Mutations in von Recklinghausen neurofibromatosis: an hypothesis

PTPN11 Mosaicism Causes a Spectrum of Pigmentary and Vascular Neurocutaneous Disorders and Predisposes to Melanoma

Phakomatosis pigmentovascularis is a diagnosis that denotes the coexistence of pigmentary and vascular birthmarks of specific types, accompanied by variable multisystem involvement, including CNS disease, asymmetrical growth, and a predisposition to malignancy. Using a tight phenotypic group and high-depth next-generation sequencing of affected tissues, we discover here clonal mosaic variants in gene PTPN11 encoding SHP2 phosphatase as a cause of phakomatosis pigmentovascularis type III or spilorosea. Within an individual, the same variant is found in distinct pigmentary and vascular birthmarks and is undetectable in blood. We go on to show that the same variants can cause either the pigmentary or vascular phenotypes alone, and drive melanoma development within pigmentary lesions. Protein structure modeling highlights that although variants lead to loss of function at the level of the phosphatase domain, resultant conformational changes promote longer ligand binding. In vitro modeling of the missense variants confirms downstream MAPK pathway overactivation and widespread disruption of human endothelial cell angiogenesis. Importantly, patients with PTPN11 mosaicism theoretically risk passing on the variant to their children as the germline RASopathy Noonan syndrome with lentigines. These findings improve our understanding of the pathogenesis and biology of nevus spilus and capillary malformation syndromes, paving the way for better clinical management.

Mosaicism in Tumor Suppressor Gene Syndromes: Prevalence, Diagnostic Strategies, and Transmission Risk

A mosaic state arises when pathogenic variants are acquired in certain cell lineages during postzygotic development, and mosaic individuals may present with a generalized or localized phenotype. Here, we review the current state of knowledge regarding mosaicism for eight common tumor suppressor genes-NF1, NF2, TSC1, TSC2, PTEN, VHL, RB1, and TP53-and their related genetic syndromes/entities. We compare and discuss approaches for comprehensive diagnostic genetic testing, the spectrum of variant allele frequency, and disease severity. We also review affected individuals who have no mutation identified after conventional genetic analysis, as well as genotype-phenotype correlations and transmission risk for each tumor suppressor gene in full heterozygous and mosaic patients. This review provides new insight into similarities as well as marked differences regarding the appreciation of mosaicism in these tumor suppressor syndromes.

Mosaic abnormalities of the skin: review and guidelines from the European Reference Network for rare skin diseases

BACKGROUND

Cutaneous mosaicism is an area of dermatology in which there has been an explosion of knowledge within the current decade. This has led to fundamental changes in the understanding of the conditions in this field, and to an ongoing paradigm shift in the approach to management of mosaic skin disorders.

OBJECTIVES

To lay out the general principles of mosaicism as they are currently understood, summarize the known cutaneous mosaic abnormalities of the skin with associated phenotypic and genotypic information, review the latest trials on targeted therapies and propose guidelines for the general approach to a patient with suspected mosaicism.

METHODS

This was a consensus expert review as part of the European Reference Network project (ERN-Skin).

CONCLUSIONS

This study provides clinicians with a practical approach to the patient with suspected mosaicism, redefines mosaicism for the modern genetic era, and proposes a new classification system based on genetic mechanism. What's already known about this topic? Cutaneous mosaicism is a complex field of dermatology that encompasses most birthmarks, and many rare syndromes. Some cutaneous patterns are known to be seen in mosaicism. Very few treatment options are available for most mosaic abnormalities of the skin. Recent high-sensitivity genetic techniques have led to an explosion of knowledge about genotype and phenotype in the literature. What does this study add? Expert consensus from the European Reference Network project. Review of knowledge of confirmed mosaic abnormalities of the skin, including cutaneous phenotype, extracutaneous associated features and genotype. Proposed new classification of mosaic abnormalities of the skin by genetic mechanism and therefore inheritance potential. Practical tips on correct sample collection and genetic investigation. Review of trials of targeted therapies. Guidelines for a practical clinical approach to the patient with suspected mosaicism.

