Assignment-2

Topic: Molecular Pathway of a rare disease

Chosen rare disease- Polio

Team members-Diptarka Saha- 15BBT0101Raunak Bose- 15BBT0051

POLIO VIRUSPoliovirus, the causative agent of 

poliomyelitis (commonly known as polio), is a human enterovirus and member of the family of Picornaviridae.

TEM photograph of polio virus virions

A type 3 polio virus capsid coloured by chains

Introduction:Poliovirus, a human enterovirus that belongs to the family Picornaviridae, is the causative agent of poliomyelitis. Humans are the only natural hosts of poliovirus. The virus, however, can be transferred to monkeys when it is directly inoculated into the central nervous system (CNS). The species specificity of this virus is governed by a specific cell surface molecule that serves as the poliovirus receptor (PVR).1),2) Indeed, transgenic mice carrying the human poliovirus receptor (hPVR/CD155) gene show susceptibility to poliovirus infection, although mice in general are not susceptible to poliovirus.

BASIC STRUCTURE OF POLIO VIRUSPoliovirus is composed of an RNA genome and a protein capsid. The

genome is a single-stranded positive-sense RNA genome that is about 7500 nucleotides long. The viral particle is about 30 nanometres in diameter with icosahedral symmetry. Because of its short genome and its simple composition—only RNA and a non-enveloped icosahedral protein coat that encapsulates it—poliovirus is widely regarded as the simplest significant virus.Poliovirus was first isolated in 1909 by Karl Landsteiner and Erwin Popper. In 1981, the poliovirus genome was published by two different teams of researchers: by Vincent Racaniello and David Baltimore at MIT and by Naomi Kitamura and Eckard Wimmer at Stony Brook University. Poliovirus is one of the most well-characterized viruses, and has become a useful model system for understanding the biology of RNA viruses.

TYPES OF POLIO :-There are three types of polio infections:Sub-clinical: Approximately 95 percent of polio cases are sub-clinical, and patients may not experience any symptoms. This form of polio does not affect the central nervous system (the brain and spinal cord).Non-paralytic: This form, which does affect the central nervous system, produces only mild symptoms and does not result in paralysis.Paralytic: This is the rarest and most serious form of polio, which produces full or partial paralysis in the patient. There are three types of paralytic polio: spinal polio (affects the spine), bulbar polio (affects the brainstem), and bulbospinal polio (affects the spine and brainstem).CAUSES :Poliovirus is often transmitted from person-to-person through fecal matter. People living in areas with limited access to running water or flush toilets often get the virus from drinking water contaminated by human waste that contains the virus.In addition, the virus can be spread by contaminated food or water or direct contact with another infected person. According to the May Clinic, the virus that causes polio is so contagious that anyone living with an infected person will likely become infected themselves. 

Molecular pathway :-In humans, poliovirus infection usually begins with oral ingestion of the virus. After oral ingestion, the virus multiplies in the alimentary mucosa, and possibly in the tonsils and Peyer’s patches. The virus then moves into the blood stream (viremia) through the putative barrier(s) that virulent poliovirus strains cross more efficiently than attenuated strains. The circulating virus invades the CNS and replicates in neurons, particularly motor neurons. There are 2 possible dissemination routes through which the circulating polio-virus can enter the CNS.1. One is virus permeation through the blood-brain barrier (BBB), and the other is virus transmission via peripheral nerves

Paralytic poliomyelitis occurs as a result of neuronal destruction by lytic replication of the poliovirus, although paralysis develops in less than 1% of those infected.5),6) A poliovirion consists of a single-stranded RNA genome of positive polarity and a non-enveloped capsid that comprises 60 copies of each of 4 capsid proteins: VP1, VP2, VP3, and VP4. The three-dimensional structure has been elucidated,7) and the study revealed depressions called “canyons” on the virion surface, which have been shown to be attachment sites for PVR2. The genome of the poliovirus functions as mRNA in the cytoplasm of infected cells. The virus-specific translation process begins with the entry of ribosomes into the internal ribosome entry site (IRES) within the 5′ noncoding sequence of the RNA,11) and all viral proteins are translated as a large precursor protein from a single open reading frame that is cotranslationally processed into functional viral proteins 

