How the Malaria Parasite Hides Itself from the Mosquito’s Defenses

Malaria remains a potent killer to the human inhabitants of the tropical regions in the so-called “undeveloped world.”  This disease results in approximately one million deaths a year in Africa, alone.  An insect vector, the female Anopheles mosquito that requires blood to mature its eggs, is responsible for its transmission to humans.  There are a host of human maladies caused by such diverse organisms as viruses, protozoans and even worms that infect humans and that are carried by mosquitoes and other insects.

The causative agent for human malaria is a parasite referred to as Plasmodium falciparum.  The life cycle of P. falciparum is quite complex and intriguing.  When it is first inadvertently ingested by the mosquito from an infected host, the parasite undergoes a progression of transformations.  The specialized sexual precursor cells – male and female - are rapidly activated to form complete and functional sex cells – gametes.  Male and female gamete pairs subsequently fuse to from the incipient new organism called the zygote.  Within 18 – 24 hours, this zygote develops into a motile organism called the ookinete that is infectious to the mosquito.  This ookinete rapidly enters the insect’s midgut and forms an oocyst.  Within the oocyst more than 10,000 sporozoites are created within ten days.  Once this oocyst ruptures, these sporozoites navigate to the mosquito’s salivary glands where they are transferred to the next human host upon the mosquito’s bite.

Given this information, the questions that comes to mind are the following –

• What are the mosquito’s natural defenses against this parasite?
• How does the P. falciparum successfully elude these defenses?

The investigations of Dr. Alvaro Molina-Cruz and colleagues at the Laboratory of Malaria and Vector Research at the National Institute of Allergy and Infectious Diseases at the National Institute of Health in Rockville Maryland helped elucidate the mechanism by which the parasite escapes the immune defenses of its host.

It has been well established that insects have a first line of immune defense, as do vertebrates, referred to as the innate immune system (IIR).  IIR utilizes both humoral (chemical) and cellular defense mechanisms; it is quite sophisticated.  In spite of this ability to fend off foreign invaders, P. falciparum manages to survive and propagate in this hostile environment.

Molina-Cruz and the team of collaborators were able to identify a gene product from the parasite that enables the invading organism to infect its host mosquito without activating the innate immune system described earlier.  The gene responsible for producing this product was identified as Pfs47.  In fact when this gene was disrupted from its normal functionality, the parasite’s survival within the host was greatly reduced.

This finding is significant, for it establishes the fact that the Pfs47 gene contained within P. falciparum is essential for the efficient transmission of the parasite responsible for malaria from the vector to its human host.

The CCR5 Receptor and HIV/AIDS

The recently reported news that an AIDS patient has been successfully cured of his disease is a very exciting development in regards to global public health.  The methodology used to affect this outcome is directly related to the manner in which the human immunodeficiency virus (HIV) gains entry into its host cell.  HIV enters and ultimately kills the so-called,” T-Helper” cell that plays a critical role in adaptive human immunity.  Once a significant portion of these cells are destroyed, the patient displays the classic symptoms of AIDS – the inability to successfully fight off infections by invading micro-organisms.

In order to successfully invade its target cell, HIV must bind to specific proteins found on the surface of T-Helper cells.  One of these cell surface proteins is referred to CCR5 (CD195) – a member of the chemokine receptor family – and the protein product of the CCR-5 gene.  Chemokines are factors that are used to attract T cells to particular tissue or organ targets when battling infection.  If this protein is absent or the product of a mutated gene, HIV will fail to bind and therefore be unable to infect the target cell.  It is important to remember that viruses have an absolute requirement for access into a living cell in order to produce an infection; for, they need to exploit the intracellular processes of a living cell in order to make copies of themselves.  The molecular biology of HIV has been studied extensively and, as a consequence, this mechanism of action is well understood.

In addition, there is a percentage of the human population that is innately resistant to HIV infection even upon exposure to the virus.  This apparent insensitivity to HIV infection is readily explained by the fact that these individuals possess defective variants of the CCR-5 gene.

Thanks to the ingenuity and inventiveness of Drs. Gero Hutter, Eckard Thiel and colleagues from the Charite Hospital in Berlin Germany, an approach to a potential cure of AIDS was recognized that involves the widely used procedure of bone marrow transplantation – the first successful human bone marrow transplantation was accomplished in 1975 in Seattle pioneered by Dr. Donnall Thomas who won a Nobel Prize in Medicine for his efforts.

