Droplet Digital PCR for Fast and Accurate Characterization of NF1 Locus Deletions: Confirmation of the Predominant Maternal Origin of Type-1 Deletions.

Neurofibromatosis type-1 is a genetic disorder caused by loss-of-function variants in the tumor-suppressor NF1. Approximately 4% to 11% of neurofibromatosis type-1 patients have a NF1 locus complete deletion resulting from nonallelic homologous recombination between low copy repeats. Codeleted genes probably account for the more severe phenotype observed in NF1-deleted patients. This genotype-phenotype correlation highlights the need for a detailed molecular description. A droplet digital PCR (ddPCR) set along the NF1 locus was designed to delimitate the three recurrent NF1 deletion breakpoints. The ddPCR was tested in 121 samples from nonrelated NF1-deleted patients. Classification based on ddPCR versus multiplex ligation-dependent probe amplification (MLPA) was compared. In addition, microsatellites were analyzed to identify parental origin of deletions. ddPCR identified 77 type-1 (64%), 20 type-2 (16%), 7 type-3 (6%), and 17 atypical deletions (14%). The results were comparable with MLPA, except for three atypical deletions misclassified as type-2 using MLPA, for which the SUZ12 gene was not deleted. A significant maternal bias (25 of 30) in the origin of deletions was identified. This study proposes a fast and efficient ddPCR quantification to allow fine NF1 deletion classification. It indicates that ddPCR can be implemented easily into routine diagnosis to complement the techniques dedicated to NF1 point variant identification. This new tool may help unravel the genetic basis conditioning phenotypic variability in NF1-deleted patients and offer tailored genetic counseling.

Contribution of whole genome sequencing in the molecular diagnosis of mosaic partial deletion of the NF1 gene in neurofibromatosis type 1.

Neurofibromatosis type 1 (NF1) is an autosomal dominant disease with complete penetrance but highly variable expressivity. In most patients, Next Generation Sequencing (NGS) technologies allow the identification of a loss-of-function pathogenic variant in the NF1 gene, a negative regulator of the RAS-MAPK pathway. We describe the 5-year diagnosis wandering of a patient with a clear NF1 clinical diagnosis, but no molecular diagnosis using standard molecular technologies. The patient presented with a typical NF1 phenotype but NF1 targeted NGS, NF1 transcript analysis, MLPA, and array comparative genomic hybridization failed to reveal a genetic aberration. After 5 years of unsuccessful investigations, trio WGS finally identified a de novo mosaic (VAF ~ 14%) 24.6 kb germline deletion encompassing the promoter and first exon of NF1. This case report illustrates the relevance of WGS to detect structural variants including copy number variants that would be missed by alternative approaches. The identification of the causal pathogenic variant allowed a tailored genetic counseling with a targeted non-invasive prenatal diagnosis by detecting the deletion in plasmatic cell-free DNA from the proband's pregnant partner. This report clearly highlights the need to make WGS a clinically accessible test, offering a tremendous opportunity to identify a molecular diagnosis for otherwise unsolved cases.

Hemiplegic Migraine Associated With PRRT2 Variations: A Clinical and Genetic Study.

BACKGROUND AND OBJECTIVE: PRRT2 variants have been reported in a few cases of patients with hemiplegic migraine. To clarify the role of PRRT2 in familial hemiplegic migraine, we studied this gene in a large cohort of affected probands. METHODS: PRRT2 was analyzed in 860 probands with hemiplegic migraine, and PRRT2 variations were identified in 30 probands. Genotyping of relatives identified a total of 49 persons with variations whose clinical manifestations were detailed. RESULTS: PRRT2 variations were found in 12 of 163 probands who previously tested negative for CACNA1A, ATP1A2, and SCN1A variations and in 18 of 697 consecutive probands screened simultaneously on the 4 genes. In this second group, pathogenic variants were found in 105 individuals, mostly in ATP1A2 (42%), followed by CACNA1A (26%), PRRT2 (17%), and SCN1A (15%). The PRRT2 variations included 7 distinct variants, 5 of which have already been described in persons with paroxysmal kinesigenic dyskinesia and 2 new variants. Eight probands had a deletion of the whole PRRT2 gene. Among the 49 patients with variations in PRRT2, 26 had pure hemiplegic migraine and 16 had hemiplegic migraine associated with another manifestation: epilepsy (8), learning disabilities (5), hypersomnia (4), or abnormal movement (3). Three patients had epilepsy without migraine: 2 had paroxysmal kinesigenic dyskinesia without migraine, and 1 was asymptomatic. DISCUSSION: PRRT2 should be regarded as the fourth autosomal dominant gene for hemiplegic migraine and screened in any affected patient, together with the 3 other main genes. Further studies are needed to understand how the same loss-of-function PRRT2 variations can lead to a wide range of neurologic phenotypes, including paroxysmal movement disorder, epilepsy, learning disabilities, sleep disorder, and hemiplegic migraine.

Severe Phenotype in Patients with Large Deletions of NF1.

Complete deletion of the NF1 gene is identified in 5-10% of patients with neurofibromatosis type 1 (NF1). Several studies have previously described particularly severe forms of the disease in NF1 patients with deletion of the NF1 locus, but comprehensive descriptions of large cohorts are still missing to fully characterize this contiguous gene syndrome. NF1-deleted patients were enrolled and phenotypically characterized with a standardized questionnaire between 2005 and 2020 from a large French NF1 cohort. Statistical analyses for main NF1-associated symptoms were performed versus an NF1 reference population. A deletion of the NF1 gene was detected in 4% (139/3479) of molecularly confirmed NF1 index cases. The median age of the group at clinical investigations was 21 years old. A comprehensive clinical assessment showed that 93% (116/126) of NF1-deleted patients fulfilled the NIH criteria for NF1. More than half had café-au-lait spots, skinfold freckling, Lisch nodules, neurofibromas, neurological abnormalities, and cognitive impairment or learning disabilities. Comparison with previously described "classic" NF1 cohorts showed a significantly higher proportion of symptomatic spinal neurofibromas, dysmorphism, learning disabilities, malignancies, and skeletal and cardiovascular abnormalities in the NF1-deleted group. We described the largest NF1-deleted cohort to date and clarified the more severe phenotype observed in these patients.

PRRT2 mutations cause hemiplegic migraine.

OBJECTIVE: Hemiplegic migraine (HM) is a rare subtype of migraine with aura that occurs as a familial or sporadic condition. The 3 culprit genes identified so far do not account for all familial forms of HM. PRRT2 mutations have recently been shown to cause various childhood-onset episodic syndromes including paroxysmal kinesigenic dyskinesia, infantile convulsions with choreoathetosis syndrome, and benign familial infantile epilepsy. Our objective was to test the possible implication of PRRT2 in HM, another episodic disorder with early onset in most cases. METHODS: The whole genomic coding region of PRRT2 was sequenced in 101 index cases with HM that started before age 20 years and for whom no mutation was found in the 3 known HM genes. Affected relatives of mutated patients were analyzed when available. RESULTS: PRRT2 mutations were identified in 4 patients: the previously reported c.649dupC mutation was found in 2 cases, and a novel mutation, c.649delC, was found in the other 2. One patient with mutation subsequently developed paroxysmal dyskinesia, as well as generalized epileptic seizures. CONCLUSIONS: PRRT2 mutations can occasionally cause HM. This underscores the complexity of the phenotypic consequences of PRRT2 mutations.