Background: The detection rate for identifying the underlyingmutation in neurocutaneous syndromes is affected by
the sensitivity of themutation test and the heterogeneity of the disease based on the diagnostic criteria. Neurofibromatosis
type (NF1) has been defined for 29 years by the National Institutes for Health (NIH) criteria which include
≥6 Café au Laitmacules (CAL) as a defining criterion. The discovery of SPRED1 as a cause of Legius syndromewhich is
manifested by CAL, freckling and learning difficulties has introduced substantial heterogeneity to the NIH criteria.
Methods: We have defined the sensitivity of comprehensive RNA analysis on blood of presumed NF1 patientsmeeting
NIH criteria with at least one nonpigmentary criterion and determined the proportion of children with ≥6 CAL
and no family history that has an NF1 or SPRED1 genetic variant. RNA analysis was carried out from 04/2009–12/
2015 on 361 NF1 patients.
Findings: A presumed causative NF1 mutation was found in 166/171 (97.08%–95% CI 94.56–99.6%) of familial cases
and 182/190 (95.8%–95% CI 92.93–98.65%) sporadic de novo cases. Two of thirteen (15%)mutation negative individuals
had dysembryoplastic neuroepithelial tumour (DNET) compared to 2/348 (0.6%) with an NF1 variant (p =
0.007). No SPRED1 variants were found in the thirteen individuals with no NF1 variant. Of seventy-one individuals
with ≥6 CAL and no non-pigmentary criterion aged 0–20 years, 47 (66.2%) had an NF1 variant six (8.5%) a SPRED1
variant and 18 (25.3%) no disease causing variant. Using the 95.8% detection rate the likelihood of a child with ≥6
CAL having constitutional NF1 drops from 2/3 to 1/9 after negative RNA analysis.
Interpretation: RNA analysis in individuals with presumed NF1 has high sensitivity and includes a small subset with
DNET without an NF1 variant. Furthermore negative analysis for NF1/SPRED1 provides strong reassurance to children
with ≥6 CAL that they are unlikely to have NF1.