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Clinical Handbook › Genetics & Family Planning

Genetics, Mosaicism, and Family Planning (PGT-M) in Neurofibromatosis

Autosomal dominant transmission probabilities, why half of all cases arise spontaneously with unaffected parents, clinically actionable NF1 genotype-phenotype correlations, and reproductive testing options.

Clinical Genetics & Preconception Counseling Reference
Genetic counselor tracing an autosomal dominant family pedigree diagram on white paper with an adult couple

1. Inheritance Probabilities: Inherited vs De Novo & Mosaic NF

Transmission Probability: Each pregnancy of an adult with constitutional (full) NF1 or NF2-related schwannomatosis carries an independent 50% (1 in 2) chance of inheriting the pathogenic variant. Conversely, when a child has a confirmed de novo variant and both parents test negative with no clinical signs, sibling recurrence risk drops to roughly 1% to 2% (attributable solely to rare parental gonadal mosaicism).
Parent with NF1 (N/v) Unaffected Parent (N/N) 50% Inherit Variant (N/v) ~100% penetrance in NF1 50% Unaffected (N/N) Cannot pass variant onward Clinical note: ~50% of newly diagnosed cases arise de novo (evaluate parental mosaicism).
Figure 1. Autosomal dominant Punnett transmission in constitutional NF1/NF2-SWN. Inheriting the unaltered chromosome completely breaks transmission to future generations.

Segmental / Mosaic NF1 and NF2-SWN: When a post-zygotic mutation occurs during embryonic development rather than in the egg or sperm, clinical signs may be confined to one body quadrant (segmental CALMs or localized neurofibromas) and routine blood leukocyte DNA sequencing often returns normal unless deep NGS sequencing is performed on affected skin/tumor tissue. Offspring transmission risk in mosaic individuals is lower than 50% (typically 1%–12% depending on gonadal involvement), Note however that if a child does inherit the variant from a mosaic parent, the child will carry constitutional (non-mosaic) NF in every cell.

2. Clinically Actionable NF1 Genotype-Phenotype Correlations

While the majority of the >3,000 known NF1 variants show variable intrafamilial expressivity, five specific genotypes carry strong predictive weight that directly alters surveillance:

Specific NF1 Variant Distinct Clinical Phenotype Surveillance Impact
1.4 Mb NF1 Whole-Gene Microdeletion (~5% of cases) Early heavy plexiform & cutaneous burden, tall stature, congenital heart defects, intellectual disability, 16%–26% lifetime MPNST risk (co-deletion of SUZ12) Heightened sarcoma vigilance, baseline whole-body MRI, early developmental support, cardiology echocardiogram.
p.Met992del (3-bp in-frame deletion) & p.Arg1809 missense Pigmentary signs (CALMs + freckling) ± Noonan features, virtually zero cutaneous or plexiform neurofibromas Major prognosis reassurance for families; standard ophthalmic check still maintained in childhood.
Codons 844–848 Missense Variants Elevated prevalence of symptomatic spinal/plexiform neurofibromas, optic gliomas, and skeletal dysplasia Closer spinal and volumetric MRI monitoring.

3. Preconception Counseling, IVF with PGT-M, and Prenatal Testing

Prospective parents where either partner carries a known pathogenic NF1, NF2, SMARCB1, or LZTR1 variant have several reproductive pathways—requiring pinpointing the exact familial DNA variant prior to conception (comprehensive RNA + DNA sequencing identifies >95% of constitutional NF1 mutations):

  • In Vitro Fertilization with Preimplantation Genetic Testing for Monogenic Disorders (PGT-M): Blastocyst trophectoderm biopsy at Day 5/6 allows transfer strictly of embryos confirmed negative for the familial variant (>98%–99% diagnostic accuracy when paired with linkage haplotyping).
  • Prenatal Diagnostic Testing: Chorionic villus sampling (CVS at 11–13 weeks) or amniocentesis (15–20 weeks) tests fetal DNA for the known familial mutation; note that prenatal DNA testing confirms presence/absence of the condition but cannot predict mild versus complex clinical severity.
  • Maternal Pregnancy Care in NF1: Pregnant women with NF1 may experience growth or new eruption of cutaneous neurofibromas and require careful blood pressure monitoring (pre-eclampsia overlap and pre-delivery exclusion of pheochromocytoma plus lumbar spine MRI check if epidural analgesia is planned and lumbar neurofibromas are suspected).

Frequently Asked Clinical Questions

If neither parent has signs of NF1, what is the chance their next child will also have NF1?

When a child's NF1 arises from a spontaneous (de novo) variant and both parents have completely normal eye/skin exams and negative genetic testing, the recurrence risk for future siblings is approximately 1% to 2% due to rare germ-cell mosaicism.

Can genetic testing predict how severe NF1 will be?

For most truncating NF1 variants, severity varies widely even within the same family. However, specific genotypes—such as the 1.4 Mb whole-gene microdeletion (more severe) or p.Met992del and p.Arg1809 variants (mild pigmentary phenotype without neurofibromas)—provide strong prognostic guidance.

Key Consensus Guidelines & Primary Sources

  1. Legius E, Messiaen L, Wolkenstein P, et al. Revised diagnostic criteria for neurofibromatosis type 1 and Legius syndrome: an international consensus recommendation. Genetics in Medicine. 2021;23(8):1506–1513. PubMed 34012067.
  2. Plotkin SR, Messiaen L, Legius E, et al. Updated diagnostic criteria and nomenclature for neurofibromatosis type 2 and schwannomatosis: an international consensus recommendation. Genetics in Medicine. 2022;24(9):1967–1977. PubMed 35674741.
  3. Gross AM, Wolters PL, Dombi E, et al. Selumetinib in children with inoperable plexiform neurofibromas (SPRINT Phase II). New England Journal of Medicine. 2020;382(15):1430–1442. PubMed 32187457.
  4. Moertel CL, Hirbe AC, Shuhaiber HH, et al. ReNeu: A pivotal phase IIb trial of mirdametinib in adults and children with symptomatic neurofibromatosis type 1-associated plexiform neurofibroma. Journal of Clinical Oncology. 2025;43(7):758–769. PubMed 39514828.
  5. Miller DT, Freedenberg D, Schorry E, et al. Health supervision for children with neurofibromatosis type 1 (American Academy of Pediatrics). Pediatrics. 2019;143(5):e20190660. PubMed 30988031.