MRI characterization of cerebral dysgenesis in maternal PKU

Neurometabolic disorders and congenital malformations of the central nervous system

Both malformations of the central nervous system and neurometabolic disorders are common, mainly in highly consanguineous populations. Both metabolic pathways and developmental pathways are closely related and interact with each other. Neurometabolic disorders can lead to disturbances in brain development through multiple mechanisms that include deficits in energy metabolism, critical nutrient deficiency, accumulation of neurotoxic substrates, abnormality in cell membrane constituents, and interference in cell-to-cell signaling pathways. The anomalies observed include absent or hypoplastic corpus callosum, midline brain defects, and malformations of the cortex, the cerebellum and the brain stem. Early diagnosis of an underlying inherited neurometabolic disorders is critical for the institution of treatment, which may positively influence prognosis, and allow for proper genetic counseling. In this review, we discuss those disorders in which the structural brain malformation is a dominant feature, and propose a practical approach that will permit a physician to investigate, and treat these disorders.

Elevated phenylalanine levels interfere with neurite outgrowth stimulated by the neuronal cell adhesion molecule L1 in vitro

Elevated levels of phenylalanine (Phe) as observed in patients with phenylketonuria interfere with proper neuronal development, leading to severe psychomotor deficits and mental retardation. We have analyzed the effects of Phe on neurite outgrowth in vitro. When expressed in fibroblasts, the neuronal cell adhesion molecules L1 and plexin B3 strongly increase the length of neurites emanating from cerebellar neurons in co-culture experiments. Elevated Phe blocks L1-mediated, but not plexin B3-mediated outgrowth, whereas tyrosine is ineffective. Elevated Phe also interferes with aggregation of fibroblasts overexpressing L1, suggesting that the pathological effect of elevated Phe occurs by interfering with L1-mediated cell adhesion.

Teratogenic alleles and neurodevelopmental disorders

The genetic interaction between mother and fetus during pregnancy is discussed, focusing on teratogenic alleles that act in the mother to alter fetal development and contribute to a neurodevelopmental disorder. For these alleles, the mother is the genetic patient. Teratogenic alleles interact with modifying and specificity alleles that act in the fetus and with environmental factors. Based on examples of the model, two candidate mechanisms emerge as contributors to neurodevelopmental disorders, folate-homocysteine pathways and immune/inflammatory mechanisms. Both, acting in mothers, affect fetal development and contain many polymorphic genes. These two systems interact with each other. Common functional polymorphisms of mild effect in these two systems of interacting genes are good candidates for teratogenic alleles. The presence of teratogenic alleles complicates gene identification for neurodevelopmental disorders. However, using the special methods required to identify teratogenic alleles is important because this could lead to new approaches to prevention and improved therapy of these disorders.