Dysbiosis of the intestinal microbiome as a component of pathophysiology in the inborn errors of metabolism

Inborn errors of metabolism (IEMs) represent monogenic disorders in which specific enzyme deficiencies, or a group of enzyme deficiencies (e.g., peroxisomal biogenesis disorders) result in either toxic accumulation of metabolic intermediates or deficiency in the production of key end-products (e.g., low cholesterol in Smith-Lemli-Opitz syndrome (Gedam et al., 2012 [1]); low creatine in guanidinoacetic acid methyltransferase deficiency (Stromberger, 2003 [2])). Some IEMs can be effectively treated by dietary restrictions (e.g., phenylketonuria (PKU), maple syrup urine disease (MSUD)), and/or dietary intervention to remove offending compounds (e.g., acylcarnitine excretion with the oral intake of L-carnitine in the disorders of fatty acid oxidation). While the IEMs are predominantly monogenic disorders, their phenotypic presentation is complex and pleiotropic, impacting multiple physiological systems (hepatic and neurological function, renal and musculoskeletal impairment, cardiovascular and pulmonary activity, etc.). The metabolic dysfunction induced by the IEMs, as well as the dietary interventions used to treat them, are predicted to impact the gut microbiome in patients, and it is highly likely that microbiome dysbiosis leads to further exacerbation of the clinical phenotype. That said, only recently has the gut microbiome been considered as a potential pathomechanistic consideration in the IEMs. In this review, we overview the function of the gut-brain axis, the crosstalk between these compartments, and the expanding reports of dysbiosis in the IEMs recently reported. The potential use of pre- and probiotics to improve clinical outcomes in IEMs is also highlighted.