Inborn errors of mitochondrial fatty acid oxidation (FAO) comprise the most common group of disorders identified through expanded newborn screening mandated in all 50 states in the United States, affecting 1:10,000 newborns. While some of the morbidity in FAO disorders (FAODs) can be reduced if identified through screening, a significant gap remains between the ability to diagnose these disorders and the ability to treat them. At least 25 enzymes and specific transport proteins are responsible for carrying out the steps of mitochondrial fatty acid metabolism, with at least 22 associated genetic disorders. Common symptoms in long chain FAODs (LC-FAODs) in the first week of life include cardiac arrhythmias, hypoglycemia, and sudden death. Symptoms later in infancy and early childhood may relate to the liver or cardiac or skeletal muscle dysfunction, and include fasting or stress-related hypoketotic hypoglycemia or Reye-like syndrome, conduction abnormalities, arrhythmias, dilated or hypertrophic cardiomyopathy, and muscle weakness or fasting- and exercise-induced rhabdomyolysis. In adolescent or adult-onset disease, muscular symptoms, including rhabdomyolysis, and cardiomyopathy predominate. Unfortunately, progress in developing better therapeutic strategies has been slow and incremental. Supplementation with medium chain triglyceride (MCT; most often a mixture of C8–12 fatty acids containing triglycerides) oil provides a fat source that can be utilized by patients with long chain defects, but does not eliminate symptoms. Three mitochondrial metabolic pathways are required for efficient energy production in eukaryotic cells: oxidative phosphorylation (OXPHOS), FAO, and the tricarboxylic (TCA) cycle, also called the Krebs cycle. Cell and mouse studies have identified a deficiency in TCA cycle intermediates in LC-FAODs, thought to be due to a depletion of odd chain carbon compounds in patients treated with a predominantly MCT fat source. Triheptanoin (triheptanoyl glycerol; UX007, Ultragenyx Pharmaceuticals) is chemically composed of three heptanoate (seven carbon fatty acid) molecules linked to glycerol through ester bonds that has the potential to replete TCA cycle intermediates through production of both acetyl-CoA and propionyl-CoA through medium chain FAO. Compassionate use, retrospective, and recently completed prospective studies demonstrate significant reduction of hypoglycemic events and improved cardiac function in LC-FAOD patients, but a less dramatic effect on muscle symptoms.

The most well-studied gene associated with MMIHS is ACTG2 (OMIM: 102545) (Thorson et al., 2014; Wangler et al., 2014), which has autosomal dominant inheritance. To date, four other genes are also linked to MMIHS among children born to consanguineous parents, suggesting autosomal recessive inheritance:

Purpose We characterize the clinical and molecular phenotypes of six unrelated individuals with intellectual disability and autism spectrum disorder who carry heterozygous missense variants of the PRKAR1B gene, which encodes the R1β subunit of the cyclic AMP-dependent protein kinase A (PKA). Methods Variants of PRKAR1B were identified by single- or trio-exome analysis. We contacted the families and physicians of the six individuals to collect phenotypic information, performed in vitro analyses of the identified PRKAR1B-variants, and investigated PRKAR1B expression during embryonic development. Results Recent studies of large patient cohorts with neurodevelopmental disorders found significant enrichment of de novo missense variants in PRKAR1B. In our cohort, de novo origin of the PRKAR1B variants could be confirmed in five of six individuals, and four carried the same heterozygous de novo variant c.1003C>T (p.Arg335Trp; NM_001164760). Global developmental delay, autism spectrum disorder, and apraxia/dyspraxia have been reported in all six, and reduced pain sensitivity was found in three individuals carrying the c.1003C>T variant. PRKAR1B expression in the brain was demonstrated during human embryonal development. Additionally, in vitro analyses revealed altered basal PKA activity in cells transfected with variant-harboring PRKAR1B expression constructs. Conclusion Our study provides strong evidence for a PRKAR1B-related neurodevelopmental disorder.

Purpose: We characterize the phenotypes of six unrelated individuals with intellectual disability and autism spectrum disorder, who carry heterozygous missense-variants of the PRKAR1B gene. Methods: Variants of PRKAR1B were identified by single-exome or trio-exome analysis. We contacted the families and physicians of the six individuals in order to collect clinical and phenotypic information. Results: PRKAR1B encodes the R1β subunit of the cyclic AMP-dependent protein kinase A (PKA), and is predominantly expressed in the central nervous system. Recent studies of patient cohorts with neurodevelopmental disorders found significant enrichment of de novo missense variants in PRKAR1B, and in vivo studies of the murine ortholog demonstrated altered hippocampal function and reduced neurogenic inflammation and long-term nociceptive pain in R1β-deficient mice. In our study, de novo origin of the PRKAR1B-variants could be confirmed in five out of six individuals, and four carried the same heterozygous de novo variant c.1003C>T (p. Arg335Trp; NM_001164760). Global developmental delay, autism spectrum disorder, and apraxia/dyspraxia has been reported in all six, and reduced pain sensitivity was found in three individuals carrying the c.1003C>T variant. Conclusion: Our study provides strong evidence for a novel, PRKAR1B-related neurodevelopmental disorder.

No abstract