Introduction

The field of human molecular genetics has advanced significantly revealing many gene-based disease mechanisms in various areas of medicine. This is why the study of both diagnostic and prognostic markers are important to help translate new findings in basic science and apply it to clinical practice. The increasing use of new techniques in molecular biology has revolutionized the investigation of pathogenesis and disease progression. It is important to understand the fundamental molecular mechanisms involved in the progression of normal to malignant tissue as it may lead to improved detection and treatment for cancer. Studies have found multiple novel markers which are mostly at the gene level. Authentication of these markers via standard techniques can be time-consuming, costly, and labor intensive. Tissue microarray is a technique used in the field of pathology to overcome these significant issues. Designed as a high-throughput molecular biology technique, it allows the simultaneous assessment of expression in interesting candidate disease-related genes on hundreds of tissue samples. It also allows molecular profiling at the DNA, RNA, and protein level. Tissue microarray is a technique that enables pathologists to perform large-scale analyses using RNA in situ hybridization, fluorescence in situ hybridization, and immunohistochemistry at lower costs and faster duration. 

History

The technique was first reported by Battifora who described a method where he wrapped 1millimeter rods of tissues in small intestine sheets which were subsequently embedded in a paraffin block. This was then cut and examined. In 1987, the array format was conceived by Wan and colleagues. While there is a significant advantage of being able to simultaneously examine multiple specimens under the same conditions, there is the inability to identify individual rods resulting in unmeaningful interpretation. However, in 1998, Kononen et al were able to address this issue by inventing a device that could rapidly and accurately construct tissue microarrays in a way that is accessible to most pathology labs. This subsequently led to a dramatic increase in the popularity of the tissue microarray technique. 

Tissue Microarray Technique

The tissue microarray technique is useful in the organization of minute amounts of biological samples on a solid support. They are composite paraffin blocks that are constructed through the extraction of cylindrical tissue core “biopsies” from other paraffin donor blocks which are then re-embedded into a single microarray block. The donor blocks are first retrieved and sectioned to produce standard microscopic slides which are then stained with hematoxylin and eosin. Next, it is examined by a pathologist to mark the area of interest. The samples are then arrayed. 

A tissue microarray instrument is used to obtain a tissue core from the donor block. It is then placed in the recipient block, an empty paraffin block. The core is then placed at a specific coordinate which is recorded on a spreadsheet. The sampling process is repeated using different donor blocks until they are all placed into one recipient block. This produces the final tissue microarray block. The tissue microarray is then sectioned using a microtome to generate tissue microarray slides for molecular and immunohistochemical analyses. This method allows an entire cohort of cases to be analyzed by staining one or two master array slides. 

Advantages and Applications of Tissue Microarrays

There are many advantages and applications of tissue microarrays compared to other standard techniques. This includes:

• The amplification of a scarce resource

• Experimental uniformity

• Simultaneous analysis of large numbers of specimens

• Conservation of valuable tissue as the technique does not destroy the original block

• Decreases the time, cost, and assay volume

• Facilitates the standardization of fluorescence in situ hybridization, immunohistochemical, and other molecular assays allowing results to be reproducible between laboratories. 

• Can be used for internal quality control and optimization of diagnostic reagents.

• Facilitates the translation of new molecular discoveries to clinical applications.

• Clinical validation of newly identified genes in histopathological specimens. 

• Screening for presence or absence of novel markers in multi tumor arrays. 

• Assessment of molecular and morphological changes in tumor progression microarrays.

• Assessment of prognosis or patient outcome in prognostic arrays. 

Tissue Heterogeneity and Tissue Microarray Disadvantages

One of the commonest criticisms of the tissue microarray technique is that the sampled cores may not represent the entire tumor especially in heterogeneous cancers such as Hodgkin lymphoma and prostate adenocarcinoma. However, there are many groups that have proven excellent concordance between the spots and whole sections in immunohistochemical studies involving multiple tumor types. Another minor disadvantage of the technique is the absence of some core sections on the immunostained slide. While this may occur, the statistical analysis of many other cases eliminates the effect of a single data point in the conclusion. 

Conclusion

The tissue microarray technique is practical and effective for high throughput molecular analysis of tissues in the identification of new diagnostic and prognostic markers in human cancers. With varying degrees of use and range of potential applications, this technique is anticipated to become a widely used tool for various types of tissue-based research. It is believed that this technique will lead to a significant acceleration in the process of translating basic research findings into clinical applications. 

References:

1. Jawhar NMT. Tissue microarray: a rapidly evolving diagnostic and research tool. Ann Saudi Med. 2009; 29(2): 123-127. 

2. Tissue Microarray: An Evolving Diagnostic and Research Tool. Accessed 3/13/2019. https://www.geneticistinc.com/blog/tissue-microarray-an-evolving-diagnostic-and-research-tool

3. Galdiero, Maria, et al. “Potential Involvement of Neutrophils in Human Thyroid Cancer.” PLoS One, vol. 13, no. 6, Public Library of Science, June 2018, p. e0199740.