Molecular diagnosis of genodermatoses

The progress of molecular genetics helps clinicians to prove or exclude a suspected diagnosis for a vast and yet increasing number of genodermatoses. This leads to precise genetic counselling, prenatal diagnosis and preimplantation genetic haplotyping for many inherited skin conditions. It is also helpful in such occasions as phenocopy, late onset and incomplete penetrance, uniparental disomy, mitochondrial inheritance and pigmentary mosaicism. Molecular methods of two genodermatoses are explained in detail, i.e. genodermatoses with skin fragility and neurofibromatosis type 1.

A mild neurofibromatosis type 1 phenotype produced by the combination of the benign nature of a leaky NF1-splice mutation and the presence of a complex mosaicism

Here we analyze the genetic and molecular basis responsible for a very benign phenotype observed in an NF1 patient. Quantification of cells carrying the NF1 mutation in different samples derived from the three embryonic layers revealed mosaicism. Furthermore, the construction of a minigene with patient's mutation (c.3198 - 314G>A) confirmed its benign nature due to the leakiness of the splicing mechanism that generated a proportion of correctly spliced transcripts. Hence, we concluded that the mild phenotype observed in this patient is the result of the presence of mosaicism together with the benign nature of a leaky NF1-splice mutation. Finally, with the aim of developing a personalized therapeutic approach for this patient, we demonstrated correction of the splicing defect by using specific antisense morpholino oligomers. Our results provide an example of the molecular complexity behind disease phenotypes and highlight the importance of using comprehensive genetic approaches to better assess phenotype-genotype correlations.

Clinical, pathologic, and genetic features of massive soft tissue neurofibromas in a Sicilian patient

Neurofibromatosis type 1 (NF1) is an autosomal dominant condition seen in all ethnic groups affecting approximately one in 4000 individuals. The disease is caused by mutations in the NF1 gene, one of the largest human genes. In this report we present one case of massive soft tissue neurofibromas arising in a Sicilian patient with NF1. The sequence of NF1 gene in this patient revealed a cluster of variants distributed along the whole gene. Among these we identified two heterozygous frameshift mutations in exon 19 (c.2480delA) and a novel mutation in exon 21 (c.2632delC). Ten novel nonsynonymous missense variations were also identified in this patient.

Example of somatic mosaicism in a series of de novo neurofibromatosis type 1 cases due to a maternally derived deletion

Neurofibromatosis type 1 (NF1), affecting primarily the growth of neural crest-derived tissues, is one of the most common autosomal dominant genetic disorders with an unusually high spontaneous mutation rate. In four cases of sporadic NF1, demonstrated by hemizygosity to have a deletion involving the NF1 gene, we were able to assign the deletion event to the maternally derived chromosome. One of these individuals was determined to be a somatic mosaic for NF1, as a trace of the maternally derived haplotype was detected at the NF1 locus. This indicated a postzygotic, as opposed to gametic, deletion event. It may be that somatic mosaicism is more common in NF1 than has hitherto been appreciated and may be responsible in part for the high mutation rate in this disorder. In addition, it is suggested that the mechanism(s) of gene deletion is subject to a parent of origin effect, being more frequent on the maternally derived chromosome. This is in contrast to the other types of mutations which, in sporadic NF1, have been found to occur preferentially on the paternally derived chromosome.

Paternal age and sporadic neurofibromatosis 1: a case-control study and consideration of the methodologic issues

Sporadic neurofibromatosis 1 (NF1) occurs in the absence of a family history of the disease and usually results from a new mutation in the germ cell of one of the parents, most commonly the father. Older paternal age may increase the risk for a new germinal NF1 mutation, but the results of studies to address this question conflict. We investigated paternal age in sporadic NF1 by using a case-control study design. Patients who were seen at two specialty NF clinics in Houston, Texas, born between 1970 and 1992 and living in the Houston area and surrounding counties, were studied. Birth certificates with information on the father were found for 89 cases. For each case, two birth certificates were chosen at random from the same year and county of birth. In this way, the control group of 178 individuals was formed. Fathers of patients with NF1 were 1.5 years older than fathers of control subjects at the birth of the child, but the difference was only of borderline statistical significance (P = 0.07). This paternal age difference was not changed by adjustment for socioeconomic status or maternal age. These and previous data are consistent with either a small paternal age effect in sporadic NF1 or a bias such as that resulting from the selection of cases and/or controls.

