Mosaic RASopathies: how postzygotic mutations rewrite diagnosis and prognosis

When a child is born with a greasy, hairless patch of skin running across the scalp and face, clinicians have long known to look beyond the surface. These congenital epidermal nevi, particularly linear nevus sebaceous lesions of the craniofacial region, are the visible signature of a family of rare disorders known as mosaic RASopathies, in which a mutation arises not in the egg or sperm but during embryonic development itself, after fertilization. Because the mutation occurs postzygotically, it is carried only by a subset of the body’s cells, creating a patchwork organism in which some tissues carry an oncogenic change and others remain genetically normal. A new study published in Frontiers in Cell and Developmental Biology has now analyzed 18 such patients in molecular detail and combined the findings with nearly a century of published cases, arriving at a conclusion that could reshape how these children are diagnosed, monitored, and treated: the specific gene that is mutated matters enormously, and genetic testing of the affected skin, not the blood, is the key that unlocks the diagnosis.

The research team, led by investigators from the N.N. Blokhin National Medical Research Center of Oncology and the Engelhardt Institute of Molecular Biology in Moscow, enrolled 18 patients ranging in age from one month to 21 years. All had congenital craniofacial nevi and a variable array of extracutaneous abnormalities affecting the brain, eyes, heart, kidneys, bones, and endocrine system. The central diagnostic challenge in this field is that the clinical entities overlap so heavily that the nevus subtype, whether sebaceous, spilus, or melanocytic, has traditionally been the main feature used to classify the disease. Schimmelpenning–Feuerstein–Mims syndrome, for instance, is defined by linear nevus sebaceous, while phacomatosis pigmentokeratotica requires that same nevus in combination with a speckled lentiginous nevus, or nevus spilus. Yet the same mosaic variants in RAS/MAPK pathway genes, primarily HRAS and KRAS, have been found across all of these conditions, blurring the boundaries between them.

To cut through this ambiguity, the team turned to targeted next-generation sequencing. DNA was extracted from peripheral blood leukocytes, from the nevus tissue itself, from unaffected skin, and, where available, from tumor tissue. Libraries were prepared with the KAPA HyperPrep kit and hybridized to customized capture probes covering the coding regions of RAS/MAPK pathway genes. Sequencing was performed on an Illumina NextSeq 2000 instrument with an average coverage of 250 to 350 times, ensuring that at least 90 percent of the target region was covered at 50-fold or greater depth. The pipeline aligned reads to the GRCh38/hg38 reference genome, called variants with GATK, and applied a minimum variant allele frequency threshold of 2 percent, a cutoff chosen deliberately because mosaic variants are present in only a fraction of cells and can be missed by conventional methods. All candidate variants were then verified by Sanger sequencing, although the authors acknowledge that Sanger has a practical detection limit of roughly 15 to 20 percent variant allele frequency, meaning low-level mosaics rest on the NGS findings alone.

The results were striking in their consistency. Every pathogenic or likely pathogenic variant, in the genes HRAS, KRAS, BRAF, PIK3CA, and FGFR2, was present exclusively in affected tissues and entirely absent from peripheral blood DNA and normal skin. Variant allele frequencies ranged from 17 percent to 84 percent in the lesions. Eight patients received a diagnosis of Schimmelpenning–Feuerstein–Mims syndrome, four carrying postzygotic HRAS variants and four carrying KRAS variants. Seven patients met the clinical criteria for phacomatosis pigmentokeratotica, with mosaic variants identified in HRAS in four cases, KRAS in two, and BRAF in one. In two additional patients with similar skin phenotypes, the culprit turned out to be a somatic mutation in PIK3CA in one case and FGFR2 in the other, pointing to entirely different diagnostic categories, while in one final patient no clinically significant variant was detected at all.

The clinical narratives behind these genetic findings are extraordinary. One nine-year-old boy with craniofacial linear nevus sebaceous developed skeletal deformities and classic rickets from the age of one year. Standard treatment with phosphate and alfacalcidol failed, so at age four he was started on burosumab, a monoclonal antibody against fibroblast growth factor 23. After fifteen months of therapy, his pain resolved, independent walking returned, and his alkaline phosphatase, serum phosphorus, and calcium levels normalized. Genetic testing of his nevus revealed a pathogenic HRAS variant, c.182A>G (p.Gln61Arg), with a variant allele frequency of 62 percent. Another four-year-old boy with the same syndrome developed alveolar rhabdomyosarcoma, a malignant soft tissue tumor, in the paratesticular region, with metastases to the femur and hand. After nine courses of polychemotherapy he has remained in remission for more than two years, and sequencing showed the HRAS variant at 17 to 22 percent in the nevus but a striking 84 percent in the tumor itself, a pattern consistent with a two-stage mechanism in which the HRAS mutation drives tumor formation and subsequent loss of heterozygosity delivers the second hit.

Patients with KRAS variants told a different clinical story. One five-year-old girl with a KRAS c.35G>A (p.Gly12Asp) variant in her nevus had seizures from four months of age, MRI findings of bilateral trigeminal nerve lipomatosis, ventriculomegaly, and hydrocephalus, a rhabdomyoma in the right ventricle of the heart, epibulbar dermoids, retinal calcifications, and severe cognitive impairment. A twelve-year-old boy with the identical KRAS variant had epilepsy from early infancy, underwent temporal lobe resection that revealed a gangliocytoma, and experienced profound developmental delay, walking at nearly two years and speaking only at five. A twenty-one-year-old woman with pharmacoresistant epilepsy, focal cortical dysplasia, an arteriovenous malformation of the spine, renal angiomyolipomas, retinal hamartomas, and self-aggression requiring antipsychotic therapy also carried a KRAS variant in her nevus, undetectable in blood exome sequencing.

