Mosaic Mutations Research Articles

Progressive multifocal leukoencephalopathy (PML) treated with pembrolizumab in a patient with mosaic STAT3 gain-of-function mutation

Progressive multifocal leukoencephalopathy (PML) is a rare and often fatal disease of the central nervous system caused by JC virus. It occurs exclusively in immunocompromised hosts and is strongly associated with HIV infection and iatrogenic immunosuppression. Monogenic defects underlying the pathogenesis and reactivation of JC virus are not well understood, although some reports of primary immunodeficiencies (including four cases of STAT1 gain-of-function) have been previously described.A 77 year old male with history of prostate cancer (in remission; treated with radiation and hormonal therapy >10 years ago) presented with dysarthria, ataxia, left-sided weakness, diagnosed with CSF and biopsy-confirmed PML. He underwent an extensive workup upon which no other chronic infectious or oncologic processes were identified. He has never received any form of immunomodulation (including chemotherapy) in the past. Workup revealed low CD4 (347/mm3) and CD8 (282/mm3) T cell counts with normal proportions of naive vs. memory populations. Mitogen stimulation demonstrated poor CD8T cell proliferation. Targeted sequencing revealed a heterozygous mosaic mutation in the DNA-binding domain of STAT3 (c.1229 A > G p.H410R, 14% in 96 reads). This variant had been previously identified in two individuals with large granular lymphocyte (LGL) leukemia and validated as a gain-of-function via downstream signaling assays.Our patient demonstrated a favorable response to PD-1 blockade with pembrolizumab (2 mg/kg) and was discharged to a rehabilitation facility. His MRI brain showed no further progression of cerebellar white matter lesions following three doses of pembrolizumab.To our knowledge, this is the first case to implicate that STAT3 gain-of-function may confer a predisposition to JC virus reactivation and/or control in the pathogenesis of PML. Furthermore, we propose that this patient’s response to pembrolizumab may be partially attributed to STAT3-mediated regulation of the PD-1/PD-L1 axis. Further studies are currently underway to characterize this patient’s mosaic variant in flow cytometry-sorted subsets (including CD8T cells). Additional studies may include measurement of PD-1/PD-L1 expression and basal vs. induced STAT3 phosphorylation in sorted cell populations.

Developmental expression of the Sturge-Weber syndrome-associated genetic mutation in Gnaq: a formal test of Happle's paradominant inheritance hypothesis.

Sturge-Weber Syndrome (SWS) is a sporadic (non-inherited) syndrome characterized by capillary vascular malformations in the facial skin, leptomeninges, or the choroid. A hallmark feature is the mosaic nature of the phenotype. SWS is caused by a somatic mosaic mutation in the GNAQ gene (p.R183Q), leading to activation of the G protein, Gαq. Decades ago, Rudolf Happle hypothesized SWS as an example of "paradominant inheritance", that is, a "lethal gene (mutation) surviving by mosaicism". He predicted that the "presence of the mutation in the zygote will lead to death of the embryo at an early stage of development". We have created a mouse model for SWS using gene targeting to conditionally express the GNAQ p.R183Q mutation. We have employed two different Cre-drivers to examine the phenotypic effects of expression of this mutation at different levels and stages of development. As predicted by Happle, global, ubiquitous expression of this mutation in the blastocyst stage results in 100% embryonic death. The majority of these developing embryos show vascular defects consistent with the human vascular phenotype. By contrast, global but mosaic expression of the mutation enables a fraction of the embryos to survive, but those that survive to birth and beyond do not exhibit obvious vascular defects. These data validate Happle's paradominant inheritance hypothesis for SWS and suggest the requirement of a tight temporal and developmental window of mutation expression for the generation of the vascular phenotype. Furthermore, these engineered murine alleles provide the template for the development of a mouse model of SWS that acquires the somatic mutation during embryonic development, but permits the embryo to progress to live birth and beyond, so that postnatal phenotypes can also be investigated. These mice could then also be employed in pre-clinical studies of novel therapies.

