The prevalence of neurofibromatosis type 1 (NF1) is about 1/3,000. There are no known ethnic groups in which NF1 does not occur or is unusually common. The prevalence is somewhat higher in young children than in adults, a difference that probably results at least in part from the early death of some NF1 patients. NF1 is fully penetrant in adults, but many disease features increase in frequency or severity with age. The reproductive fitness of NF1 patients is reduced by about one-half. About half of all cases result from new mutations. The estimated rate of new NF1 mutations is unusually high, but the basis for this high mutation rate is not known. Am. J. Med. Genet. (Semin. Med. Genet.) 89:1-6, 1999.

Histone lysine methyltransferases (KMTs) and demethylases (KDMs) underpin gene regulation. Here we demonstrate that variants causing haploinsufficiency of KMTs and KDMs are frequently encountered in individuals with developmental disorders. Using a combination of human variation databases and existing animal models, we determine 22 KMTs and KDMs as additional candidates for dominantly inherited developmental disorders. We show that KMTs and KDMs that are associated with, or are candidates for, dominant developmental disorders tend to have a higher level of transcription, longer canonical transcripts, more interactors, and a higher number and more types of post-translational modifications than other KMT and KDMs. We provide evidence to firmly associate KMT2C, ASH1L, and KMT5B haploinsufficiency with dominant developmental disorders. Whereas KMT2C or ASH1L haploinsufficiency results in a predominantly neurodevelopmental phenotype with occasional physical anomalies, KMT5B mutations cause an overgrowth syndrome with intellectual disability. We further expand the phenotypic spectrum of KMT2B-related disorders and show that some individuals can have severe developmental delay without dystonia at least until mid-childhood. Additionally, we describe a recessive histone lysine-methylation defect caused by homozygous or compound heterozygous KDM5B variants and resulting in a recognizable syndrome with developmental delay, facial dysmorphism, and camptodactyly. Collectively, these results emphasize the significance of histone lysine methylation in normal human development and the importance of this process in human developmental disorders. Our results demonstrate that systematic clinically oriented pathway-based analysis of genomic data can accelerate the discovery of rare genetic disorders.

Neurofibromatosis 1 occurs in 2 to 3 people per 10,000. The most frequent clinical features are café-au-lait macules, neurofibromas, intertriginous freckling, Lisch nodules, and learning disabilities, but optic and other gliomas, malignant peripheral nerve sheath tumors, and characteristic osseous lesions also can be present. Two striking aspects of neurofibromatosis 1 are its progressive nature and its extreme variability. This article reviews the natural history and some important clinical manifestations of neurofibromatosis 1, with emphasis on features that constitute the standard diagnostic criteria. The pathogenic implications of these clinical manifestations are also considered.

UBOs are difficult to identify with certainty and occur in children with suspected CNS disorders who do not have NF1, but they tend to occur in different brain regions. UBOs may be more useful for diagnosis of NF1 in young children if they can be defined precisely and if only the cerebellum, brainstem, and basal ganglia are considered.

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We conclude that inadequate information is available for pregnant women and their physicians to determine whether the benefits exceed the teratogenic risks for most drug treatments introduced in the past 20 years.

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