Emerging biosensor technology for rapid TB diagnosis

The aptamer technology research team. In front is Dr Makobetsa Khati (group leader), flanked by intern Charlotte Maserumule. At the back are senior researchers Lionel Gresh and Jabulani Nhlapo.

 

Tuberculosis remains a major public health problem. Approximately 1.7 billion people worldwide are infected with TB, with 8 million new cases and 3 million deaths per year. It is estimated that 35 million people will have died of TB by the year 2020.

In 2004, it was estimated that more than 4% of the world’s infected people living with active TB were in South Africa. During this period, South Africa accounted for about 2% of the world’s new TB cases and approximately 3% of the total TB deaths. Most deaths occur mainly because the infection goes undiagnosed, or is diagnosed too late to be cured.

The rapid and accurate diagnosis of patients is therefore the cornerstone of global strategies for TB control. While progress in TB diagnostics has been made in developed countries, in the rest of the world the techniques used for diagnosing TB have remained relatively unchanged for the past century.

Many of the existing tests are slow and lack sensitivity. For example: sputum smear microscopy is insensitive; the culture method is technically complex and slow; chest radiography is non-specific, and the tuberculin skin test is imprecise and the results non-specific. The purified protein derivative (PPD) test is technically simple, relatively cheap, and reproducible if performed by skilled personnel. However, it lacks specificity in individuals vaccinated with Mycobacterium bovis Bacillus Calmette-Guérin (BCG) and its sensitivity in detecting active TB is limited to 75%. In addition, negative results are often associated with extensive disease. It is also reported in individuals vaccinated with BCG or exposed to environmental mycobacterium, that PPD skin testing is found to be unreliable due to cross-reactive (false positive) immune responses to antigens common to Mycobacterium tuberculosis (MTB) and non-pathogenic mycobacterium.

Alternatively, T-cell assay may be used, but the requirements for in vitro diagnosis are more complex and include highly purified MTB-specific antigens and facilities for cell culture and cytokine assays. Hence its use as a routine diagnostic tool is limited. Despite the huge need for new, improved, affordable and readily accessible point-of-care TB diagnostics, there is generally a lack of interest. Moreover, the high-technology, rapid tests available in developed countries – where less than 5% of global TB cases are found – are too complex and costly for the resource-poor settings where TB is most prevalent.

Although an estimated US$1 billion is spent annually around the world on TB diagnostics, World Health Organization statistics of 2006 also indicate that only one-third of this is spent in low or middle-income countries where 73% of TB diagnostic testing takes place. A rapid, more sensitive and specific diagnostic test needs to be developed as a matter of urgency to enable early and accurate diagnosis to help control the spread of the disease, especially in developing countries. For these reasons, the CSIR is currently exploiting the high affinity, sensitivity and specificity of synthetic nucleic acid ligands – called aptamers – to develop a simple, inexpensive, rapid and accurate point-of-care TB diagnostic prototype kit.

Aptamer technology is a relatively new technology that emerged in the 1990s with a wide range of potential applications, including drug discovery, therapeutics and diagnostics.

Aptamers are selected according to their ability to bind to a target molecule, which varies depending on the application. Also known as chemical antibodies, aptamers are emerging as a class of molecules that rivals conventional antibodies in both therapeutic and diagnostic applications.

Like antibodies, aptamers recognise a target with high affinity and specificity, but can also discriminate between very subtle structural differences. The main competitive advantage of aptamers over conventional approaches includes their high specificity, high sensitivity, relatively low production costs, convenience and simplicity, which allows for rapid point-of-care diagnosis.

Following the establishment of the CSIR aptamer technology group in 2006 under the leadership of Dr Makobetsa Khati, and the subsequent launch of a TB project in 2007, significant progress has been made in identifying aptamers that can be used in the diagnosis of TB. The group’s first phase of research has focused on selecting suitable aptamers that bind to relevant TB antigens. These target proteins are secreted by the TB bacteria in infected patients, so their presence indicates active TB infection.

Dr Khati explains that “aptamers could potentially fill the current gap in the availability of optimum TB diagnostic kits by providing enhanced molecular recognition of biomarkers of active TB infection.” He adds, “the envisaged aptamer-based point-of- care TB diagnostic kit will be relevant for resource- poor settings because it embraces important considerations such as sensitivity, specificity, affordability by those at risk of infection, ease of performance and reliability.”

The next stage of research in developing this technology platform, funded by BioPAD for three years, will enable further characterisation of the aptamers identified, leading to the development of a novel, aptamer-based, TB diagnostic prototype kit. Once the prototype kit has been validated, optimised and benchmarked, it is set to be packaged and commercialised through a spin-off company.

The initial phase of funding was supported jointly by the CSIR and DST, including additional multi-million rand funding from the DST for the establishment of a world-class P3 containment facility, required for work with clinical isolates of TB. The containment level 3-facility will be commissioned by the end of the year.

This research group is the only one of its kind focusing on aptamer technology in Africa and one of the few in the world. If successful, this TB diagnostic kit could significantly impact the lives of South Africans and contribute to the development of the South African biotechnology sector.

Article courtesy of the CSIR ScienceScope www.csir.co.za

May 2010