MECP2 germline mosaicism plays an important part in the inheritance of Rett syndrome: a study of MECP2 germline mosaicism in males

BackgroundGermline mosaicisms could be inherited to offspring, which considered as “de novo” in most cases. Paternal germline MECP2 mosaicism has been reported in fathers of girls with Rett syndrome (RTT) previously. For further study, we focused on MECP2 germline mosaicism in males, not only RTT fathers.MethodsThirty-two fathers of RTT girls with MECP2 pathogenic mutations and twenty-five healthy adult males without history and family history of RTT or other genetic disorders were recruited. Sperm samples were collected and ten MECP2 hotspot mutations were detected by micro-droplet digital PCR (mDDPCR). And routine semen test was performed at the same time if the sample was sufficient. Additionally, blood samples were also detected for those with sperm MECP2 mosaicisms.ResultsNine fathers with RTT daughters (28.1%, 9/32) were found to have MECP2 mosaicism in their sperm samples, with the mutant allele fractions (MAFs) ranging from 0.05% to 7.55%. Only one father with MECP2 c.806delG germline mosaicism (MAF 7.55%) was found to have mosaicism in the blood sample, with the MAF was 0.28%. In the group of healthy adult males, MECP2 mosaicism was found in 7 sperm samples (28.0%, 7/25), with the MAFs ranging from 0.05% to 0.18%. None of the healthy adult males with MECP2 germline mosaicisms were found with MECP2 mosaicism in blood samples. There were no statistical differences in age, or the incidence of asthenospermia between fathers with RTT daughters and healthy adult males with MECP2 germline mosaicisms. Additionally, there was no linear correlation between MAFs of MECP2 mosaicisms and the age of males with germline MECP2 mosaicisms.ConclusionsGermline MECP2 mosaicism could be found not only in fathers with RTT daughters but also in healthy adult males without family history of RTT. As germline mosaic mutations may be passed on to offspring which commonly known as “de novo”, more attention should be paid to germline mosaicism, especially in families with a proband diagnosed with genetic disorders.

Efficient reconstruction of cell lineage trees for cell ancestry and cancer.

Mosaic mutations can be used to track cell ancestries and reconstruct high-resolution lineage trees during cancer progression and during development, starting from the first cell divisions of the zygote. However, this approach requires sampling and analyzing the genomes of multiple cells, which can be redundant in lineage representation, limiting the scalability of the approach. We describe a strategy for cost- and time-efficient lineage reconstruction using clonal induced pluripotent stem cell lines from human skin fibroblasts. The approach leverages shallow sequencing coverage to assess the clonality of the lines, clusters redundant lines and sums their coverage to accurately discover mutations in the corresponding lineages. Only a fraction of lines needs to be sequenced to high coverage. We demonstrate the effectiveness of this approach for reconstructing lineage trees during development and in hematologic malignancies. We discuss and propose an optimal experimental design for reconstructing lineage trees.

Expression of RANKL in breast cancer tissue in patients with fibrous dysplasia/McCune-Albright syndrome.

