Megencephaly, significance, biology and identification of a genetic cause

Abstract

Enlarged brain is termed megencephaly or megalencephaly, and is defined as a brain volume above the 98th percentile. It is a major sign in several neurological diseases with symptoms ranging from very mild to lethal, including mental retardation, low intelligence, motoric and neurological disturbances, seizures and dysmorphic features. The etiology of megalencephaly syndromes is in many cases unknown, but they are often heritable.

The megencephaly mouse model, mceph/mceph, displays continuous brain growth and progressive motoric and neurological disturbances. Symptoms are present from approximately three weeks of age. Histological and compositional examinations of the brain revealed overall brain cell hypertrophy and slight hippocampal astrocytosis, but absence of hydrocephalus, edema. and obvious structural abnormalities. The recessive phenotype results from a spontaneous mutation, previously mapped to a three-centiMorgan interval on Chromosome 6.

The objectives of this thesis were to evaluate inheritance and effects of primary megalencephaly in humans and to identify the genetic cause of megencephaly and associated molecular aberrations in the mceph/mceph mouse.

In paper I, the association between primary megalencephaly (PMG) at birth and later psycho- sensory conditions as well as the maternal inheritance of PMG at birth were studied, using population based data sets. In this study, PMG was significantly associated with a low intelligence level and displayed a trend for association to mental retardation. Associations between mother and child PMG and support for a multifactorial inheritance of PMG were found.

In paper II and III, dramatic changes in the expression of molecules in the neurotrophic insulin- like growth factor system, transforming growth factor 01 and the neuropeptides cholecystokinin, enkephalin, galanin and neuropeptide Y were found in the adult mceph/mceph brain. Changes were mainly confined to the hippocampus, the parietal cortex and the piriform/entorhinal cortex and displayed significant similarities to alterations found in induced rodent epileptic models.

In paper IV, the mceph mutation was identified, using a positional cloning approach. mceph/mceph mice carry an eleven base pair deletion in the Shaker voltage-gated potassium channel gene, Kcnal. The mutation leads to a frameshift and premature termination at amino acid 230s (out of 495). The putative truncated MCEPH protein may retain the domains for assembly and may congregate non-functional complexes of multiple Shaker family subunits. Kcnal mRNA levels were increased in mceph/mceph brains. mceph/mceph mice was found to experience recurrent epileptic events and abnormal electroencephalograms. However, in contrast to the commonly demonstrated epilepsy-induced neurodegeneration, the mceph mutation leads to seizures with a concomitant increase in brain size, without overt neural atrophy.

In paper V, the development and magnitude of the mceph/mceph brain enlargement was studied using magnetic resonance imaging (MRI). Increases in the ventral cortex, the hippocampus and the parietal cortex contributed to a 23% increased total brain sectional area in the adult mceph/mceph brain. Regional increases appeared already at three weeks of age. Also, altered expression of neuroactive molecules was detected in mceph/mceph already at two and three weeks of age.

In summary, megalencephaly-related seizures have generally been considered pathological consequences of the enlarged brain, whereas epilepsy is often associated with shrinkage and degeneration of neural tissue. The mceph/mceph mouse is a model where the mutation of an ion channel causes abnormal electrical brain activity, brain enlargement and typical signs of reflective neural plasticity but no overt neural atrophy. These findings may be of significance for the diagnosis and possibly even future treatment of similar disorders in human.