Identification of a recurrent mosaic KRAS variant in brain tissue from an individual with nevus sebaceous syndrome

Abstract

Nevus sebaceous syndrome (NSS) is a rare, multisystem neurocutaneous disorder, characterized by a congenital nevus, and may include brain malformations such as hemimegalencephaly or focal cortical dysplasia, ocular, and skeletal features. It has been associated with several eponyms including Schimmelpenning and Jadassohn. The isolated skin lesion, nevus sebaceous, is associated with postzygotic variants in HRAS or KRAS in all individuals studied. The RAS proteins encode a family of GTPases that form part of the RAS/MAPK signaling pathway, which is critical for cell cycle regulation and differentiation during development. We studied an individual with nevus sebaceous syndrome with an extensive nevus sebaceous, epilepsy, intellectual disability, and hippocampal sclerosis without pathological evidence of a brain malformation. We used high-depth gene panel sequencing and droplet digital polymerase chain reaction (PCR) to detect and quantify RAS/MAPK gene variants in nevus sebaceous and temporal lobe tissue collected during plastic and epilepsy surgery, respectively. A mosaic KRAS c.34G > T; p.(Gly12Cys) variant, also known as G12C, was detected in nevus sebaceous tissue at 25% variant allele fraction (VAF), at the residue most commonly substituted in KRAS Targeted droplet digital PCR validated the variant and quantified the mosaicism in other tissues. The variant was detected at 33% in temporal lobe tissue but was absent from blood and healthy skin. We provide molecular confirmation of the clinical diagnosis of NSS. Our data extends the histopathological spectrum of KRAS G12C mosaicism beyond nevus sebaceous to involve brain tissue and, more specifically, hippocampal sclerosis.

Keywords: focal seizures with impairment of consciousness or awareness; generalized tonic seizures; intellectual disability; linear nevus sebaceous; mild.

Figure 1.

Histopathological evaluation of the individual with nevus sebaceous syndrome (NSS). (A) Coronal fast spin echo image showing a small right hippocampus with a subtle increase in T2 signal, consistent with hippocampal sclerosis (arrow). (B) Tilted axial FSPGR (fast spoiled gradient echo) image showing a small right hippocampus (arrow); there is no radiological evidence of cortical dysplasia. (C,D) Photographs showing sebaceous nevus on the scalp and face. (E) Hematoxylin and eosin (H&E) stain; 20× original objective. Hippocampal cortex showing intact neurons (CA3) in the right half of the field (arrows) with abrupt disappearance and transition to mesial temporal sclerosis in the left half of the field. (F) H&E stain; 10× original objective. Temporal cortex representative of multiple sections, with preserved horizontal lamination; molecular (I) and external granular (II) laminae only shown. Subpial gliosis seen in protracted seizures in general is shown with a broad arrow. (G) Images of fundi showing anomalous optic nerves. (H,I) Photographs of lesions affecting the limbus, sclera, and conjunctiva of both eyes.

Figure 2.

Molecular evaluation of mosaicism in the individual with nevus sebaceous syndrome (NSS). (A) Sanger sequencing of the KRAS c.34G > T; p.(G12C) mosaic variant. (B) The level of mosaicism of the KRAS G12C variant was quantified by droplet digital polymerase chain reaction (ddPCR) in nevus sebaceous, brain, normal skin, blood, and saliva DNA. (Blue droplets) Mutant DNA copies, (green droplets) wild-type DNA copies, (gray droplets) droplets without DNA copies, (orange droplets) droplets with multiple DNA copies. The x-axis shows the amplitude of wild-type fluorescent probe; the y-axis shows the amplitude of mutant fluorescent probe.