Proteomics techniques for the identification of new biomarkers in chronic disease: alzheimer's disease and allergy to olive pollens as models

Abstract

The term “biomarker” can be defined as any characteristic which can be objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, orpharmacological responses to a therapeutic intervention. Biomarkers can be highly diverse, and include specific molecules such as proteins (protein biomarkers), which can be used in screening tests for diagnosis. Proteomics consists of the large-scale characterization of all the expressed proteins in a cell, tissue or living organism at a specific moment and condition, using techniques such as two-dimensional electrophoresis (2-DE), mass-spectrometry (MS) and protein microarrays. During the last years, these high-throughput screening techniques have been extensively used for the discovery of multiple protein biomarkers, as they allow for the identification and quantification of a high number of proteins in a single sample and with high sensitivity. Alzheimer’s disease (AD) is a chronic, progressive and unremitting neurodegenerative disorder affecting fundamental brain areas for memory and cognition, mainly the neocortex and the hippocampus. It is the most common form of dementia worldwide, with an estimated prevalence between 10% and 30% in the aging population which is expected to double in the next years. Two main histopathological hallmarks define AD: extracellular amyloid β peptide (Aβ) deposits known as amyloid plaques, and intracellular aggregates of hyperphosphorylated tau known as neurofibrillary tangles (NFTs). Currently, the only useful biomarkers for AD diagnosis are based on cerebrospinal fluid (CSF) analysis and neuroimaging techniques, which restricts their use to specialty clinics. Therefore, AD diagnosis is mainly based on clinical symptoms, sometimes leading to misdiagnosis. Besides, this method fails to detect the disease at early stages. To counter these problems, new biomarkers useable in primary care settings, such as blood-based biomarkers, are needed in the field and are the focus of many research groups worldwide. Allergic diseases, also known as type I hypersensitivity disorders, constitute another group of chronic diseases whose prevalence has alarmingly increased in the population of developed countries. It is estimated that up to 25% of the population of these countries suffer from respiratory allergies such as rhinitis, conjunctivitis and asthma, which involve the production of high-affinity Immunoglobulin E (IgE) antibodies against generally harmless environmental antigens known as aeroallergens. While in tropical areas mites are the main cause of respiratory allergies, in North America and Europe pollen plays a major role in allergic sensitization (pollinosis). In Spain, olive (Olea europaea L.) pollen is the second leading cause of pollinosis after grass pollen, becoming the first one in regions like Andalusia, where olive is extensively cultivated. Besides, the continuous advertising of the Mediterranean diet has boosted olive cultivation in regions of the United States, Japan, China, South Africa and South America, where it will presumably become a main allergenic source. Although olive pollen allergens have been deeply characterized during the last years, some allergens potentially relevant for the diagnosis and management of this allergy have remained unidentified due to technical restrictions and limited genomics and proteomics data...Se define como biomarcador a toda aquella característica que puede medirse de forma objetiva y que pueda servir como indicador de un proceso biológico, patológico o de una respuesta farmacológica tras una intervención terapéutica. Los biomarcadores son muy diversos, e incluyen moléculas específicas como las proteínas (biomarcadores proteicos), útiles para el diagnóstico de enfermedades. La proteómica consiste en la caracterización a gran escala de todas las proteínas de una célula, tejido u organismo vivo en un momento o condición determinada, mediante técnicas como la electroforesis bidimensional, la espectrometría de masas y los microarrays de proteínas. En los últimos años, estas técnicas de alto rendimiento han demostrado ser válidas para el descubrimiento de múltiples biomarcadores proteicos, ya que permiten la identificación y cuantificación de un alto número de proteínas a partir de una sola muestra y con gran sensibilidad. La enfermedad de Alzheimer (EA) es una patología neurodegenerativa crónica, progresiva y no reversible que afecta a áreas fundamentales para la memoria y la percepción, especialmente al neocórtex y al hipocampo. Se trata de la forma de demencia más común, con una prevalencia mundial estimada de entre el 10% y el 30% en la población de edad avanzada y que se duplicará en los próximos años. La EA se diferencia de otras demencias por la presencia de dos marcas histopatológicas bien definidas: depósitos extracelulares de péptido β-amiloide conocidos como placas amiloides; y agregados intracelulares de la proteína tau híper-fosforilada conocidos como ovillos neurofibrilares. En la actualidad, los únicos biomarcadores con validez clínica para el diagnóstico de la EA se basan en el análisis de líquido cefalorraquídeo y en técnicas de neuroimagen, por lo que su uso se restringe a centros médicos especializados. Por lo tanto, el diagnóstico de la EA se realiza principalmente en base a los síntomas clínicos, lo que a veces provoca diagnósticos erróneos. Además, este método de diagnóstico no permite detectar la enfermedad en sus fases tempranas. Para contrarrestar estos problemas, son necesarios nuevos biomarcadores que puedan utilizarse en centros de atención primaria, como los biomarcadores en sangre...