Known genes and mutations found on the human X chromosome. The MED12 gene is found at location q13, highlighted in green. MED12, or mediator of RNA polymerase II transcription, subunit 12 homolog of S. cerevisiae, is one of several subunits in the mammalian mediator complex, which regulates RNA polymerase II during mRNA transcription. The Mediator complex is required for polymerase II transcription and acts as a bridge between the polymerase II enzyme and different gene-specific transcription factors. Mediator can contain up to 30 subunits, but some of the subunits are only required for regulation of transcription in particular tissues or cells. Currently, the exact mechanism by which dysfunction of MED12 results in LFS and its associated neuropsychopathic and physical characteristics is unclear. Marfanoid habitus, a highly arched palate and several other features of LFS can be found with Marfan syndrome, a connective tissue disorder. The finding of aortic root dilation in both disorders suggests that a mutation in an unspecified connective tissue regulating gene may contribute to the etiology of LFS. A number of interesting experimental results have been obtained by studying MED12 mutations in the zebrafish, an animal model representing vertebrates. In zebrafish, a mutation in MED12 was found to be responsible for the mutant motionless (mot). Zebrafish with the mot mutation have neuronal and cardiovascular defects, although not all types of neurons are affected. Introduction of human MED12 mRNA into the zebrafish restores normal development. MED12 is also a critical coactivator for the gene SOX9, which is involved in the developmental regulation of neurons, cartilage and bone. In the zebrafish, MED12 defects cause maldevelopment of vertebrate embryonic structures such as the neural crest, which would alter function of the autonomic and peripheral nervous systems; and they also cause malformations of cell types serving as precursors to cartilage and bone, such as osteocytes. Some features found in LFS, like agenesis of the corpus callosum and cartilage-related craniofacial anomalies, are similar to defects found in zebrafish with MED12 and associated mutations. Genetics This condition is inherited in an X-linked dominant manner.<unk>  is inherited in an X-linked dominant manner. This means the defective gene responsible for the disorder (MED12) is located on the X chromosome, and only one copy of the defective gene is sufficient to cause the disorder when inherited from a parent who has the disorder. Males are normally hemizygous for the X chromosome, having only one copy. As a result, X-linked dominant disorders usually show higher expressivity in males than females. This phenomenon is thought to occur with LFS. As the X chromosome is one of the sex chromosomes (the other being the Y chromosome), X-linked inheritance is determined by the gender of the parent carrying a specific gene and can often seem complex. This is because, typically, females have two copies of the X-chromosome, while males have only one copy. The difference between dominant and recessive inheritance patterns also plays a role in determining the chances of a child inheriting an X-linked disorder from their parentage. In LFS, X-linked dominant inheritance was suspected, as boy and girl siblings in one family both exhibited the disorder. A scenario such as this would also be possible with X-linked recessive inheritance, but in this particular case report, the girl was believed to be a manifesting heterozygote carrying one copy of the mutated gene. Sporadic cases of LFS, where the disorder is present in an individual with no prior family history of it, have also been reported in a small number of affected males. Similarities to other genetic diseases An individual exhibiting intellectual disability and other symptoms similar to LFS was found to have a terminal deletion of the subtelomeric region in the short arm of chromosome 5. Deletion of this area of chromosome 5 is associated with intellectual disability, psychotic behavior, autism, macrocephaly and hypernasal-like speech, as well as the disorder Cri du chat syndrome.<unk>  (2006) suggests a detailed examination of chromosome 5 with FISH should be performed as part of the differential diagnosis of LFS. Mutations in the UPF3B gene, also found on the X chromosome, are another cause of X-linked intellectual disability. UPF3B is part of the nonsense-mediated mRNA decay (NMD) complex, which performs mRNA surveillance, detecting mRNA sequences that have been erroneously truncated (shortened) by the presence of nonsense mutations. Mutations in UPF3B alter and prevent normal function of the NMD pathway, resulting in translation and expression of truncated mRNA sequences into malfunctioning proteins that can be associated with developmental errors and intellectual disability. Individuals from two families diagnosed with LFS and one family with FGS were found to have mutations in UPF3B, confirming that the clinical presentations of the different mutations can overlap.
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