Next-Generation–Sequencing-Based Noninvasive Prenatal Diagnosis - Next Generation Sequencing Technologies in Medical Genetics - page 49

2. Chromosomal abnormalities such as unbalanced translocations, deletions, and
duplications will not be detected by NIPS.
3. NIPS is not able to distinguish specifi c forms of aneuploidy. For example, NIPS
cannot determine if Down syndrome is due to the presence of an extra chromo-
some (trisomy 21), a Robertsonian translocation involving chromosome 21, or
high-level mosaicism.
4. NIPS does not screen for single-gene mutations.
5. Uninformative test results due to insuffi cient isolation of cffDNA could lead to
a delay in diagnosis or eliminate the availability of information for risk
assessment.
6. Currently, it takes longer for NIPS test results to be returned than for test results
on maternal serum analytes.
7. NIPS does not screen for open neural tube defects.
8. NIPS does not replace the utility of a fi rst-trimester ultrasound examination,
which has been proven to be useful for accurate gestational dating, assessment
of the nuchal translucency region to identify a fetus at increased risk for a
chromosome abnormality, identifi cation of twins and higher-order pregnancies,
placental abnormalities, and congenital anomalies.
9. Limited data are currently available on the use of NIPS in twins and higher-
order pregnancies.
10. NIPS has no role in predicting late-pregnancy complications.

5.11 Summary

NGS technologies have been applied to the analysis of trisomies, namely, T21, T18,
and T13, with DNA extracted from maternal plasma (Chiu et al. 2008 ; Fan et al.
2008 ). Importantly, large cohort studies proved the effi cacy of NGS as applied to
chromosome counting in maternal plasma samples (Chiu et al. 2011 ). Improvements
on the original method have been attempted by implementation of new analysis
algorithms and exome enrichment technologies. Moreover, single molecule
sequencing for the detection of trisomy 21 has been successfully demonstrated.
Even though these are promising new methods, there are factors that affect aneu-
ploidy determination by NGS including fetal fraction level, maternal weight, and
mosaicism. In addition, there are a number of limitations of NIPS that the ACMG
has described. Therefore, continued research and agreement among several consor-
tia will determine the fate of this technology in clinical practice.

References

Avent ND, Chitty LS (2006) Non-invasive diagnosis of fetal sex; utilisation of free fetal DNA in
maternal plasma and ultrasound. Prenat Diagn 26:598-603. doi: 10.1002/pd.1493
Avent ND, Madgett TE, Maddocks DG, Soothill PW (2009) Cell-free fetal DNA in the maternal
serum and plasma: current and evolving applications. Curr Opin Obstet Gynecol 21:175-179.
doi: 10.1097/GCO.0b013e3283294798