Discordant puberty in monozygotic twin sisters with neurofibromatosis type 1 (NF1)

Stochastic phenotypes in RAS-dependent developmental diseases

Germline mutations upregulating RAS signaling are associated with multiple developmental disorders. A hallmark of these conditions is that the same mutation may present vastly different phenotypes in different individuals, even in monozygotic twins. Here, we demonstrate how the origins of such largely unexplained phenotypic variations may be dissected using highly controlled studies in Drosophila that have been gene edited to carry activating variants of MEK, a core enzyme in the RAS pathway. This allowed us to measure the small but consistent increase in signaling output of such alleles in vivo. The fraction of mutation carriers reaching adulthood was strongly reduced, but most surviving animals had normal RAS-dependent structures. We rationalize these results using a stochastic signaling model and support it by quantifying cell fate specification errors in bilaterally symmetric larval trachea, a RAS-dependent structure that allows us to isolate the effects of mutations from potential contributions of genetic modifiers and environmental differences. We propose that the small increase in signaling output shifts the distribution of phenotypes into a regime, where stochastic variation causes defects in some individuals, but not in others. Our findings shed light on phenotypic heterogeneity of developmental disorders caused by deregulated RAS signaling and offer a framework for investigating causal effects of other pathogenic alleles and mild mutations in general.

11-year old boy with facial hair, acne and deepened voice

11-year old twin boy found to have idiopathic precocious puberty after routine well-child examination revealed discordant pubertal growth between the two brothers.

Analysis of copy number variants in 11 pairs of monozygotic twins with neurofibromatosis type 1

Phenotypic variability among individuals with neurofibromatosis type 1 (NF1) has long been a challenge for clinicians and an enigma for researchers. Members of the same family and even identical twins with NF1 often demonstrate variable disease expression. Many mechanisms for this variability have been proposed. We have performed an exploratory study of copy number variants (CNVs) as a possible source of phenotypic variability in NF1. We enrolled 11 pairs of monozygotic (MZ) twins with NF1 and their parents, catalogued their clinical characteristics, and utilized a single nucleotide polymorphism (SNP) microarray to identify CNVs in blood and saliva. The 11 twin pairs showed high concordance for presence and number of café-au-lait spots, cutaneous neurofibromas, IQ, and ADHD. They were more likely to be discordant for optic pathway glioma, plexiform neurofibromas, skeletal manifestations, and malignancy. Microarray analysis identified a total of 81 CNVs meeting our conservative criteria, 37 of which overlap known genes. Of interest, three CNVs were previously unreported. Microarray analysis failed to ascertain any CNV differences within twin pairs, between twins and parents, or between tissues in any one individual. Results of this small pilot study did not demonstrate any de novo CNV events in our MZ twin pairs, nor were de novo CNVs overrepresented in these individuals with NF1. A much larger sample size would be needed to form any conclusions about the role of CNVs in NF1 variable expressivity. Alternative explanations for discordant phenotypes include epigenetic changes, smaller genetic alterations, or environmental factors. © 2016 Wiley Periodicals, Inc.

Variable expression of neurofibromatosis 1 in monozygotic twins

Neurofibromatosis 1 (NF1) is a common autosomal dominant disorder with high penetrance but extreme variability of expression. Monozygotic (MZ) twins with NF1 who have phenotypic discordances are a useful tool in evaluating which traits are influenced by non-hereditary influences such as second hit somatic events, environmental agents, epigenetic modification, or post-zygotic mutations. We evaluated nine sets of MZ twins and one set of MZ triplets, ages 4-18 years, for NF1 features and calculated probandwise concordance (P(C)) for each feature. MZ twins were highly concordant in numbers of café-au-lait spots (P(C) = 0.89) and cutaneous neurofibromas. IQ scores were within 10 points for all twin pairs tested, and similar patterns of learning disabilities and speech disorders were observed. Twin pairs showed significant discordance for tumors, particularly plexiform neurofibromas (P(C) = 0.40) and malignant peripheral nerves sheath tumors (MPNST), as expected if post-natal second-hit events were contributing to these features. One set of twins was concordant for multiple, large paraspinal neurofibromas, suggesting that there may be more hereditary factors involved in production of paraspinal neurofibromas. Four sets were concordant for pectus deformities of the chest (P(C) = 0.80). Three sets of twins were discordant for scoliosis (P(C) = 0.40); an additional set was concordant for scoliosis but differed in presence of dystrophic features and need for surgery. Our data suggest there are additional non-hereditary factors modifying the NF1 phenotype and causing discordancies between MZ twins. Future studies may focus on differences in epigenetic changes or somatic mosaicism which have been documented for other disease genes in MZ twins.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

Monozygotic twins discordant for neurofibromatosis 1

We present monozygotic twins discordant for the autosomal dominant disorder neurofibromatosis type 1 (NF1). The affected twin was diagnosed with NF1 at age 12, based upon accepted clinical criteria for the disorder. Both twins were re-examined at ages 35 and 57, at which times the unaffected twin continued to show no clinical manifestations of NF1. Short tandem repeat marker (STR) genotyping at 10 loci on chromosome 17 and 10 additional loci dispersed across the genome revealed identical genotypes for the twins, confirming their monozygosity. The affected twin has three children, two of whom also have NF1, while the unaffected twin has two children, both unaffected. Using lymphoblastoid, fibroblast, and buccal cell samples collected from both twins and from other family members in three generations, we discovered a pathogenic nonsense mutation in exon 40 of the NF1 gene. This mutation was found in all cell samples from the affected twin and her affected daughter, and in lymphoblastoid and buccal cells but not fibroblasts from the unaffected twin. We also found a novel non-synonymous change in exon 16 of the NF1 gene that was transmitted from the unaffected mother to both twins and co-segregated with the pathogenic mutation in the ensuing generation. All cells from the twins were heterozygous for this apparent exon 16 polymorphism and for single nucleotide polymorphisms (SNPs) within 2.5 kb flanking the site of the exon 40 nonsense mutation. This suggests that the NF1 gene of the unaffected twin differed in the respective lymphoblastoid cells and fibroblasts only at the mutation site itself, making post-zygotic mutation leading to mosaicism the most likely mechanism of phenotypic discordance. Although the unaffected twin is a mosaic, the distribution of the mutant allele among different cells and tissues appears to be insufficient to cause overt clinical manifestations of NF1.