In fibrous dysplasia/McCune-Albright syndrome (FD/MAS), mosaic mutations in the GNAS gene lead to locally abnormal bone turnover. Additionally, patients with FD/MAS, particularly with thoracic lesions, have an increased risk for breast cancer. Development and progression of breast cancer has been associated with expression of Receptor Activator of NF-κB ligand (RANKL) in mammary tissue, and due to the GNAS mutation, RANKL is systemically increased in patients with FD/MAS. Yet it is unknown whether breast cancer in FD/MAS is also dependent on RANKL. We hypothesized that the GNAS mutation might induce RANKL overproduction and an oncogenic niche in mammary tissue, and examined RANKL expression in breast cancer tissue of patients with FD/MAS compared to controls. Nine patients with FD/MAS and breast cancer were included and clinical data were retrieved. Patients were matched to controls with breast cancer without FD/MAS based on age and tumor type. Three pregnant breast cancer patients were included as positive controls. Immunohistochemical detection of RANKL was performed on formalin-fixed paraffin-embedded breast cancer specimens. Staining intensity was classified as weak, moderate or intense. The area of positive RANKL staining divided by the total ductal-lobular area was assessed (positive area percentage, PAP). Number of patients with RANKL expression was compared between FD/MAS and control group by chi-square (χ2) test, the PAP by Mann-Whitney U test (MWU). RANKL expression was observed in 3 patients with FD/MAS (38%), mainly in healthy tissue, and none of the control patients (χ2p=0.055). The FD/MAS group demonstrated considerably more intense staining than the control group, comparable to positive controls. The median PAP was 0.64% (range 0.14-2.04%) in the 3 FD/MAS patients with RANKL expression, 0.01% (Q1-Q3: 0.0003-0.514%) in the entire FD/MAS group, 0.006% (Q1-Q3: 0.001-0.012%) in the control group (MWU=0.574), and 0.19% (0.08-0.32%) in the pregnant patients. All patients with FD/MAS and RANKL expression had thoracic bone lesions, but no correlation was observed between RANKL expression and presence of the GNAS mutation or FD disease burden. The triad of a higher number of patients, higher positive area percentage and stronger intensity in the FD/MAS compared to the control group indicates that RANKL may be upregulated in mammary tissue in a subset of patients with FD/MAS, which may explain the increased risk for breast cancer, although the clinical significance remains unclear. Further research is needed to establish risk profiles for the development of RANKL-positive breast cancer and to improve early screening and treatment.

Morphologic Features in Congenital Pulmonary Airway Malformations and Pulmonary Sequestrations Correlate With Mutation Status: A Mechanistic Approach to Classification.

Congenital pulmonary airway malformations (CPAMs) have a range of morphologies with varying cyst sizes and histologic features (types 1 to 3). Evidence suggested they arise secondary to bronchial atresia, however, we recently showed that cases with type 1 and 3 morphology are driven by mosaic KRAS mutations. We hypothesized that 2 distinct mechanisms account for most CPAMs: one subset is secondary to KRAS mosaicism and another is due to bronchial atresia. Cases with type 2 histology, similar to sequestrations, would be related to obstruction and therefore negative for KRAS mutations regardless of cyst size. We sequenced KRAS exon 2 in type 2 CPAMs, cystic intralobar and extralobar sequestrations, and intrapulmonary bronchogenic cysts. All were negative. Most sequestrations had a large airway in the subpleural parenchyma adjacent to the systemic vessel, anatomically confirming bronchial obstruction. We compared morphology to type 1 and 3 CPAMs. On average, type 1 CPAMs had significantly larger cysts, but there remained substantial size overlap between KRAS mutant and wild-type lesions. Features of mucostasis were frequent in sequestrations and type 2 CPAMs, while their cysts were generally simple and round with flat epithelium. Features of cyst architectural and epithelial complexity were more common in type 1 and 3 CPAMs, which rarely showed mucostasis. Similarity in histologic features among cases that are negative for KRAS mutation support the hypothesis that, like sequestrations, the malformation of type 2 CPAMs is related to obstruction during development. A mechanistic approach to classification may improve existing subjective morphologic methods.

Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.

Gain-of-function mutations in the EPAS1/HIF2A gene have been identified in patients with hereditary erythrocytosis that can be associated with the development of paraganglioma, pheochromocytoma and somatostatinoma. In the present study, we describe a unique European collection of 41 patients and 28 relatives diagnosed with an erythrocytosis associated with a germline genetic variant in EPAS1. In addition we identified two infants with severe erythrocytosis associated with a mosaic mutation present in less than 2% of the blood, one of whom later developed a paraganglioma. The aim of this study was to determine the causal role of these genetic variants, to establish pathogenicity, and to identify potential candidates eligible for the new hypoxia-inducible factor-2 α (HIF-2α) inhibitor treatment. Pathogenicity was predicted with in silico tools and the impact of 13 HIF-2b variants has been studied by using canonical and real-time reporter luciferase assays. These functional assays consisted of a novel edited vector containing an expanded region of the erythropoietin promoter combined with distal regulatory elements which substantially enhanced the HIF-2α-dependent induction. Altogether, our studies allowed the classification of 11 mutations as pathogenic in 17 patients and 23 relatives. We described four new mutations (D525G, L526F, G527K, A530S) close to the key proline P531, which broadens the spectrum of mutations involved in erythrocytosis. Notably, we identified patients with only erythrocytosis associated with germline mutations A530S and Y532C previously identified at somatic state in tumors, thereby raising the complexity of the genotype/phenotype correlations. Altogether, this study allows accurate clinical follow-up of patients and opens the possibility of benefiting from HIF-2α inhibitor treatment, so far the only targeted treatment in hypoxia-related erythrocytosis disease.

