Multiple congenital anomalies-hypotonia-seizures syndrome-2 (MCAHS2) is an X-linked recessive neurodevelopmental disorder characterized by dysmorphic features, neonatal hypotonia, early-onset myoclonic seizures, and variable congenital anomalies involving the central nervous, cardiac, and urinary systems. Some affected individuals die in infancy (summary by Johnston et al., 2012). The phenotype shows clinical variability with regard to severity and extraneurologic features. However, most patients present in infancy with early-onset epileptic encephalopathy associated with developmental arrest and subsequent severe neurologic disability; these features are consistent with a form of developmental and epileptic encephalopathy (DEE) (summary by Belet et al., 2014, Kato et al., 2014). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of nomenclature and genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Hereditary diffuse leukoencephalopathy with spheroids-2 (HDLS2) is an autosomal dominant neurodegenerative disorder characterized by progressive cognitive and executive dysfunction, psychiatric disturbances, and neurologic symptoms, such as gait abnormalities, paresis, seizures, and rigidity. Symptom onset is usually in adulthood, although earlier onset has been reported. Some patients have an acute encephalopathic course with severe neurologic decline resulting in early death, whereas other patients have a more protracted and chronic disease course. Neuropathologic examination shows a leukoencephalopathy with axonal spheroids and myelination defects (summary by Sundal et al., 2012). For a discussion of genetic heterogeneity of HDLS, see HDLS1 (221820).