Genetic Differences Between the Polioviruses Polioviruses, like other RNA viruses, have errorprone virus encoded RNA polymerase enzyme, which lacks proof reading activities. This results in rapid accumulation of mutations upon replication [18] Epidemiologically, there are two categories of the poliovirus(WPV) which are also known as non- Sabin-like (NSL) and the Vaccine virus also known as Sabin –like (SL). These two categories are common to all the 3 serotypes. These two types of viruses are detected by intratypic differentiation tests (ITD) which are based on one antigenic method, the polyclonal or monoclonal ELISA[19] and one molecular method which can enter the polymerase chain reaction (PCR) or RNA probe hybridization technique [20,21]. From this concondant non Sabin like ITD results are classified as wild, concordant Sabin-like as vaccine virus while any discordant results or sabin-like isolates lacking the two ITD tests are subjected to sequencing analysis of the major viral capsid surface protein, the VP1 [22]. In the sequence analysis, isolates with the P2 wild and the P2 Sabin while the neurovirulence is determined by a changein position 481 and one other in the VP1. Only 11 nucleotide changes differentiate the wild P3 from its P3 sabin counterpart while the amino acid determining the neurovirulence is situated at position 472. The wild polioviruses irrespective of their serotypes consists of many genotypes, they are of high genetic diversity. They are highly transmissible and are usually highly neurovirulent.

Polio- The Molecular Clock Polioviruses are among the most rapidly evolving viruses known. The rapid evolution permits the pattern of poliovirus transmission to be followed with precision [25, 26]. Several factors combine to determine the overall rate of virus evolution. These include the replication error rates, the virus population size and growth rate, the frequency of genetic bottlenecks, the intensity of selective forces and the mechanism for genetic exchange [27]. Error rates for the poliovirus replicase have been estimated to ing region are synonymous codons) accumulate at overall rate of 10-2 substitutions per site per year and at 3x10-2 substitutions per year at synonymous sites [25, 31, 32, 33, and 34]. Evolution rates are similar across serotypes and between wild and vaccine-derived polioviruses. Interestingly the bottlenecks driving the rapid evolution of polioviruses appear also to occur during replication in the human intestine in addition to that which happens during person-to person transmission [35, 36]. Many poliovirus clinical isolates are recombinants [25, 35, 36]. Heterotrophic recombinants are frequently isolated from vaccinees given trivalent OPV [35, 37]. All wild polioviruses probably have a recent history of recombination because frequent genetic exchange with other species C enteroviruses and vaccine derived polioviruses appear to be typical of circulating polioviruses [25,38]. Crossover is most common in the non-capsid region, less common in the 5` nontraslated region and very rare nontranslated within the capsid region [39].Molecular clock data have been used to estimate the dates of the common ancestors to wild [25,26] and vaccine derived [34] polioviruses.For example it was possible to determine the date of receipt of an OPV dose that eventually gave rise to a type 2 vaccine derived poliovirus in a vaccinated Nigerian child that developed acute flaccid paralysis following vaccination with OPV [40].