Comprehensive multi-omic profiling of somatic mutations in malformations of cortical development.

Malformations of cortical development (MCD) are neurological conditions involving focal disruptions of cortical architecture and cellular organization that arise during embryogenesis, largely from somatic mosaic mutations, and cause intractable epilepsy. Identifying the genetic causes of MCD has been a challenge, as mutations remain at low allelic fractions in brain tissue resected to treat condition-related epilepsy. Here we report a genetic landscape from 283 brain resections, identifying 69 mutated genes through intensive profiling of somatic mutations, combining whole-exome and targeted-amplicon sequencing with functional validation including in utero electroporation of mice and single-nucleus RNA sequencing. Genotype-phenotype correlation analysis elucidated specific MCD gene sets associated with distinct pathophysiological and clinical phenotypes. The unique single-cell level spatiotemporal expression patterns of mutated genes in control and patient brains indicate critical roles in excitatory neurogenic pools during brain development and in promoting neuronal hyperexcitability after birth.

Mosaic TP53 Mutation on Tumour Development in Pigs: A Case Study.

Pigs rarely develop cancer; however, tumour protein p53 (TP53)-modified pigs may have an increased incidence of cancer. In this study, two pigs with mosaic mutations induced by gene editing were compared to determine the role of the wild-type TP53 sequence in tumorigenesis and to speculate how amino acid changes in TP53 sequences are related to tumorigenesis. The pig without tumours had a wild-type TP53 sequence and a 1-bp deletion in the TP53 sequence that resulted in a premature stop codon. In contrast, the pig with nephroblastoma had 6- and 7-bp deletions in the TP53 sequence, resulting in the absence of two amino acids and a premature stop codon, respectively. Our results indicated that TP53 mutations with truncated amino acids may be related to tumour formation.

Linear nevus sebaceous syndrome with bilateral ocular involvement in a Bangladeshi child: A case report with literature review

The linear sebaceous nevus is one of a variety of epidermal nevi, which is a congenital ectodermal hamartoma that occurs due to post-zygotic mosaic mutation. Such sebaceous nevus is one of the commonest nevi, and up to 30% are associated with syndromic associations, which are rare. After cutaneous manifestation, central nervous system and ocular anomalies are common and found in almost every case. Multiorgan involvement can lead to various minor or major morbidities. Adequate knowledge and a high degree of suspicion can identify this rare disease and can minimize unwanted morbidity.

Numbers of Mutations within Multicellular Bodies: Why It Matters

Multicellular organisms often start life as a single cell. Subsequent cell division builds the body. Each mutational event during those developmental cell divisions carries forward to all descendant cells. The overall number of mutant cells in the body follows the Luria–Delbrück process. This article first reviews the basic quantitative principles by which one can understand the likely number of mutant cells and the variation in mutational burden between individuals. A recent Fréchet distribution approximation simplifies calculation of likelihoods and intuitive understanding of process. The second part of the article highlights consequences of somatic mutational mosaicism for understanding diseases such as cancer, neurodegeneration, and atherosclerosis.

Challenges in Gene Therapy for Somatic Reverted Mosaicism in X-Linked Combined Immunodeficiency by CRISPR/Cas9 and Prime Editing.

