Expanding Access to Noninvasive Prenatal Diagnosis for Monogenic Conditions to Consanguineous Families.

BACKGROUND: Cell-free fetal DNA exists within the maternal bloodstream during pregnancy and provides a means for noninvasive prenatal diagnosis (NIPD). Our accredited clinical service offers definitive NIPD for several autosomal recessive (AR) and X-linked conditions using relative haplotype dosage analysis (RHDO). RHDO involves next-generation sequencing (NGS) of thousands of common single nucleotide polymorphism (SNPs) surrounding the gene of interest in the parents and an affected or unaffected offspring to conduct haplotype phasing of the high- and low-risk alleles. NGS is carried out in parallel on the maternal cell-free DNA, and fetal inheritance is predicted using sensitive dosage calculations performed at sites where the parental genotypes differ. RHDO is not currently offered to consanguineous couples owing to the shared haplotype between parents. Here we test the expansion of RHDO for AR monogenic conditions to include consanguineous couples. METHODS: The existing sequential probability ratio test analysis pipeline was modified to apply to SNPs where both parents are heterozygous for the same genotype. Quality control thresholds were developed using 33 nonconsanguineous cases. The performance of the adapted RHDO pipeline was tested on 8 consanguineous cases. RESULTS: The correct fetal genotype was predicted by our revised RHDO approach in all conclusive cases with known genotypes (n = 5). Haplotype block classification accuracies of 94.5% and 93.9% were obtained for the nonconsanguineous and consanguineous case cohorts, respectively. CONCLUSIONS: Our modified RHDO pipeline correctly predicts the genotype in fetuses from consanguineous families, allowing the potential to expand access to NIPD services for these families.

Noninvasive prenatal diagnosis for single gene disorders.

PURPOSE OF REVIEW: Noninvasive prenatal diagnosis for single gene disorders is coming to fruition in its clinical utility. The presence of cell-free DNA in maternal plasma has been recognized for many years, and a number of applications have developed from this. Noninvasive prenatal diagnosis for single gene disorders has lagged behind due to complexities of technology development, lack of investment and the need for validation samples for rare disorders. RECENT FINDINGS: Publications are emerging demonstrating a variety of technical approaches and feasibility of clinical application. Techniques for analysis of cell-free DNA including digital PCR, next-generation sequencing and relative haplotype dosage have been used most often for assay development. Analysis of circulating fetal cells in the maternal blood is still being investigated as a viable alternative and more recently transcervical trophoblast cells. Studies exploring ethical and social issues are generally positive but raise concerns around the routinization of prenatal testing. SUMMARY: Further work is necessary to make testing available to all patients with a pregnancy at risk of a single gene disorder, and it remains to be seen if the development of more powerful technologies such as isolation and analysis of single cells will shift the emphasis of noninvasive prenatal diagnosis. As testing becomes possible for a wider range of conditions, more ethical questions will become relevant.

Clinical Service Delivery of Noninvasive Prenatal Diagnosis by Relative Haplotype Dosage for Single-Gene Disorders.

A relative haplotype dosage (RHDO)-based method was developed and implemented into routine clinical practice for noninvasive prenatal diagnosis (NIPD) of multiple single-gene disorders: spinal muscular atrophy, Duchenne and Becker muscular dystrophies, and cystic fibrosis. This article describes the experiences of the first 152 pregnancies to have NIPD by RHDO as part of a routine clinical service. Provision of results within a clinically useful time frame (mean, 11 calendar days) was shown to be possible, with a very low failure rate (4%), none being due to a technical failure. Where follow-up confirmatory testing was performed for audit purposes, 100% concordance was seen with the NIPD result, and no discrepancies have been reported. The robust performance of the assay, together with high sensitivity and specificity, demonstrates that NIPD by RHDO is feasible for use in a clinical setting.

Non-Invasive Prenatal Diagnosis of Retinoblastoma Inheritance by Combined Targeted Sequencing Strategies.

Retinoblastoma, the most common childhood eye cancer, presents in two forms: heritable or sporadic. Heritable retinoblastoma is caused by a germline mutation in the RB1 gene. Early diagnosis of children at risk of inheriting an RB1 mutation is crucial to achieve optimal clinical outcome. Currently, the majority of genetic testing is performed on newborns, which has multiple disadvantages for both families and the healthcare system. We have developed a non-invasive prenatal diagnosis (NIPD) service for retinoblastoma, available from 8 weeks' gestation, which uses a combination of massively parallel sequencing (MPS) techniques, dependent on the inheritance model. Detection of paternal or suspected de novo RB1 variants is achieved through amplicon-based MPS. NIPD of a fetus at risk of maternal inheritance is performed using capture-based targeted sequencing and relative haplotype dosage analysis. In addition, we show proof of principle of how capture-based sequencing can be used for de novo variants unsuitable for amplicon-based testing. In total, we report the NIPD of 15 pregnancies, results of which show 100% concordance with all postnatal testing performed at the time of publication (n = 12) with remaining pregnancies ongoing. NIPD of retinoblastoma therefore offers a viable alternative to newborn genetic testing.

Non-invasive prenatal diagnosis of spinal muscular atrophy by relative haplotype dosage.

Although technically possible, few clinical laboratories across the world have implemented non-invasive prenatal diagnosis (NIPD) for selected single-gene disorders, mostly owing to the elevated costs incurred. Having previously proven that NIPD for X-linked disorders can be feasibly implemented in clinical practice, we have now developed a test for the NIPD of an autosomal-recessive disorder, spinal muscular atrophy (SMA). Cell-free DNA was extracted from maternal blood and prepared for massively parallel sequencing on an Illumina MiSeq by targeted capture enrichment of single-nucleotide polymorphisms across a 6 Mb genomic window on chromosome 5 containing the SMN1 gene. Maternal, paternal and proband DNA samples were also tested for haplotyping purposes. Sequencing data was analysed by relative haplotype dosage (RHDO). Six pregnant SMA carriers and 10 healthy pregnant donors were recruited through the NIPSIGEN study. Inheritance of the maternally and paternally derived alleles of the affected SMN1 gene was determined in the foetus by RHDO analysis for autosomal-recessive disorders. DNA from the proband (for SMA carriers) or an invasively obtained foetal sample (for healthy pregnant donors) was used to identify the maternal and paternal reference haplotypes associated with the affected SMN1 gene. Results for all patients correlated with known outcomes and showed a testing specificity and sensitivity of 100%. On top of showing high accuracy and reliability throughout the stages of validation, our novel test for NIPD of SMA is also affordable and viable for implementation into clinical service.