Siblings of retinoblastoma patients: are we underestimating their risk?

PURPOSE

To describe the clinical presentation of probable germ-line mosaicism in four retinoblastoma kindreds.

METHODS

Review of 255 retinoblastoma patients and their family records in a University of California, San Francisco-Bascom Palmer database to identify those with potential germ-line mosaicism. Parents and siblings of retinoblastoma patients were given comprehensive ophthalmologic examinations.

RESULTS

Four kindreds were identified, wherein retinoblastoma was diagnosed in two siblings and both parents demonstrated no evidence of retinoblastoma or retinocytoma.

CONCLUSION

Clinical appearance of germ-line mosaicism is demonstrated in our retinoblastoma patient populations. We recommend routine clinical screening of all parents and siblings of retinoblastoma patients to provide more accurate genetic counseling and to allow earlier examination and treatment of children at presymptomatic disease stages. Germ-line mosaicism must be considered as a genetic transmission pattern in these patients, and genetic counseling should specifically recognize this possibility. If a parent is germ-line mosaic, the possibility of bearing a second child with retinoblastoma is clearly higher than conventionally believed.

Recurrent NF1 gene mutation in a patient with oligosymptomatic neurofibromatosis type 1 (NF1)

We report a 21-year-old male with symptomatic optic glioma who does not fulfill the diagnosis of neurofibromatosis 1 (NF1) according to standard NIH criteria. Analysis of the NF1 gene revealed a recurrent mutation in exon 37 (C6792A or Y2264X). This nonsense mutation causes skipping of exon 37 during the splicing process and is predicted to result in a protein shortened by 34 amino acid residues. The mutation was detected in all tissues examined (blood lymphocytes, oral mucosa, and dermal fibroblasts). The same mutation was previously found in 3 patients with clinically confirmed NF1. To our knowledge, this is the first report of an adult patient carrying a putative (non-mosaic) NF1 gene mutation in multiple tissues but not fulfilling the NIH criteria for the clinical diagnosis of NF1.

The von Hippel-Lindau tumor suppressor gene. A rare and intriguing disease opening new insight into basic mechanisms of carcinogenesis

The von Hippel-Lindau (VHL) disease is an inherited tumor susceptibility syndrome featuring a high variety of benign and malignant tumors. The gene has been localized and cloned at 3p25-26. Recent functional analysis defined the VHL gene product as an inhibitor of the transcription elongation process. Its possible involvement in the vascularization process may explain the histologic features of VHL tumors providing insight into basic mechanism of tumorigenesis. Direct genetic testing is available for patients affected with VHL. Seventy to eighty percent of the germline mutations expected could be detected. As first geno/phenotype correlations have been established, we are now beginning to understand the diversity of this fascinating disease at the molecular level. As mutational analysis proved to be of striking prognostic significance, gene testing became an important tool for the management of the disease. The VHL gene was also found to be responsible for tumorigenesis in the corresponding sporadic tumors, especially in the clear cell type of renal cell carcinomas. The understanding of the normal and disturbed function of the VHL gene product will enable us to develop treatment strategies based on and targeted at the molecular cause of the disease. In this review we summarize the current knowledge about genetics, clinics, and function of VHL.

Molecular basis of neurofibromatosis type 1 (NF1): mutation analysis and polymorphisms in the NF1 gene

Neurobromatosis type 1 (NF1) is one of the commonest genetic disorders in humans. The gene for NF1 was cloned in 1990. The protein encoded by the gene (neurofibromin) has extensive sequence homology with GTPase-activating protein (GAP). Despite screening the whole coding region of the gene for large and medium size rearrangements and approximately 40% of the coding region of the gene for small alterations, only 45 germ-line mutations have been reported in more than 500 unrelated patients. Of these, 25 mutations involve small changes in the gene, of which 17 (68%) result in the formation of an inappropriate stop codon. A "hot spot" for mutations has not been identified. The high mutation rate at this locus and the general difficulty in identifying mutations are discussed. A complete understanding of the structure and function of the NF1 gene awaits further detailed studies of both naturally occurring and in vitro-generated mutations.