be described as organ-specific but not disease-specific.