X-linked severe combined immunodeficiency (X-SCID) is a primary immunodeficiency that is caused by mutations in the interleukin-2 receptor gamma (IL2RG) gene. Some patients present atypical X-SCID with mild clinical symptoms due to somatic revertant mosaicism. CRISPR/Cas9 and prime editing are two advanced genome editing tools that paved the way for treating immune deficiency diseases. Prime editing overcomes the limitations of the CRISPR/Cas9 system, as it does not need to induce double-strand breaks (DSBs) or exogenous donor DNA templates to modify the genome. Here, we applied CRISPR/Cas9 with single-stranded oligodeoxynucleotides (ssODNs) and prime editing methods to generate an in vitro model of the disease in K-562 cells and healthy donors' T cells for the c. 458T>C point mutation in the IL2RG gene, which also resulted in a useful way to optimize the gene correction approach for subsequent experiments in patients' cells. Both methods proved to be successful and were able to induce the mutation of up to 31% of treated K-562 cells and 26% of treated T cells. We also applied similar strategies to correct the IL2RG c. 458T>C mutation in patient T cells that carry the mutation with revertant somatic mosaicism. However, both methods failed to increase the frequency of the wild-type sequence in the mosaic T cells of patients due to limited in vitro proliferation of mutant cells and the presence of somatic reversion. To the best of our knowledge, this is the first attempt to treat mosaic cells from atypical X-SCID patients employing CRISPR/Cas9 and prime editing. We showed that prime editing can be applied to the formation of specific-point IL2RG mutations without inducing nonspecific on-target modifications. We hypothesize that the feasibility of the nucleotide substitution of the IL2RG gene using gene therapy, especially prime editing, could provide an alternative strategy to treat X-SCID patients without revertant mutations, and further technological improvements need to be developed to correct somatic mosaicism mutations.

MPC-10 RECURRENT ASTROCYTOMA WITH SOMATIC IDH MOSAICISM: A CASE REPORT AND WARRANT FOR MOLECULAR DIAGNOSTICS

Abstract Introduction Acquisition of somatic mutations is essential for the development of malignant tumors such as diffuse gliomas. Recent single-cell analyses have pointed out the importance of somatic mosaicism in the pathogenesis of cancer. Here we report a case of IDH (isocitrate dehydrogenase)-mutant astrocytoma that has relapsed after intensive chemoradiation with somatic mosaicism for the IDH1 R132H mutation. Clinical and Pathological Findings A 36-year-old male presented with epileptic symptoms and a mass lesion in the right frontal lobe. A needle biopsy demonstrated IDH-mutant astrocytoma (grade 2). At the age of 41, relapsed tumor was completely resected, and the tumor cells were diffusely positive for IDH1 R132H with grade 4 morphology. Chemoradiotherapy (PAV + IMRT 60Gy/30fr) was added, but contrast-enhanced lesions reappeared 5 months after the initial surgery. The re-excised specimen was grade 4 astrocytoma, and the number of IDH1 R132H-positive tumor cells was very small compared to the total number of tumor cells, suggesting somatic mosaicism for IDH mutations. Conclusion It is imperative to clarify the induction mechanism and significance of somatic cell mosaicism in cancer, and molecular genetic assessment with sequencing and digital PCR would be necessary in addition to immunostaining for accurate molecular diagnoses of the brain tumors.

Defining the spatial landscape of KRAS mutated congenital pulmonary airway malformations: a distinct entity with a spectrum of histopathologic features