 

In this particular case, the patient presented with acute myelogenous leukemia (AML) as well as AIDS.  The protocol that was developed involved the destruction of the AIDS patient’s immune system followed by recovery using bone marrow from a suitable donor who was also immune to HIV infection on account of a defect in the CCR-5 gene as described above.  As a result of this approach, the patient became asymptomatic and free of HIV.  This is possible because all the immuno-competent cells in the human immune system repertoire are derived from progenitors found in the bone marrow and once the patient’s own immune cells were successfully replaced by the donor’s, then he became HIV resistant.

Since bone marrow transplantation has inherent risks, it is not the recommended approach for AIDS patients; unless, they also suffer from leukemia.  However, there is considerable hope in developing methodologies to use gene modification techniques to modify an AIDS patient’s own immune cells so that they would exhibit the CCR-5 gene variant and render them HIV resistant.

Autism and Genetic Mutations

Autism spectrum disorders (ASD) is a general term that encompasses a group of disorders that have as their point of origin anomalies in the development of the human brain.  The neurological symptoms that have been associated with ASD include: impairment of normal social development, difficulties with verbal and nonverbal communication and marked repetitive behaviors.  In terms of the latter symptom, anomalies in the brain circuits that are also implicated in obsessive compulsive disorder (OCD) have been identified.  The syndromes that fall within the general definition of ASD include autistic disorder, Rett syndrome, childhood disintegrative disorder, pervasive developmental disorder-not otherwise specified (PDD-NOS) and Asperger syndrome. Paradoxically, certain individuals sometimes display uncanny abilities in visual skills, music, math and art.

In general, the disease is discovered in young children, usually between 2 and 3 years old.  ASD has been diagnosed in over 2 million individuals in the U.S. and many millions throughout the world.  Evidence obtained through the statistics compiled by the Center for Disease Control (CDC) indicates that the incidents of ASD are increasing.  The reasons for this are unclear.

What has been ascertained is that there is strong genetic component in this disease.  What makes ASD particularly refractory to the development of a definitive cure is that the molecular biology is quite complex and involves a multiplicity of genes.  In this report, the findings of Dr. Brian J O’Roak and his colleagues, from the Department of Genome Studies at the University of Washington in Seattle and other collaborating institutions, will be examined.

Autism spectrum disorders (ASD) is a general term that encompasses a group of disorders that have as their point of origin anomalies in the development of the human brain.  The neurological symptoms that have been associated with ASD include: impairment of normal social development, difficulties with verbal and nonverbal communication and marked repetitive behaviors.  In terms of the latter symptom, anomalies in the brain circuits that are also implicated in obsessive compulsive disorder (OCD) have been identified.  The syndromes that fall within the general definition of ASD include autistic disorder, Rett syndrome, childhood disintegrative disorder, pervasive developmental disorder-not otherwise specified (PDD-NOS) and Asperger syndrome. Paradoxically, certain individuals sometimes display uncanny abilities in visual skills, music, math and art.

In general, the disease is discovered in young children, usually between 2 and 3 years old.  ASD has been diagnosed in over 2 million individuals in the U.S. and many millions throughout the world.  Evidence obtained through the statistics compiled by the Center for Disease Control (CDC) indicates that the incidents of ASD are increasing.  The reasons for this are unclear.

What has been ascertained is that there is strong genetic component in this disease.  What makes ASD particularly refractory to the development of a definitive cure is that the molecular biology is quite complex and involves a multiplicity of genes.  In this report, the findings of Dr. Brian J O’Roak and his colleagues, from the Department of Genome Studies at the University of Washington in Seattle and other collaborating institutions, will be examined.

In this exhaustive study, samples from 2446 ASD patients were used.  Of the 192 candidate genes, 44 genes were selected.  This selection was based on the determination to look at disruptive mutations, known associations with ADS-related symptoms, overlap with genes known to be involved in neurodevelopment and gene copy number variation (CNV).

A summary of the results of this work follows –

• Discovery of 27 de-novo genetic mutation events in 16 genes
• 59% of these mutations predicted to either shorten the protein gene products or disrupt the splicing that is required to yield functional protein products
• Recurrent disruptive mutations in six genes – CHD8, DYRK1A, GRIN2B, TBR1, PTEN AND TBL1XR1.  Five of these genes are contained within the catenin-chromatin-remodeling network.

This data confirms associations with specific genes and particular phenotypic expressions of ASD – CHD8 has been linked to macrocephaly and DYRK1A with microcephaly.  In addition, these results confirm the possible role of the catenin-chromatin-remodeling network - a critical network of genes and gene products involved in embryonic development.  However, these data do not establish an unambiguous causal relationship between these genetic anomalies and ASD.  This work represents an important contribution in the search for such direct causal link that may eventually lead to therapeutic relief for individuals with ASD.

Alternative Genetic Splicing – What is its Significance?

