Homozygous loss-of-function mutations in the HOXA1 gene result in disorders with variable phenotypic expressivity that span a spectrum. Two related, but somewhat distinctive, phenotypes have been described in different populations: the Athabaskan brainstem dysgenesis syndrome (ABDS) in Native Americans, and Bosley-Salih-Alorainy syndrome (BSAS) in individuals from the Middle East, including Turkey and Saudi Arabia. Features common to both disorders include Duane retraction syndrome with variable gaze palsies, sensorineural deafness associated with inner ear abnormalities, and delayed motor development. More variable features, observed in both disorders, include conotruncal cardiac malformations, cerebral vascular malformations, and impaired intellectual development with autism. Unique to ABDS are central hypoventilation, often resulting in early death, facial weakness, and more severe cognitive deficits. These features are thought to be due to a more severe malformation of the hindbrain in ABDS compared to BSAS (summary by Tischfield et al., 2005).

The spectrum of DCTN1-related neurodegeneration includes Perry syndrome, distal hereditary motor neuronopathy type 7B (dHMN7B), frontotemporal dementia (FTD), motor neuron disease / amyotrophic lateral sclerosis (ALS), and progressive supranuclear palsy. Some individuals present with overlapping phenotypes (e.g., FTD-ALS, Perry syndrome-dHMN7B). Perry syndrome (the most common of the phenotypes associated with DCTN1) is characterized by parkinsonism, neuropsychiatric symptoms, hypoventilation, and weight loss. The mean age of onset in those with Perry syndrome is 49 years (range: 35-70 years), and the mean disease duration is five years (range: 2-14 years). In most affected persons, the reported cause/circumstance of death relates to sudden death/hypoventilation or suicide.

The spectrum of MECP2-related phenotypes in females ranges from classic Rett syndrome to variant Rett syndrome with a broader clinical phenotype (either milder or more severe than classic Rett syndrome) to mild learning disabilities; the spectrum in males ranges from severe neonatal encephalopathy to pyramidal signs, parkinsonism, and macroorchidism (PPM-X) syndrome to severe syndromic/nonsyndromic intellectual disability. Females: Classic Rett syndrome, a progressive neurodevelopmental disorder primarily affecting girls, is characterized by apparently normal psychomotor development during the first six to 18 months of life, followed by a short period of developmental stagnation, then rapid regression in language and motor skills, followed by long-term stability. During the phase of rapid regression, repetitive, stereotypic hand movements replace purposeful hand use. Additional findings include fits of screaming and inconsolable crying, autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, seizures, and acquired microcephaly. Males: Severe neonatal-onset encephalopathy, the most common phenotype in affected males, is characterized by a relentless clinical course that follows a metabolic-degenerative type of pattern, abnormal tone, involuntary movements, severe seizures, and breathing abnormalities. Death often occurs before age two years.

Congenital central hypoventilation syndrome-3 (CCHS3) is an autosomal recessive disorder characterized by slow and shallow breathing due to a deficiency in autonomic control of respiration. Affected individuals present in the neonatal period with respiratory insufficiency and absence of the hypercapnic reflex that stimulates breathing. Patients also have gastrointestinal problems manifest as feeding difficulties and diarrhea or constipation. Other features may include poor heat tolerance and paroxysmal hypertension (Hernandez-Miranda et al., 2018). For a discussion of genetic heterogeneity of CCHS, see CCHS1 (209880).

Congenital central hypoventilation syndrome (CCHS) represents the extreme manifestation of autonomic nervous system dysregulation (ANSD) with the hallmark of disordered respiratory control. The age of initial recognition of CCHS ranges from neonatal onset (i.e., in the first 30 days of life) to (less commonly) later onset (from 1 month to adulthood). Neonatal-onset CCHS is characterized by apparent hypoventilation with monotonous respiratory rates and shallow breathing either during sleep only or while awake as well as asleep; ANSD including decreased heart rate beat-to-beat variability and sinus pauses; altered temperature regulation; and altered pupillary response to light. Some children have altered development of neural crest-derived structures (i.e., Hirschsprung disease, altered esophageal motility/dysphagia, and severe constipation even in the absence of Hirschsprung disease) and/or tumors of neural crest origin (neuroblastoma, ganglioneuroma, and ganglioneuroblastoma). Neurocognitive delay is variable, and possibly influenced by cyanotic breath holding, prolonged sinus pauses, need for 24-hour/day artificial ventilation, and seizures. Later-onset CCHS is characterized by alveolar hypoventilation during sleep and attenuated manifestations of ANSD.

Combined oxidative phosphorylation deficiency-57 (COXPD57) is an autosomal recessive multisystem mitochondrial disease with varying degrees of severity from premature death in infancy to permanent disability in young adulthood (Lee et al., 2022). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).