The potential pathogenetic mechanisms underlying the varied morphology of congenital pulmonary airway malformations (CPAMs) have not been molecularly determined, but a subset have been shown to contain clusters of mucinous cells (MCC). These clusters are believed to serve as precursors for potential invasive mucinous adenocarcinoma, and they are associated with KRAS codon 12 mutations. To assess the universality of KRAS mutations in MCCs, we sequenced exon 2 of KRAS in 61 MCCs from 18 patients, and we found a KRAS codon 12 mutation in all 61 MCCs. Furthermore, all MCCs from a single patient always had the same KRAS mutation, and the same KRAS mutation was also found in non-mucinous lesional tissue. Next generation sequencing of seven MCCs showed no other mutations or copy number variations. Sequencing of 46 additional CPAMs with MCCs revealed KRAS mutations in non-mucinous lesional tissue in all cases. RNA in situ hybridization confirmed widespread distribution of cells with mutant KRAS RNA, even extending outside of the bronchiolar type epithelium. We identified 25 additional CPAMs with overall histologic architecture similar to CPAMs with KRAS mutations but without identifiable MCCs, and we found KRAS mutations in 17 (68%). The histologic features of these KRAS mutated CPAMs included type 1 and type 3 morphology, as well as lesions with an intermediate histologic appearance, and analysis revealed a strong correlation between the specific amino acid substitution and histomorphology. These findings, together with previously published model organism data, suggests that the formation of type 1 and 3 CPAMs is driven by mosaic KRAS mutations arising in the lung epithelium early in development and places them within the growing field of mosaic RASopathies. The presence of widespread epithelial mutation explains late metastatic disease in incompletely resected patients and reinforces the recommendation for complete resection of these lesions.

260 Mosaic BRAF fusions are a recurrent cause of multiple congenital melanocytic naevi

Congenital melanocytic naevi (CMN) are moles present at birth, and when multiple or very extensive can involve other organ systems as well as predisposing to melanoma. Some CMN develop a highly proliferative phenotype whose progression is poorly understood. Genotypically, CMN are caused by mosaic NRAS missense mutations in 67% of cases and BRAF missense mutations in 7% of cases. Single cases of mosaic gene fusions have been previously described. To investigate the remaining 25%, skin biopsies from 19 patients shown to be double wildtype for NRAS/BRAF underwent transcriptome-wide paired-end RNA sequencing to detect gene fusion transcripts.

GNAQ mutations drive port wine birthmark-associated Sturge-Weber syndrome: A review of pathobiology, therapies, and current models.

Port-wine birthmarks (PWBs) are caused by somatic, mosaic mutations in the G protein guanine nucleotide binding protein alpha subunit q (GNAQ) and are characterized by the formation of dilated, dysfunctional blood vessels in the dermis, eyes, and/or brain. Cutaneous PWBs can be treated by current dermatologic therapy, like laser intervention, to lighten the lesions and diminish nodules that occur in the lesion. Involvement of the eyes and/or brain can result in serious complications and this variation is termed Sturge-Weber syndrome (SWS). Some of the biggest hurdles preventing development of new therapeutics are unanswered questions regarding disease biology and lack of models for drug screening. In this review, we discuss the current understanding of GNAQ signaling, the standard of care for patients, overlap with other GNAQ-associated or phenotypically similar diseases, as well as deficiencies in current in vivo and in vitro vascular malformation models.

PIK3CA Mutational Analysis in Patients With Macrodactyly.

Somatic mosaicism for PIK3CA mutations causes various types of growth disorders, which have been summarized under the term PROS (PIK3CA related overgrowth spectrum). Targeted therapy with PI3K inhibitors seems to be a promising alternative for severe PROS cases. Therefore, PIK3CA testing may become more relevant in the future. We report on 14 PROS patients, who had surgery for macrodactyly in the majority of cases. Clinical data were retrieved from the patient's records. Macroscopic and microscopic findings were retrospectively reviewed. Mutational analysis was performed on formalin-fixed paraffin-embedded (FFPE) material. Patient age ranged from 7 months to 35 years. Five patients showed additional anomalies. One patient had CLOVES syndrome. The majority of the specimens were ray resections characterized by hypertrophic fat tissue. Overall, microscopy was subtle. The abnormal adipose tissue showed lobules exhibiting at least focally fibrous septa. In each case, we could detect a PIK3CA mutation. Histology of affected fat tissue in PROS patients is overall nonspecific. Therefore, mutational analysis represents the key to the diagnosis, especially in unclear clinical cases. We demonstrated that FFPE material is suitable for PIK3CA testing, which can be considered as basis for targeted therapy with PI3K inhibitors.