There exists a seeming paradox among the many species that constitute vertebrates – animals with backbones.  The fact is that although so many of these species share similar families of protein-coding genes, they are very distinct in terms of their physical characteristics – their phenotypes.  There is no clear explanation of how this phenomenon can be defined on the molecular level.

What has been well established is the fact that over the evolutionary time scale for these various species – approximately 350 million years – significant changes have occurred in their respective transciptomes.  A transcriptome represents the entire set of all RNA molecules within a cell including Messenger RNA (mRNA), Ribosomal RNA (rRNA) and Transfer RNA (tRNA) and the so-called “non-coding” RNA.  It can either be used to represent the complete set of transcripts for an entire organism or for a particular cell type – for a particular organ, for example.  The transcriptome, in essence, is a quantitative measure of the genes that are actually being expressed at any given time and as such can very dependent upon the particular set of environmental conditions that the organism is subjected to.

From the accumulated data, it seems unlikely that changes in gene expression can account for the diversity of characteristics found among members of the vertebrate family.  It seems more likely that changes in alternative splicing (AS) may be responsible for the species-specific differences found in nature.  AS is a process in which a single gene can, in fact, result in the production of more than one protein.  This is a regulated process that determines what exons – that portion of a gene that that has coding information for the final protein product - will be included or excluded in the final m-RNA.  It is this mechanism that accounts for the fact that far more proteins are produced from the ~ 20,000 protein-coding genes found in the human genome than would ordinarily be expected.

Given these assumptions arrived at from well-supported evidence, Dr. Nuno L. Barbosa-Morais and colleagues, from the Banting and Best Department at the Donnelly Centre of Medical Research at the University of Toronto, Canada, conducted a genome-wide investigation of AS differences among equivalent organs from vertebrate organisms encompassing all vertebrates higher on the evolutionary scale than fishes – tetrapods.  The organs that they used for the investigation were whole brain, forebrain cortex, cerebellum, heart, skeletal muscle, liver, kidney, and testis.  This approach engendered a significant amount of precise and demanding work

From this exhaustive study, they found significant differences in the complexity of AS between the various lineages studied with the highest complexity found within primates.  The species examined spanned ~ 350 million years in the evolutionary time scale.  Furthermore, within 6 million years the splicing patterns of the individual organs studied diverged to the extent that they were more closely correlated with the identity of the species than to the organ type.

These results add significantly to the understanding of the underlying genetic mechanisms that account for the diversity of characteristics found within the vertebrate family of organisms.

The Relationship between Obesity and Insulin Resistance

Metabolism – the sum of all the chemical reactions in the body necessary for sustaining life - and immunity – the ability of the organism to defend against threats posed by the invasion of microorganisms such as bacteria and viruses – are intimately linked in the biology of mammals. It is this relationship that allows the organism to adapt to changes in both the internal and external environments. However, within the modern western diet and lifestyle that promotes the development of obesity, this close association of metabolism and immunity can have deleterious consequences. Through the evolutionary history of the human species, humanity has had to survive in the face of the possibility of death as a result of starvation, infection and predation. It is only relatively recently in human societal development that such threats have been significantly lessened due to the introduction of agriculture to meet the nutritional needs of human populations and significant progress in technology and medicine. These age-old threats have been supplanted, however, by new concerns regarding individual mortality posed by cardiovascular disease, diabetes and cancer. The evidence now strongly indicates that obesity is an essential component of these troubling diseases. Between 1980 and 2008, the total number of obese individuals has essentially doubled worldwide to .5 billion individuals and the global death rate attributed to obesity is currently at ~ 3 million people per year. Drs. Justin Odegaard and Ajay Chawla from the Cardiovascular Research Institute at the University of California at San Francisco have examined the “cellular and molecular connections between chronic low-grade inflammation, insulin resistance and obesity-induced metabolic disease.” The focus for this report will be on the relationship between obesity and insulin resistance. Within this context, obesity can be defined as an imbalance between caloric intake and energy expenditure – calories in/calories out. This state of imbalance leads to the storage of excess nutrients in white adipose tissue (WAT). For lean individuals this imbalance is readily compensated by metabolic adjustments in WAT, liver and skeletal muscles. However, in a state of chronic over-nutrition, these pathways are overwhelmed leading to wide-ranging intracellular and extracellular dysfunction. Although these deleterious effects are the result of complex metabolic processes, the end product of these disturbances leads to the inhibition of insulin signaling – insulin resistance - through the modification of the insulin receptor substrate resulting in diabetes. In addition, the metabolic stress responses that are a product of chronic obesity, leads to the triggering of the innate immune receptors resulting in inflammation. This is, indeed, troubling data especially in regards to global public health. It appears that chronic over-nutrition is a cause for concern since the health ramifications that result from obesity have a major impact on individual quality of life and mortality.

