INSIGHTS on Next-Generation Sequencing

Next-generation (NGS) sequencing brings scalability and sensitivity to diagnostics in ways that traditional DNA analysis could not

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Enabling Technology for Diagnosis, Prognosis, and Personalized Medicine

Significantly higher speed, lower cost, smaller sample size, and higher accuracy compared with conventional Sanger sequencing make next-generation sequencing (NGS) an attractive platform for medical diagnostics. By practically eliminating cost and time barriers, NGS allows testing of virtually any gene or genetic mutation associated with diseases.

Scalability and Sensitivity

NGS brings scalability and sensitivity to diagnostics in ways that traditional DNA analysis could not. “NGS analyzes hundreds of gene variants or biomarkers simultaneously. Traditional sequencing is better suited for analysis of single genes or fewer than 100 variants,” notes Joseph Bernardo, president of next-generation sequencing and oncology at Thermo Fisher Scientific (Waltham, MA).

Related Article: Computational Changes in Next-Generation Sequencing

Thermo Fisher’s Oncomine Focus Assay for NGS, for example, analyzes close to 1,000 biomarkers associated with the 52-gene panel. These biomarkers constitute about 1,000 different locations on the 52 genes that correlate with the efficacy of certain drugs. The assay allows single-workflow concurrent analysis of DNA and RNA, enabling sequencing of 35 hot-spot genes, 19 genes associated with copy number gain, and 23 fusion genes.

NGS is also better suited to detect lower levels of variants present in heterogeneous material, such as tumor samples. And while both NGS and Sanger sequencing are versatile, NGS can analyze both DNA and RNA, including RNA fusions, at a much more cost-efficient price point.

“When interrogating a limited number of analytes, Sanger sequencing is the standard for many laboratory- developed tests, offering fast turnaround times and lower cost than NGS,” Bernardo says. “We view the two methods as complementary.”

Diagnostic NGS is moving inexorably toward targeted sequencing, particularly for tumor analysis. The targets are specific regions within a tumor’s DNA or individual genes, or specific locations on single genes.

“Targeted sequencing lends itself to diagnostic testing, particularly in oncology, as the goal is to analyze multiple genes associated with cancer using a platform that offers high sensitivity, reliability, and rapid turnaround time,” Bernardo tells Lab Manager. “It is simply more cost-effective.”

That is why the National Cancer Institute (NCI) chose Thermo Fisher’s Ion Torrent sequencing system and the Oncomine reagents for NCI-MATCH, the most ambitious trial to date of NGS oncology diagnostics.

NCI-MATCH will use a 143-gene panel to test submitted tumor samples at four centers (NCI, MD Anderson Cancer Center, Massachusetts General Hospital, and Yale University). The centers then provide sequencing data that helps direct appropriate treatments.

The NCI test protocol ensures consistency across multiple instruments and sites.

Personalized Treatments

Another great opportunity for NGS-based diagnostics is in personalized or precision medicine for both new and existing drugs. Companion diagnostics—co-approved with the relevant drug—drive this entire business. “The only way personalized medicine can succeed commercially is if pharmaceutical companies incorporate a universal assay philosophy in their development programs instead of developing a unique assay for each new drug,” Bernardo explains. For example, in late 2015, Thermo Fisher partnered with Pfizer and Novartis to develop a universal companion diagnostic with the goal of identifying personalized therapy selection from a menu of drugs targeting non-small-cell lung cancer, which annually causes more deaths than breast, colon, and prostate cancer combined.

While advances in sequencing have been remarkable in recent years, the eventual success of NGS-based diagnostics will not depend on instrumentation alone. “What [ensures] ease of use and commonality of results is the cohesiveness of the entire workflow, from sample prep to rapid sequencing systems and bioinformatics,” Bernardo says. “Those components working together will drive NGS into a realizable solution for the clinical market.”

In addition to confirming a disease condition (diagnosis), NGS also provides valuable information on disease susceptibility, prognosis, and the potential effect of drugs on individual patients. The latter idea, known as precision medicine or personalized medicine, uses an individual’s molecular profile to guide treatment. The idea is to differentiate diseases into subtypes based on molecular (usually genetic) characteristics and tailor therapies accordingly.

Precision medicine is still in its infancy, but dozens of pharmaceutical, diagnostics, and genetics firms have bought into the idea.

“We are just at the beginning of connecting genomic and genetic information to target specific therapies for patients,” says T.J. Johnson, president and CEO of HTG Molecular Diagnostics (Tuscon, AZ). “Precision medicine will have a bright future as we gain better understanding of the root causes of disease.”

In 2013, HTG commercialized its HTG Edge instrument platform and a portfolio of RNA assays, which fully automate the company’s proprietary nuclease protection chemistry. This chemistry measures mRNA and miRNA gene expression levels from very small quantities of difficult-to-handle samples.

HTG entered the NGS market in 2014 with the launch of the first HTG EdgeSeq product, an assay that targets and digitally measures the expression of more than 2,000 microRNAs. The assay utilizes the HTG Edge for sample and library preparation, and it uses a suitable NGS instrument (from either Illumina or Thermo Fisher) for quantitation. The data is imported back into the HTG EdgeSeq instrument for analytics and reporting.

