Fetal Genotyping in Maternal Blood by Digital PCR: Towards NIPD of Monogenic Disorders Independently of Parental Origin.

PURPOSE: To date, non-invasive prenatal diagnosis (NIPD) of monogenic disorders has been limited to cases with a paternal origin. This work shows a validation study of the Droplet Digital PCR (ddPCR) technology for analysis of both paternally and maternally inherited fetal alleles. For the purpose, single nucleotide polymorphisms (SNPs) were studied with the only intention to mimic monogenic disorders. METHODS: NIPD SNP genotyping was performed by ddPCR in 55 maternal plasma samples. In 19 out of 55 cases, inheritance of the paternal allele was determined by presence/absence criteria. In the remaining 36, determination of the maternally inherited fetal allele was performed by relative mutation dosage (RMD) analysis. RESULTS: ddPCR exhibited 100% accuracy for detection of paternal alleles. For diagnosis of fetal alleles with maternal origin by RMD analysis, the technology showed an accuracy of 96%. Twenty-nine out of 36 were correctly diagnosed. There was one FP and six maternal plasma samples that could not be diagnosed. DISCUSSION: In this study, ddPCR has shown to be capable to detect both paternal and maternal fetal alleles in maternal plasma. This represents a step forward towards the introduction of NIPD for all pregnancies independently of the parental origin of the disease.

Non-invasive prenatal diagnosis of single-gene disorders from maternal blood.

Prenatal diagnosis (PD) is available for pregnancies at risk of monogenic disorders. However, PD requires the use of invasive obstetric techniques for fetal-sample collection and therefore, involves a risk of fetal loss. Circulating fetal DNA in the maternal bloodstream is being used to perform non-invasive prenatal diagnosis (NIPD). NIPD is a challenging discipline because of the biological features of the maternal blood sample. Maternal blood is an unequal mixture of small (and fragmented) amounts of fetal DNA within a wide background of maternal DNA. For this reason, initial NIPD studies have been based on the analysis of specific paternally inherited fetal tracts not present in the maternal genome so as to ensure their fetal origin. Following this strategy, different NIPD studies have been carried out, such as fetal-sex assessment for pregnancies at risk of X-linked disorders, RhD determination, and analysis of single-gene disorders with a paternal origin. The study of the paternal mutation can be used for fetal diagnosis of dominant disorders or to more accurately assess the risk of an affected child in case of recessive diseases. Huntington's disease, cystic fibrosis, or achondroplasia are some examples of diseases studied using NIPD. New technologies are opening NIPD to the analysis of maternally inherited fetal tracts. NIPD of trisomy 21 is the latest study derived from the use of next-generation sequencing (NGS).

Noninvasive prenatal diagnosis of monogenic disorders.

INTRODUCTION: Since the presence of circulating cell-free fetal DNA (ccffDNA) in maternal peripheral blood was demonstrated in 1997, great efforts have been done in order to use this source of fetal material for noninvasive prenatal diagnosis. The advantage that it represents is avoiding the obstetric invasive procedures required for conventional prenatal diagnosis. AREAS COVERED: Efforts are mainly focused on finding the most accurate way to diagnose the most common fetal aneuploidies, paying special attention to trisomy 21. Recent advances in technology offer new diagnostic tools with high degrees of sensitivity thus generating great expectations for this type of diagnosis. However, there are other reasons why pregnant women undergo conventional prenatal diagnosis. Being at risk of transmitting a monogenic disorder is one of them. And although the percentage of those pregnancies may represent a small percentage of the diagnosis performed in the first trimester, these numbers should not be underestimated. EXPERT OPINION: Management of pregnancies at risk of an X-linked Mendelian disorder has changed thanks to the noninvasive fetal sex assessment. As for other Mendelian disorders, until recently, their study was limited to those cases paternally inherited. Nevertheless, the new emerging technologies are also opening the scope to maternally inherited disorders.