Genetic Mutations Associated with Human Melanoma

As we have described in previous articles, there is a strong association between cancer and genetic mutations that disrupt the normal constraints placed upon cell growth and division.  In this article, evidence will be presented that links a particular set of unusual genetic mutations with human melanoma. 

Melanoma is a particularly deadly cancer of the skin.  The cells that become cancerous in melanoma are the so-called melanocytes that elaborate melanin – the pigment responsible for skin color.  The precise etiology of melanoma is not known; however, exposure to ultraviolet (UV) radiation either from natural sunlight or derived artificially from tanning beds increases the risk of developing this cancer.  The particular danger inherent in melanoma is the propensity of cancerous cells to travel from the initial site of development to other tissues of the body – a process referred to as metastasis.  It is therefore important to detect the presence of the cancerous mass before it has the opportunity to spread.

The research to date has revealed that most genetic mutations associated with various cancers seem to reside within the protein-coding regions of genes or at the splice junctions.  However Dr. Franklin W. Huang and his colleagues at the Broad Institute of Harvard and MIT, Cambridge MA were interested in determining whether any mutations consistent with tumor production appeared outside of the protein-coding regions.

To arrive at an answer to this question, the investigators performed an exhaustive analysis of whole-genome sequencing data from 70 individual cancerous melanomas.  From this analysis they discovered two independent mutations that reside within the promoter region – the promoter region of the genome lies outside of the protein coding region of the genes and is responsible for the initiation of gene transcription – for that region of the genome responsible for the production of the telemorase reverse transcriptase enzyme (TERT).  These mutations were found in 71% of the melanomas examined – this represents a remarkably high association.    In addition, they found an elevated frequency of these mutations in human bladder and liver cancer cells grown in culture.  TERT is of particular significance because this enzyme is responsible for lengthening telomeres in DNA strands and promoting cells to grow out of control.  It would, therefore, be a reasonable candidate for the mechanism of tumorigenesis. 

Single Nucleotide Polymorphisms and Intestinal Cancer

The complete sequencing of the human genome has revolutionized the study of human biology especially in relation to the understanding of the etiology of cancer.  This has been made possible by the fact that data can be accumulated from the DNA of cancer patients and studied to determine if there are any underlying patterns in regards to genetic anomalies that correlate with the different types of cancers.  Cancer results from a particular cell type growing out of control of the usual biological restraints placed upon such growth.  Cancer can arise from any tissue in the human body. 

For many years it has been known that there are certain genes that correlate with cancer development; these genes are referred to as oncogenes.  One such oncogene is referred to as myc found on human chromosome 8 (there are 23 chromosome pairs that make up the human genome – 22 pairs are so-called somatic chromosomes and 1 pair is the sex chromosomes).  It has been clearly shown that the activated product of the deregulated myc oncogene interferes with controlled cell growth and apoptosis – programmed cell death.  The net effect of these actions is uncontrolled cell growth and ultimately carcinogenesis.  Furthermore, single nucleotide polymorphisms (SNPs) – SNPs are alterations in genetic structure that represent a change in a single nucleotide – have been found upstream from the myc gene that strongly correlate with increased incidences  of different types of human cancer, including cancers of the breast, bladder and prostate.

The hypothetical causal relationship of the existence of myc-related SNPs to cancer has been extremely difficult to unambiguously confirm.  For this reason, Dr. Inderpreet Kaur Sur and his colleagues at the Science for Life Center at the Department of Biosciences and Nutrition, Karolinska Institutet in Stockholm, Sweden studied in detail the relationship between SNPs associated with the myc oncogene and intestinal tumors using the mouse model.  For the purposes of this study, the team generated mice deficient in a myc regulatory element called rs6983267.  This element is, in fact, a known SNP that is associated with more human cancer-related deaths than any other studied genetic mutation.

In addition, the investigators discovered that myc transcripts – mRNAs generated from the myc gene locus - were expressed in the intestinal crypts indicating that the myc gene was active in these genetically modified mice but at lower levels.  Most importantly, these mice proved to be remarkably resistant to the expression of intestinal tumors even when they were crossed with mice possessing the APCmin mutation – a mutation known to cause spontaneous intestinal tumors.

These results are extremely important.  They confirm the relationship between a particular SNP associated with the myc oncogene and tumorigenesis.  Although these results were obtained using the mouse as the model organism, for obvious reasons, the SNP studied has been well-established in human cancers.  Furthermore, these results show the immense benefits now being realized from the exhaustive study of the human genome; for, the etiology of cancer has been strongly linked to genetic anomalies.