Poliovirus (PV), the causative agent of poliomyelitis, is a single stranded (plus sense) RNA virus. Humans are the only natural hosts of PV. After oral ingestion, the virus multiplies in the alimentary mucosa, and possibly in the tonsils and Peyer’s patches. The virus then moves into the deep cervical and mesenteric lymph nodes and into the blood stream (viremia). The circulating virus invades the central nervous system (CNS) and replicates in neurons, particularly motor neurons. There are two possible dissemination routes through which PV can enter the CNS. One is virus permeation through the blood-brain barrier (BBB) and the other is virus transmission via peripheral nervous (neural pathway). Paralytic poliomyelitis occurs as a result of neuronal destruction by lytic replication of PV. The development of a mouse model for poliomyelitis that is transgenic for the human PV receptor (CD155) has made it much easier to investigate the efficiency of the viral dissemination process in a whole organism. These studies have given an insight into the mechanisms of BBB permeation and neural transport. Strain specific neurovirulence levels, however, appears to depend mainly on the replicating capacity of the virus in the CNS rather than the dissemination efficiency. The attenuating mutation within the IRES (internal ribosome entry site) on the RNA of the Sabin 1 vaccine strain of poliovirus was identified by constructing recombinant viruses between the Sabin 1 strain and the parental virulent Mahoney strain followed by neurovirulence test. This work opened a new avenue for elucidating the molecular mechanisms of the poliovirus pathogenesis. This discovery led to a concept “IRES activity-dependent virus tropism”.

Molecular Mechanism of Poliovirus Replication and

Pathogenesis

CD155 is not involved in the BBB permeation, but in the neural pathway. As for BBB permeation, PV is suggested to be incorporated into endosomes through a common pathway to that of transferrin. On the other hand, PV inoculated into skeletal muscle is incorporated into endosomes at synapses in a CD155-dependent manner as an infectious particle, and the PV-containing endosomes are retrogradely conveyed along microtubules in the axons with the aid of Tctex-1, a light chain-1 of cytoplasmic dynein complex, to the neuron cell body. In addition, neural cells were found to possess protective response mechanisms against PV infection.

Types of PolioSub-Clinical Polio :

If patients do have symptoms, they usually last for 72 hours or less and may include:•headache•sore, red throat•slight fever•vomiting•general discomfort

Non-Paralytic Polio :

The symptoms of non paralytic polio may last for a couple of days to a week or two and includes :•fever•sore throat in the absence of upper respiratory infection•headache•vomiting•fatigue•abnormal reflexes•problems swallowing and/or breathing•back and neck pain and stiffness•arm and leg pain or stiffness•muscle tenderness and spasms

Paralytic Polio :

People with paralytic polio experience the symptoms associated with non-paralytic polio first. Soon after, the following symptoms appear:•loss of reflexes•severe spasms and muscle pain•loose and floppy limbs, sometimes on just one side of the body, this is due to the weakness which results from the involvement of the spine•sudden paralysis (temporary or permanent)•deformed limbs (especially the hips, ankles, and feet due to prolonged weakenss and the lack of appropriate orthopedic bracing

Post-Polio Syndrome :-The symptoms of post-polio syndrome are:-•continuing muscle and joint weakness•muscle pain that gets worse•becoming easily exhausted or fatigued•muscle wasting, also called muscle atrophy•trouble breathing and/or swallowing•sleep related breathing problems (sleep apnea)•becoming easily cold or•new onset of weakness in previously uninvolved muscles

How Do Doctors Diagnose Polio?Doctors will use the patient’s reported symptoms to help determine whether he or she has polio. During a physical examination, a doctor may notice that the patient has impaired reflexes, back and neck stiffness, or difficulty lifting his or her head while lying flat.To definitively diagnose polio, a doctor will take a sample of the patient’s throat secretions, stool, or cerebrospinal fluid (fluid surrounding the brain and spinal cord). The sample is then tested to see if it contains poliovirus and if the cells in the cerebrospinal fluid demonstrate changes consistent with what is called aseptic meningitis (a brain infection)

How Do Doctors Treat Polio?

There is no cure for polio. Doctors can only treat the symptoms while the infection runs its course. The most common treatments include:•rest•painkillers to relieve headaches, muscle aches, and muscle spasms•antibiotics for urinary tract infections•portable ventilators to help with breathing•physical therapy and/or corrective braces to help with walking•heating pads or warm towels to ease muscle aches and spasms•physical therapy to treat pain in the affected muscles•physical therapy to address breathing and pulmonary problems and then pulmonary rehabilitation to increase the patient’s pulmonary endurance as the acute breathing problems improve•In advanced cases of leg weakness, when a patient has difficulty walking he or she may need a wheelchair or other mobility device