P-562 Molecular aspects of preimplantation genetic testing (PGT) for Neurofibromatosis type 1 (NF1) in three large European PGT centers: challenges and pitfalls
Authors: Vernimmen V , De Rycke M , Moutou C , Dreesen J , De Die-Smulders C , Paulussen A 
Published: July 2024

Abstract

Study question
What are the technical challenges and pitfalls of PGT for NF1 (n = 296 families) as experienced by three large European PGT centers?

Summary answer
The high (de novo) mutation rate and presence of germline mosaicism, especially in case of deletions, is challenging in PGT test development for NF1.

What is known already
NF1 is a neurocutaneous disorder with complete penetrance and extreme clinical variability. It is one of the most frequently requested autosomal dominant indications for PGT. The NF1 gene is large and has a high mutation rate. A high number of variants occur de novo, are mostly unique and more frequently associated with germline mosaicism in the index. These characteristics can pose difficulties in counseling, test development and PGT treatment. Large NF1 deletions are associated with a more severe phenotype and relative frequent mosaicism, but this has not been described in literature for intragenic deletions.

Study design, size, duration
Retrospective analysis of data on all test development procedures with the indication NF1 completed by August 2022 (n = 296). The data on test development were collected from medical files by the laboratories of the three participating PGT centers. The study was approved by the Medical and Ethical Review Committee of all three centers. The Medical Research Involving Human Subjects Act was not applicable.

Participants/materials, setting, methods
The three participating PGT centers were Maastricht University Medical Center (the Netherlands), University Hospital Brussels (Belgium) and Strasbourg University Hospital (France). Techniques applied were locus specific PCR-STR with or without the specific causative variant or genome wide haplotyping methods (NGS based OnePGT or karyomapping) or a combination of the above.

Main results and the role of chance
Test development was completed for 296 cases involving 228 different variants in the NF1 gene (125 truncating, 31 missense, 47 splice and 7 in-frame or synonymous variants, 8 larger deletions, 1 translocation, 9 awaiting further classification). We observed 36 recurring variants for 104 couples, of which 55 were sporadic cases,
The majority of the 296 cases were sporadic (60%), signs of mosaicism were observed in 14 of these cases (8%). In our cohort, 6 sporadic cases involved a complete gene deletion (n = 1) or a deletion of ≥ 1 exons in the NF1 gene (n = 5). Mosaicism was detected in 4 of these cases (67%). In one case, the mutation was not detected in sperm cells during test set-up. PGT treatment was started for 10 couples with signs of mosaicism, in 6 no affected embryos were detected in 1-2 cycles and 1 resulted in a misdiagnosis because of phasing based on unaffected offspring. Two cases had NF1 caused by a different variant in the NF1 gene than their affected family members.
A successful PGT test was developed in 289 cases (98%), 5 couples opted out during test development and in only 2 cases, development of a test was not possible for technical reasons.

Limitations, reasons for caution
The observed challenges and pitfalls depend on the techniques used and local procedures, so results may not be applicable to all PGT laboratories. However, due to the large sample size with exhaustive data and the fact that ESHRE guidelines were followed, there is probably little reason for caution.

Wider implications of the findings
Our findings could improve broader reproductive counseling of NF1 patients. Also, this large cohort contains previously unpublished variants in the NF1 gene, which is relevant for genetic diagnosis in NF1 patients in general. Our findings could possibly aid in test development for other autosomal dominant PGT indications with similar characteristics.

Trial registration number
Not applicable.