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17q11 microdeletion syndrome
17q11 microdeletion syndrome is a rare severe form of neurofibromatosis type 1 (NF1; see this term) characterized by mild facial dysmorphism, developmental delay, intellectual disability, increased risk of malignancies, and a large number of neurofibromas.
The prevalence of 17q11 microdeletion syndrome is not known. About 5% of NF1 cases are reported to have deletions of the entire NF1 gene. More than 170 affected patients have been reported to date.
Affected individuals often have unusual body habitus and facial dysmorphism including facial coarsening, prominent forehead, ptosis, down-slanting palpebral fissures, hypertelorism, broad nose and nasal bridge, low set ears, and micrognathia. Patients develop a large number of neurofibromas, often with early onset, including multiple cutaneous neurofibromas, and less commonly plexiform neurofibromas. Other characteristic features include attention deficit/hyperactivity disorder (AD/HD), delayed cognitive development and intellectual disability. Some patients are reported to have microcephaly or macrocephaly, optic pathway glioma, iris coloboma (see these terms), heart defects (mitral valve prolapse, aortic dilatation), large hands and feet, connective tissue dysplasia (joint hyperflexibility, soft palm skin), muscular hypotonia, scoliosis, pectus excavatum, and bone cysts. A higher risk of malignancy for NF1 and non-NF1 tumors is reported: malignant peripheral nerve sheath tumors (lifetime risk of 16-26%), retroperitoneal fibrosarcoma, and medulloblastoma with extensive nodularity (see this term).
Germline and mosaic microdeletions of the NF1 gene and its flanking regions caused by non-allelic homologous recombination are reported in patients with this disorder. Most occur de novo.
As most cases are de novo, recurrence risk for offspring of unaffected parents is very low. Affected individuals have a 50% risk of transmitting the microdeletion, and prenatal and preimplantation genetic diagnosis is possible.