To move beyond individual anecdotes, the researchers conducted a systematic literature review spanning 1946 to 2025, searching PubMed and ResearchGate for genetically confirmed cases of epidermal nevus syndrome, Schimmelpenning–Feuerstein–Mims syndrome, phacomatosis pigmentokeratotica, and related mosaic RASopathies. They ultimately combined their own data with published cases to yield 53 cases with postzygotic HRAS variants and 41 cases with KRAS variants for genotype–phenotype correlation analysis. Statistical comparisons used Fisher’s exact test with Benjamini–Hochberg correction for multiple comparisons across eleven clinical features, controlling the false discovery rate at a threshold of q less than 0.05. Odds ratios were calculated with KRAS as the reference group, and the Haldane correction was applied to contingency tables containing zero cells to enable stable estimation.

The correlations that emerged were both statistically robust and clinically actionable. Ophthalmological disorders were dramatically overrepresented in KRAS-mutated patients, occurring in 71 percent versus 21 percent of HRAS-mutated cases, with an odds ratio of 9.2 and a confidence interval of 3.6 to 23.8. Coloboma, choristoma, and epibulbar dermoid were detected only in patients with KRAS variants. Neurological disorders, including epilepsy and various brain malformations, were also more common in KRAS-mutated patients, at 78 percent versus 40 percent, with an odds ratio of 5.4. Cardiovascular disorders and lymphatic malformations followed the same pattern, with odds ratios of 3.7 and 7.2 respectively. In sharp contrast, hypophosphatemic rickets was diagnosed exclusively in HRAS-mutated patients, occurring in 30 to 40 percent of cases, and malignant tumors were also more frequent in the HRAS group, at 36 percent versus 12 percent, with an odds ratio of 4.0. Among the malignancies, embryonal and alveolar rhabdomyosarcoma predominated in childhood, while skin cancer, urothelial carcinoma, and astrocytoma have been described in adults.

The mutational spectrum itself showed recurring hotspots. In the study cohort, the most common HRAS substitution was Q61R at 50 percent, followed by G13R at 37 percent, whereas in the literature the most frequent substitution was HRAS G13R at 57 percent. The G12D substitution predominated among all KRAS pathogenic variants, both in the study, where it accounted for 83 percent of cases, and in the literature, where it accounted for 64 percent. All six patients with KRAS variants in the study had congenital craniofacial linear nevus sebaceous, and two also had nevus spilus. In the published KRAS cases, linear nevus sebaceous was noted in 68 percent, and only one patient also had nevus spilus, a finding indicative of phacomatosis pigmentokeratotica.

Based on these findings, the authors propose a practical algorithm for establishing somatic mosaicism in epidermal nevus syndromes. Because variant allele frequencies can vary across a lesion, biopsies should be taken from at least four different areas of the nevus, and ideally from different types of nevi when feasible, and genetic testing of peripheral blood should always be performed in parallel to exclude germline pathogenic variants associated with hereditary syndromes that can mimic the mosaic phenotype. This dual approach proved decisive in several cases: a patient initially suspected of having Schimmelpenning–Feuerstein–Mims syndrome turned out to carry a PIK3CA variant, placing her in the PIK3CA-Related Overgrowth Spectrum category, while another patient with a mosaic BRAF variant presented a differential diagnosis between mosaic cardiofaciocutaneous syndrome and phacomatosis pigmentokeratotica. The authors also emphasize that these genotype–phenotype correlations are probabilistic rather than deterministic, and that all patients with characteristic skin lesions should be referred for assessment by a multidisciplinary team of specialists. Nevertheless, the specific mosaic genotype can, with appropriate caution, inform prognosis, guide surveillance for the complications most likely to arise, and support the development of targeted therapies, from MEK inhibitors for refractory epilepsy to anti-FGF23 antibodies for hypophosphatemic rickets. In a field where the visible lesion is only the tip of the molecular iceberg, sequencing the affected tissue has become the compass that points the way forward.

Subject of Research: Mosaic RASopathies and genotype–phenotype correlations in postzygotic RAS/MAPK pathway mutations

Article Title: Mosaic RASopathies: comprehensive genetic and clinicopathological analysis for diagnosis and management

Article References: Belysheva, T., Nasedkina, T., Zelenova, E., Semenova, V., Semenova, N., Sagoyan, G., Kletskaya, I., Vishnevskaya, Y., Sharapova, E., Kulikova, K., Vlasov, P., Botkina, A., Belogurova, M., Dorofeeva, M., Valiev, T., & Varfolomeeva, S. (2026). Mosaic RASopathies: comprehensive genetic and clinicopathological analysis for diagnosis and management. Frontiers in Cell and Developmental Biology, 14, Article 1908228. https://doi.org/10.3389/fcell.2026.1908228

Image Credits: AI Generated

DOI: 10.3389/fcell.2026.1908228

Keywords: mosaic RASopathies, epidermal nevus syndrome, Schimmelpenning–Feuerstein–Mims syndrome, phacomatosis pigmentokeratotica, HRAS, KRAS, genotype–phenotype correlation, next-generation sequencing, rhabdomyosarcoma, hypophosphatemic rickets, Mosaic, RASopathies