In 2015, the company launched four additional HTG EdgeSeq panels: immuno- oncology and pan-oncology biomarker panels, a lymphoma profiling panel, and a classifier for subtyping diffuse large B-cell lymphomas (DLBCL).

Eliminating Biopsies?

Traditional biopsies for tumor DNA analysis are invasive, risky, and often impossible to obtain, and they may not uncover the heterogeneity often present in tumors. It was recently discovered that dying tumor cells release small pieces of DNA into the bloodstream. This cell-free circulating tumor DNA (ctDNA) is detectable in samples through NGS.

In September 2015, Memorial Sloan Kettering Cancer Center (MSK) and NGS leader Illumina (San Diego, CA) entered a collaboration to study ctDNA for cancer diagnosis and monitoring. The aim is to establish ctDNA as a relevant cancer biomarker.

Heterogeneity as it pertains to cancer traditionally refers to multiple tissues located within a tumor, as determined histologically. A number of recent studies suggest that tumor heterogeneity occurs at the genetic level as well. “In particular, there appears to be a tremendous variety of sequence variants within the same tumor, even resulting in situations where one tumor can have multiple mutated genes that have been demonstrated to drive cancer,” says John Leite, PhD, vice president, oncology—market development and product marketing at Illumina.

Heterogeneity challenges the search for treatments that target a specific gene product or pathway. Once the patient is treated, biopsies tell very little about how that patient is responding. “Our hope is that ctDNA provides clinicians with a real-time measure of the abundance of those mutated genes and that a decrease in the relative abundance is synonymous with a lower tumor burden,” Leite adds.

Clinical trials are needed to demonstrate that patients whose therapy was selected using ctDNA versus traditional tissue biopsy testing had a significantly improved outcome or that the analysis might be informative for prognosis.

What about cancer cells that do not release DNA? “Studies show that tumors from different organs or tissues release more or less ctDNA into the peripheral blood,” Leite explains, “but in general the possibility that some cells might not release ctDNA is an open area of research.”

For the MSK-Illumina collaboration, the cancer center will collect samples, and Illumina will apply its sequencing technology to detect ctDNA in those samples. The big draw here is the potential to reduce the number of invasive, expensive diagnostic and monitoring procedures with a simple blood test. This would not be possible without deep next-generation sequencing—the genomics vernacular for sequencing at great depths of coverage.

“Whereas sequencing to identify germline variants can be performed at a nominal depth of coverage—for example, reading a DNA strand 30 times—sequencing rare variants such as in ctDNA requires a much higher level of sensitivity, which is driven by increasing depth of coverage [as much] as 25,000 times,” Leite tells Lab Manager.

In addition to the Illumina MSK collaboration and the work of Thermo Fisher Scientific described above, many more studies involving research consortia and pharmaceutical companies are under way.

“This is a really exciting time for oncology,” Leite says.

Reducing Sample Size

Similarly, in November 2015, Circulogene Theranostics (Birmingham, AL) launched its cfDNA (cell-free DNA) liquid biopsy products for testing ten tumor types, including breast, lung, and colon cancers. The company claims the ability to enrich circulating cfDNA from a single drop of blood.

“While all liquid biopsy companies are focusing on the downstream novel technologies to selectively enrich or amplify tumor-specific cfDNA from a dominantly normal population, the upstream 40 to 90 percent material loss during cfDNA extraction leads to potential false negative results of cancer mutation detection,” explains Chen Yeh, Circulogene’s chief scientific officer. “This is why 10 to 20 mL of blood [are] generally required for conventional cfDNA liquid biopsies.”

Related Article: Researcher Using Next-Generation Sequencing, Other New Methods to Rapidly Identify Pathogens

Released cfDNA fragments often complex with proteins and lipids, which shift their densities to values much lower than those of pure DNA or protein while protecting the corresponding cfDNA from attack by circulating nucleases. Circulogene’s cfDNA breakthrough concentrates and enriches these genetic fragments through density fractionation followed by enzyme-based DNA modification and manipulation, eliminating extraction-associated loss. The technology ensures near-full recovery of both small-molecular-weight (apoptotic cell death) and high-molecular-weight (necrotic cell death) cell-free DNA species from droplet volumes of plasma, serum, or other body fluids.

“The 50-gene panel is our first offering,” says Yeh. “We will continue to develop and cover more comprehensive, informative, and actionable genes and tests.”

The current bottleneck in personalized and precision medicine is the severe shortage of anticancer drugs. Yeh provides perspective, saying, “We have about 60 FDA-approved drugs for cancer-targeted therapies on market, while there are approximately 150 cancer driver genes to aim for. If counting all mutations within these 150 genes, the numbers will be overwhelming.”

Circulogene’s cell-free DNA enrichment technology may be followed up with NGS, conventional Sanger sequencing, or any DNA assay based on PCR or mass spectrometry. However, the sensitivity of Sanger sequencing is insufficient for detecting variants with volumes below 15 percent. Moreover, the company’s multiplex NGS liquid biopsy test captures and monitors real-time, longitudinal tumor heterogeneity or tumor clonal dynamic evolution, which goes well beyond testing of a single mutation on a single sample in traditional sequencing.

Categories: INSIGHTS

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Published: March 11, 2016

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