Listen to the Clinical Chemistry Podcast
Article
Rui Zhang, et al. Generation of Highly Biomimetic Quality Control Materials for Noninvasive Prenatal Testing Based on Enzymatic Digestion of Matched Mother–Child Cell Lines. Clin Chem 2019;65:761-70.
Guests
Drs. Jinming Li and Rui Zhang of the Department of Immunoassay and Molecular Diagnosis at Beijing Hospital.
Transcript
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Bob Barrett:
This is a podcast from Clinical Chemistry, sponsored by the Department of Laboratory Medicine at Boston Children’s Hospital. I am Bob Barrett.
Non-invasive prenatal testing based on cell-free DNA is now
a widely used technique. However, quality control materials
that have properties identical to clinical samples and that
are applicable to a wide range of procedures are not
available to support assay development, internal or external
quality control, and proficiency testing.
The June 2019 issue of Clinical Chemistry includes a study
describing the development of such quality control materials
that comprise simulated human plasma and mixtures of
mother cell line derived cell-free DNA based on DNA
fragmentation factor digestion.
Today, we are joined by two authors of that study, Dr.
Jinming Li serves as Associate Director of the National
Center for Clinical Laboratories and Director of the
Department of Immunoassay and Molecular Diagnosis at
Beijing Hospital.
He is joined by his colleague, Dr. Rui Zhang, who is
Associate Professor in the same department. And we will
start with you, Dr. Li. Before we get into the generation of
quality control materials, how are they usually produced and
used? And how do quality control materials for non-invasive
prenatal testing differ from other types of laboratory
testing?
Jinming Li:
Quality control materials have one or more well-established
properties which are homogenous and stable and fit for
measurement. For NIPT, quality control materials are
homogenous and stable samples of normal chromosomal
abnormalities with different fetal fraction. Quality control
materials are used by clinical laboratories for assay
development, assay validation, internal quality control, and
proficiency testing to assure quality of their performance.
As we know, the development of a new drug usually
requires in vitro models such as cell lines, animal models to
demonstrate the effects and side effects, and then clinical trials to approve the validity in patients. Diagnostic assays
are somewhat similar. For NIPT, is the established method
accurate and reliable, including the steps of blood collection,
transport, storage, cfDNA extraction, and analysis? Quality
control materials with known results need to be analyzed
before the assays are used in clinical testing. It is essential
to demonstrate the diagnostic performance, including
precision, accuracy, sensitivity, and specificity. If the
performances are not satisfactory, the assays should be
adjusted. This process is assay development and assay
validation. Quality control materials are also required for
tests to monitor the reliability of each batch in daily
detection. Errors may occur in all the procedures, especially
during the handling of operator and the work of equipment.
Positive controls can confirm successful pipelines, such as
cfDNA extraction, library construction and so on. Negative
controls can identify contamination. This is the internal
quality control. External quality assessment organizers
distribute quality control materials to clinical laboratories,
evaluate the detecting capacity of laboratories based on the
degree of consistency between laboratory results and
expected results, in order to promote awareness of causes
and improvement. This process is also called proficiency
testing.
Bob Barrett:
So, the quality control is very important during clinical tests.
Dr. Zhang, how did the idea of this novel quality control
material come about?
Rui Zhang:
One of the main problems we encountered in NIPT
standardization is the lack of proper quality control materials
used for monitoring the performance over time. And
apparently, a large amount of such characterized samples is
needed for regular application in clinical practice. Whereas,
currently the quality control materials for NIPT available
include natural maternal plasma samples, and simulated
samples based on fragmentation of human genomic DNA.
As we know, maternal plasma with a particular
chromosomal abnormality is very limited, and shearing-based
DNA products have dissimilar biological properties
from those of circulating cfDNA, with differing characteristic
size profile of circulating fragmented nucleosomes. Besides,
the synthetic samples are not matched for mother-child
relationships and therefore, do not allow single-nucleotide,
polymorphism-based methods.
So, what can we do next? Many studies have revealed that
the mechanism for cfDNA origins is related to apoptosis.
Therefore, we envisaged if cfDNA could be generated
through simulating the process of apoptosis. We then
examined the mechanism of apoptosis and found that DNA fragmentation factor prefers to cleave in the
internucleosomal linker region, which generates a size
distribution similar to maternal cfDNA.
Meanwhile, MNase produces mononucleosomes and then
proceeds to trim the DNA of nucleosome monomers to the
core particles with 146 base pair, it is similar to fetal cfDNA.
By this way, we created materials based on paired mother-child cell lines, just as they would be in real NIPT samples.
Bob Barrett:
Okay, that’s very impressive. What quality control materials
have your team designed before? For example, quality
control materials and other tests related to molecular
diagnostics.
Rui Zhang:
We have developed a set of cfDNA quality control materials
based on MNase digestion and matched genomic DNA,
which was published in Clinical Chemistry back in 2017. It is
mainly used in liquid biopsy for cancer, aiming at somatic
mutation detection, covering a wide range of oncogenic
alterations ranging from single-nucleotide changes to
rearrangements. Because MNase can digest the DNA
between nucleosomes, it can produce a size profile
analogous to cfDNA in patients with cancer.
The quality control materials meet the requirements of
various ctDNA testing methods, including NGS. Using the
quality control materials, we have provided 3 runs of
proficiency testing in recent 3 years, and around 400
laboratories have participated in this scheme, which would
be challenging if we depend on clinical plasma samples from
patients with tumor. Laboratories have validated their
methods and implemented internal quality control
procedures with the materials, which is critical for ensuring
laboratory quality and patient safety.
This exploration to create cfDNA quality control materials
have laid a foundation for us to prepare simulated maternal
plasma free DNA in this research. In addition, our team
have generated quality control products for molecular
testing. These include molecular oncology for solid tumors,
hematologic oncology, pharmacogenetics, genetic testing,
and microbiology. The materials cover biological samples
and bioinformatics files for NGS. Accordingly, proficiency
testing programs are also provided for clinical laboratories
to improve their performance.
Bob Barrett:
Finally, Dr. Li, what do you see in the future for your
biomimetric quality control materials?
Jinming Li:
As for future applications, we think their potentials are not
limited in fetal aneuploidy detection of NIPT, they’re going
to shine in other non-invasive areas as well. In a similar approach, as previously described, the quality control
product can also be applied to other NIPT-related fields,
such as single genetic disease. We’re not just talking about
envisioning. We’re now working in this direction and believe
there will be new progress. Considering the scarce positive
samples of noninvasive prenatal diagnostics for single gene
disorders, the quality control materials created will play
important role in promoting the detection in this field.
Therefore, we hope to find solutions to the problems in this
area as soon as possible to promote its faster clinical
application, for the benefit of people's health.
Bob Barrett:
That was Dr. Xiu-Min Li, Associate Director of the National
Center for Clinical Laboratories and Director of the
Department of Immunoassay and Molecular Diagnosis at
Beijing Hospital. He was joined by his colleague, Dr. Ri
Zhang, from the same institution. They have been our
guests in this podcast on the development of a new type of
quality control material for cell-free DNA testing. Their
paper describing that approach appears in the June 2019
issue of Clinical Chemistry. I'm Bob Barrett. Thanks for
listening!