La libertad de los modernos. Humanismo e individualismo

ADARs have been identified in all metazoan species, including C. elegans, D.

melanogaster, X. laevis and humans (Bass, 1997). The biological relevance of the enzymes was investigated by generating knock-out animals in several species.

Although the invertebrate species remain viable upon ADAR deletion, they exhibit severe phenotypes. ADAR elimination in mammals, on the other hand, results in lethality, proving the adenosine deamination to be essential for normal mammalian development. Altogether, it can be concluded that during the course of evolution the role of ADAR-mediated RNA editing became increasingly important, providing higher vertebrates with yet another sophisticated mode of posttranscriptional gene regulation.

Despite their viability, both adr-1 and adr-2 knock-outs in C. elegans have difficulties detecting and tracking chemoattractants, a phenotype described as aberrant chemotaxis (Tonkin et al., 2002). Strikingly, this phenotype could be partially rescued when animals deficient in one of the adr genes were crossed with the strains mutated in one of the genes responsible for the effective RNAi response. These findings, together with reports on low Dicer cleavage efficiency of hyperedited dsRNAs in vitro, have provoked a setting of a theory according to which RNA editing and RNAi act as two antagonistic processes (Scadden and Smith, 2001a; Tonkin and Bass, 2003).

A-to-I RNA editing has also been reported for several D. melanogaster ion channel gene transcripts. The deletion mutants of the responsible enzyme, dADAR, that lack deamination activity in extracts lack all known site-specific RNA editing (25 sites in three ion channel transcripts). Although D. melanogaster larvae homozygous for the deletion in their adar locus have no phenotypical changes, adults lacking dADAR show no morphological abnormalities, but exhibit extreme behavioural deficits including temperature-sensitive paralysis, locomotor uncoordination and tremors and, finally, changes in grooming and mating behaviour. Neurodegeneration, affecting particularly the retina and causing ubiquitous brain lesions, accompanies the increase in phenotypic severity. This study provided clear evidence that pre-mRNA editing in D. melanogaster alters the nervous system targets to affect adult nervous system function, integrity, and behaviour (Palladino et al., 2000b).

23 In contrast to severe but viable phenotypes of invertebrate ADAR knock-outs, such deletions in mammals have been shown to be lethal. In the case of ADAR1 locus deletion, heterozigosity lead to birth of apparently completely healthy mice.

Homozygous deletants, on the other hand, die between days 11.5 and 12.5. Death occurs because of the halted developmental processes, distorted fetal liver structure and widespread programmed cell death in tissues including heart and vertebra. The apoptotic events, however, are most severely expressed in both hepatocyte and haematopoietic cell lineages of the fetal liver (Hartner et al., 2004; Wang et al., 2004a; Wang et al., 2000).

Recently, experiments with the inducible gene disruption in mice have shown that ADAR1 is not only essential for the maintenance of both fetal liver and adult bone marrow haematopoietic stem cells, but it can also act as a crucial suppressor of interferon signaling and, thus, protect cells from the deleterious consequences of the activation of interferon-stimulated pathways. The latter role of ADAR1 is attributed to the editing of as yet unidentified transcripts involved in the interferon-induced cellular response (Hartner et al., 2009).

Point mutations in locus coding for ADAR1 are associated with dyschromatosis symmetrica hereditaria (DSH), a disease that leads to altered pigmentation pattern of the skin on hands and feet. Patients display only a mild heterozygous dominant phenotype, with their mental and physical condition seemingly unaffected by the mutation (Miyamura et al., 2003).

Similar to ADAR1 mutants, mice heterozygous for the ADAR2 deletion/disruption are viable and appear normal. Their homozygous mutant counterparts, however, die not long after birth, between postnatal days 0 and 20, exhibiting severe neurological symptoms, among them frequent epileptic seizures.

This phenotype is a consequence of underediting at exclusively ADAR2-targeted Q/R site of the GluR-B pre-mRNA, giving rise to incorrectly spliced mRNA and, subsequently, receptors with increased Ca²⁺ permeability in hippocampal neurons, leading to hippocampal neurodegeneration. Interestingly, the viability and completely normal phenotype are reverted once both alleles for this underedited transcript get substituted with the genomically pre-edited ones, i.e. where target adenosine is mutated to guanosine. This observation defines the glutamate receptor subunit B pre-mRNA as the most important, if not the only essential, ADAR2 target in vivo (Higuchi et al., 2000).

24 ADAR2 dysfunction in humans has been implicated in couple of serious conditions, most of them unsurprisingly accompanied by neurological and psychiatric symptoms, considering that ADAR2 expression is highest in the brain where most of its substrates reside. Amyotropic lateral sclerosis (ALS), for example, is an illness characterized by the degeneration of motor neurons, leading to the muscular atrophy and respiratory failure. Impaired ADAR2 activity has been implicated in ALS because the editing at Q/R site of GluR-B pre-mRNA is incomplete in 56 % of ALS patients and the corresponding symptoms can be attributed to glutamate excitotoxicity and the excessive Ca²⁺ influx (Kawahara et al., 2004; Kwak and Kawahara, 2005).

5-HT2C receptor pre-mRNA, on the other hand, is edited at five positions and, considering the role that serotonin plays in the central nervous system, it is believed that reduced or disrupted ADAR activity could play a role in psychiatric disorders like depression or schizophrenia. In comparison with healthy individuals, the 5-HT2C

receptor pre-mRNA editing pattern is significantly changed in suicide victims with documented chronic major depression. These patients had overedited E and C sites and underedited D site when compared to samples of individuals without similar condition. Since all of the affected sites are ADAR2 targets, altered function of this protein seems to be a likely cause of the symptoms. Moreover, studies on mice have shown that the treatment with the antidepressant fluoxetine leads to exactly reverse changes in editing frequency at E, C and D sites when compared to the suicide victims, implying one of the possible mechanisms of action for this type of medication (Gurevich et al., 2002).

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