A disease caused by mosaic gain-of-function (GoF) of several genes in the MTOR pathway (MTOR, PIK3CA, PIK3R2 and AKT3) are functionally the same despite significant phenotypic variability. These GoF variants result in overgrowth due to an over-activation of key genes in this pathway. The phenotypic variability is generally attributed to the mosaic fraction and affected tissue types. For example, macrocephaly is noted if the variant is identified in the brain, but non symmetric overgrowth of that limb is noted when the variant is only present in the affected limb. The pathologies of the affected tissue often reveal similar characteristics such as cellular overgrowth. However, this is not always the case especially with focal cortical dysplasia. At times the characteristics pathologies are not present in the tissue but sampling biases are an issue. FCD resections often involve a very small area and so a very small amount of tissue is available for pathology and it is not guaranteed that lesional tissue is sent. Therefore, having a single disease term which can encompass the phenotypic variability yet provide a unifying molecular diagnosis name makes sense given the common functional mechanism. [from MONDO]

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