Recurring germline mosaicism in a family due to reversion of an inherited derivative chromosome 8 from an 8;21 translocation with interstitial telomeric sequences

BackgroundMosaicism for chromosomal structural abnormalities, other than marker or ring chromosomes, is rarely inherited.MethodsWe performed cytogenetics studies and breakpoint analyses on a family with transmission of mosaicism for a derivative...

Genotype and Phenotype Landscape of 283 Japanese Patients with Tuberous Sclerosis Complex.

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by multiple dysplastic organ lesions and neuropsychiatric symptoms, caused by loss of function mutations in either TSC1 or TSC2. Genotype and phenotype analyses are conducted worldwide, but there have been few large-scale studies on Japanese patients, and there are still many unclear points. This study analyzed 283 Japanese patients with TSC (225 definite, 53 possible, and 5 genetic diagnoses). A total of 200 mutations (64 TSC1, 136 TSC2) were identified, of which 17 were mosaic mutations, 11 were large intragenic deletions, and four were splicing abnormalities due to deep intronic mutations. Several lesions and symptoms differed in prevalence and severity between TSC1 and TSC2 patients and were generally more severe in TSC2 patients. Moreover, TSC2 missense and in-frame mutations may attenuate skin and renal symptoms compared to other TSC2 mutations. Genetic testing revealed that approximately 20% of parents of a proband had mild TSC, which could have been missed. The patient demographics presented in this study revealed a high frequency of TSC1 patients and a low prevalence of epilepsy compared to global statistics. More patients with mild neuropsychiatric phenotypes were diagnosed in Japan, seemingly due to a higher utilization of brain imaging, and suggesting the possibility that a significant amount of mild TSC patients may not be correctly diagnosed worldwide.

P11.58.A Case of a complex neurocutaneous syndrome characterized by extensive peripheral nerve sheath tumors and somatic ERBB2 mutation

Abstract Schwannomatosis is a rare genetic tumor predisposition syndrome characterized by the presence of multiple non-intradermal schwannomas and the definitive absence of vestibular nerve involvement. Though considered benign, the burden of tumors can cause significant morbidity in the form of motor dysfunction and refractory neuropathic pain. Treatment is focused on mitigating these symptoms, which includes resection of offending tumors when feasible and anti-angiogenesis therapy with bevacizumab. Options for targeted medical therapies are lacking. Schwannomatosis is molecularly distinct from the other neurofibromatoses and by definition lacks constitutional mutations in NF1 and NF2. Instead, constitutional mutations in SMARCB1 or LZTR1 are often the “first hit” in tumorigenesis. Activating mutations in ERBB2 (also designated HER2) have been identified as oncogenic drivers in peripheral nerve sheath tumors in patients who lack these typical constitutional tumor suppressor gene mutations. Here we report a case of extensive peripheral nerve sheath tumors in the setting of apparent somatic mosaicism of an ERBB2 mutation. A 48-year-old man with history of chronic back pain presented with diffuse enlarging soft tissues masses primarily involving the left neck and occiput. Examination revealed areas of cutaneous heterogeneous hyperpigmentation, decreased muscle bulk and dorsiflexor weakness in the bilateral lower extremities, and steppage gait. Neuroimaging showed extensive T2 hyperintense, mildly enhancing masses within the bilateral thoracic and lumbar spine neuroforamina, intercostal nerves, paraspinal musculature, and lumbosacral plexus. Tissue from two separate tumor sites was morphologically consistent with schwannoma with some features of neurofibroma. Immunohistochemistry revealed partial loss of staining for SMARCB1. Whole exome sequencing (WES) on blood and tumor tissue did not show pathogenic germline or somatic variants for NF1, NF2, SMARCB1, LZTR1, or an array of tumor predisposition syndromes. WES on tumor tissue from both sites however did reveal a somatic ERBB2 variant (p.D769Y), suggesting mosaicism. ERBB2 D769Y has previously been classified as an activating mutation that confers sensitivity to some small molecule receptor tyrosine kinase inhibitors. Patients with ERRB2-mutated peripheral nerve sheath tumors may have broader therapeutic options in the variety of available tyrosine kinase inhibitors studied in other cancers.