Mucopolysaccharidosis type II (MPS II; also known as Hunter syndrome) is an X-linked multisystem disorder characterized by glycosaminoglycan (GAG) accumulation. The vast majority of affected individuals are male; on rare occasion heterozygous females manifest findings. Age of onset, disease severity, and rate of progression vary significantly among affected males. In those with the neuronopathic phenotype, central nervous system (CNS) involvement (manifesting primarily as progressive cognitive deterioration), progressive airway disease, and cardiac disease usually results in death in the first or second decade of life. In those with the non-neuronopathic phenotype, the CNS is minimally or not affected. However, the effect of GAG accumulation on other organ systems can be severe. Survival into the early adult years with normal intelligence is common in the non-neuronopathic phenotype. Additional findings in neuronopathic and non-neuronopathic MPS II include: short stature, macrocephaly with or without communicating hydrocephalus, macroglossia, hoarse voice, conductive and sensorineural hearing loss, dysostosis multiplex, spinal stenosis, carpal tunnel syndrome, and hepatosplenomegaly.

Curry-Jones syndrome (CRJS) is a multisystem disorder characterized by patchy skin lesions, polysyndactyly, diverse cerebral malformations, unicoronal craniosynostosis, iris colobomas, microphthalmia, and intestinal malrotation with myofibromas or hamartomas (summary by Twigg et al., 2016).

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. In individuals with early-onset disease (prior to age 12 years), liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. In the juvenile/adult-onset form (age 12-40 years), disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years) is characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis.

Autosomal recessive familial visceral neuropathy-1 (VSCN1) is characterized by a broad spectrum of developmental anomalies associating neural crest and extraneural crest features, including intestinal dysmotility due to aganglionosis (Hirschsprung disease), hypoganglionosis, and/or chronic intestinal pseudoobstruction. Some patients develop progressive peripheral neuropathy, and arthrogryposis has been observed. Hypoplasia or aplasia of the olfactory bulb and of the external auditory canals, as well as microtia or anotia, have been reported. Patients also exhibit facial dysmorphisms, including microretrognathia in most; other variable features include structural cardiac anomalies and arthrogryposis with multiple pterygia (Le et al., 2021). Genetic Heterogeneity of Familial Visceral Neuropathy Autosomal recessive familial visceral neuropathy-2 (VSCN2; 619465) is caused by mutation in the ERBB2 gene (164870) on chromosome 17q12. Also see VSCN3 (609629) for an autosomal dominant form of the disorder.

Mungan syndrome (MGS) is characterized by chronic intestinal pseudoobstruction (CIPO), megaduodenum, long-segment Barrett esophagus, and cardiac abnormalities of variable severity (summary by Bonora et al., 2015).

FLNA deficiency is associated with a phenotypic spectrum that includes FLNA-related periventricular nodular heterotopia (Huttenlocher syndrome), congenital heart disease (patent ductus arteriosus, atrial and ventricular septal defects), valvular dystrophy, dilatation and rupture of the thoracic aorta, pulmonary disease (pulmonary hypertension, alveolar hypoplasia, emphysema, asthma, chronic bronchitis), gastrointestinal dysmotility and obstruction, joint hypermobility, and macrothrombocytopenia.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. In individuals with early-onset disease (prior to age 12 years), liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. In the juvenile/adult-onset form (age 12-40 years), disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years) is characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis.

Syndrome with characteristics of sick sinus syndrome and intestinal pseudo-obstruction. The heart and digestive issues develop at the same time, usually by age 20. The syndrome is caused by mutations in the SGO1 gene. This gene provides instructions for making part of a protein complex cohesin. This protein complex helps control the placement of chromosomes during cell division. Research suggests that SGO1 gene mutations may result in a cohesin complex that is less able to hold sister chromatids together, resulting in decreased chromosomal stability during cell division. This instability is thought to cause senescence of cells in the intestinal muscle and in the sinoatrial node, resulting in problems maintaining proper rhythmic movements of the heart and intestines.

Mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease is characterized by progressive gastrointestinal dysmotility (manifesting as early satiety, nausea, dysphagia, gastroesophageal reflux, postprandial emesis, episodic abdominal pain and/or distention, and diarrhea); cachexia; ptosis/ophthalmoplegia or ophthalmoparesis; leukoencephalopathy; and demyelinating peripheral neuropathy (manifesting as paresthesias (tingling, numbness, and pain) and symmetric and distal weakness more prominently affecting the lower extremities). The order in which manifestations appear is unpredictable. Onset is usually between the first and fifth decades; in about 60% of individuals, symptoms begin before age 20 years.

ACTG2 visceral myopathy is a disorder of smooth muscle dysfunction of the bladder and gastrointestinal system with phenotypic spectrum that ranges from mild to severe. Bladder involvement can range from neonatal megacystis and megaureter (with its most extreme form of prune belly syndrome) at the more severe end, to recurrent urinary tract infections and bladder dysfunction at the milder end. Intestinal involvement can range from malrotation, neonatal manifestations of microcolon, megacystis microcolon intestinal hypoperistalsis syndrome, and chronic intestinal pseudoobstruction (CIPO) in neonates at the more severe end to intermittent abdominal distention and functional intestinal obstruction at the milder end. Affected infants (with or without evidence of intestinal malrotation) often present with feeding intolerance and findings of non-mechanical bowel obstruction that persist after successful surgical correction of malrotation. Individuals who develop manifestations of CIPO in later childhood or adulthood often experience episodic waxing and waning of bowel motility. They may undergo frequent abdominal surgeries (perhaps related to malrotation or adhesions causing mechanical obstruction) resulting in resection of dilated segments of bowel, often becoming dependent on total parenteral nutrition (TPN).

Visceral myopathy-2 (VSCM2) is characterized by gastrointestinal symptoms resulting from intestinal dysmotility and paresis, including abdominal distention, pain, nausea, and vomiting. Some patients exhibit predominantly esophageal symptoms, with hiatal hernia and severe reflux resulting in esophagitis and stricture, whereas others experience chronic intestinal pseudoobstruction. Bladder involvement resulting in megacystis and megaureter has also been observed and may be evident at birth (Dong et al., 2019; Gilbert et al. (2020)).

Mitochondrial DNA depletion syndrome-20 (MTDPS20) is an autosomal recessive multisystem disorder with variable manifestations and severity. Most patients develop symptoms in childhood, although the onset can range from infancy to the teenage years. Prominent features include severe gastrointestinal dysmotility often requiring parenteral nutrition, neurogenic bladder, and muscle weakness and atrophy. Neurologic involvement manifests as headaches, stroke-like episodes, seizures, pyramidal signs, and learning difficulties or cognitive decline. Brain imaging usually shows diffuse leukoencephalopathy and may show cerebellar atrophy. The disorder results from a defect in the maintenance and repair of mitochondrial DNA, resulting in mtDNA depletion and impaired mitochondrial function (summary by Bonora et al., 2021). For a discussion of genetic heterogeneity of mtDNA depletion syndromes, see MTDPS1 (603041).