28 Document heading Molecular evolutionary studies of Lassa virus nucleoprotein 2 Okoror LE 1 * , Eniolorunda TA 1 , Okoror OI 2 1 Department of Microbiology and Biotechnology, College of Natural Sciences, Joseph Ayo Babalola University, PMB 5006, Ilesha, Osun State, Nigeria 2 Department of Microbiology, Faculty of Natural Sciences, Ambrose Alli University, PMB 14, Ekpoma, Edo State, Nigeria Asian Pacific Journal of Tropical Disease (2011)28-34 Asian Pacific Journal of Tropical Disease journal homepage:www.elsevier.com/locate/apjtd *Corresponding author: Okoror LE, Department of Microbiology and Biotechnology, College of Natural Sciences, Joseph Ayo Babalola University, PMB 5006, Ilesha, Osun State, Nigeria. E-mail: OLE LEOkoror@gmail.com; OOI Larison86@yahoo.com 1. Introduction Lassa virus belongs to a very large group of heamorrhagic fever viruses-arena viruses which are responsible for high rate of morbidity and mortality in areas of West Africa. Among the six arena viruses known so far only one-Lassa virus (LASV) is known to cause illness in humans. Lassa virus is the etiological agent of Lassa fever which is an acute and often fatal illness and is endemic to West Africa. There are an estimated 300 000 to 500 000 cases of Lassa fever each year [1-5] with mortality rate of 15% to 20% for hospitalized patients and as high as 50% during epidemics [2,6] . Presently, there is no licensed vaccine or immunotherapy available for preventing or treating this disease. Although the antiviral drug ribavirin is somewhat beneficial, it must be administered at an early stage of infection to successfully alter disease outcome, thereby limiting its utility [7] . Furthermore, there is no commercially available Lassa fever diagnostic assay, thus preventing early detection and rapid implementation of existing treatment regimens (e.g.ribavirin administration). The lack of adequate countermeasures and means of detection, coupled with the severity of disease, ARTICLE INFO ABSTRACT Article history: Received 15 January 2011 Received in revised form 21 February 2011 Accepted 10 March 2011 Available online 28 March 2011 Keywords: Lassa virus Nucleoprotein Arena viruses Molecular evolutionary MEGA Lassa fever Virulence diversity Circulating antibodies Nucleotide sequence Viral glycoprotein Phylogenetic tree Synonymous substitution Objective: To study the virulence diversity through molecular evolution, and to provide insight on circulating antibodies. Methods: The nucleotide sequences of 18 Lassa virus genomic RNA encoding Lassa virus nucleoprotein isolates collected from different parts of the world since the identification of the Josiah strain were obtained from the GenBank and nucleotide substitution among them studied using the computer program MEGA 4. The genetic distances among strains were predicted by pairwise nucleotide differences. Results: The rate of synonymous substitution was high 5.889 per nucleotide per year and nonsynonymous was higher at 49.664. The average predicted rate of synonymous and nonsynonymous using modified Nei-Gojobori (assuming transition/transversion bias=2) was 27.9 which was taken as the genetic distance between strains. The average number of synonymous sites is 150.741 while the average number of nonsynonymous sites is 392.259. The phylogenetic tree was inferred by unweighted pairwise grouping in MEGA4 and using neighbour-joining method. The time of emergence of Lassa virus was predicted to be around January 1920. However, the first human appearance of the virus was predicted to be around May (1 959 暲 24) months. In synonymous substitution the rate of (G-T) rare was high. The nucleotide frequencies were 0.314 (A), 0.246 (T/U), 0.204 (C) and 0.235 (G). The transition/transversion ratio k 1 =14.991 (purines) and k 2 =69.916 (pyrimidines). The overall transition/transversion bias R=16.662 with a total of 620 position in the final data set. These figures are far higher than an earlier study using Lassa virus glycoprotein. The nucleotide diversity were also very high using the Taijima’s model in MEGA 4. Conclusions: The divergence within strains always coincides with the period of epidemic which goes to confirm that the cause of epidemic outbreak should be the emergence of new strain and also why the infection remains endemic despite circulating antibodies. A comparison with a similar study with the viral glycoprotein concludes that the glycoprotein is more suited for vaccine development. Contents lists available at ScienceDirect