Yes or no? If yes, how much? Perfecting diagnostics for health monitoring is a quick win for human healthcare challenges in SA. Newly patented aptamer-based biorecognition agents for malaria and CD4 monitoring may open fresh opportunities for rapid diagnostics in the sector. For South Africa, the latter is particularly relevant, opening up the door to on-site diagnostics of HIV positive patients’ CD4 count.
Laboratory-based methods are already well-suited to providing many of these answers but when time is a factor or when routine monitoring is needed, rapid, cheap tests can save lives or improve quality of life. For clinics conducting on-site HIV testing, the results from crucial CD4 counts can take days. On-site diagnostics could spark a major shift in HIV care management. The same applies for rapid malaria diagnosis.
While there are several issues critical to their design, one of the big factors slowing the rollout of rapid diagnostics for a range of illnesses is centred on the biorecognition agent. This is really the heart of the sensor, the biological component (such as an antibody) capable of specifically recognising its target (e.g. antigen). For a sensor with real marketability and extended shelf-life, the device needs to be able to withstand fluctuations in temperatures within a certain range and be able to retain its activity for long periods. The limiting factor then in the realisation of such diagnostics is the access to low cost, selective and stable biorecognition agents.
Many scientists believe that molecules known as aptamers can play a lasting role. The word aptamer comes from the Latin word apto meaning “to fit”. These molecules are designed to “fit” a specific analyte or target. There are two main classes of aptamers – nucleic acid (DNA and RNA) aptamers and peptide aptamers. Focusing just on DNA and RNA aptamers here, they typically consist of between 20 – 80 nucleotides bases. Nucleotides are of course the building blocks of DNA and RNA. The beauty of these is that they can be engineered in established processes such that they are able to bind to a wide array of different targets.
Research recently patented by Rhodes University scientists from the institution’s Biotechnology Innovation Centre offers new opportunities for rapid diagnostics.
The group, led by Prof Janice Limson and managed by Dr Ronen Fogel, has patented several aptamers that show specificity for targets in malaria and HIV monitoring. For malaria, the group has patented sequences binding to the lactate dehydrogenase enzyme of Plasmodium falciparum, working in partnership with the CSIR and Prof Dean Goldring at the University of KwaZulu Natal. This three year long study forms the PhD project of student Kelly-Anne Frith.
In other research conducted by DST/Mintek Nanotechnology Innovation Centre funded student, Mary Cromhout, breakthrough research has led to the patenting of DNA aptamers that could revolutionise testing of critical CD4 levels. The group believes that it may lead to diagnostics that could offer on-site testing at clinics already offering HIV testing, to allow for rapid quantification of CD4 for effective ARV treatment.
Once specific and selective aptamers are generated, they can be readily incorporated into different sensors designs. Most people would be familiar with the so-called lateral flow sensors, devices which for the most part provide “yes or no”, or qualitative, answers. Urine strips for pregnancy tests, for example, provide a yes or no result when Human Chorionic Gonadotropin, a specific protein produced by the fertilized egg, is present and interacts with an antibody bound to the strip to give a coloured line. For HIV testing in clinics, a similar approach is followed searching for specific anti-HIV antibodies found in blood. But for those with diabetes – a yes or a no result is not sufficient – the amount, or concentration, of glucose in blood is critical. Electrochemical sensors are frequently used here, providing a quantitative result and could play a role in providing quantitative results needed for CD4 testing.
For both quantitative and qualitative sensor design, the application of nanomaterials brings the necessary sensitivity to help drive these into the commercial sector, a strong focus for Rhodes University’s Biotechnology Innovation Centre.
Writer: Janice Limson