Apolipoprotein E and Cerebral Palsy: A Cross-sectional Study of the Influence of Genetic Variation on Clinical Manifestations of CP

Abstract

Cerebral palsy (CP) is the most common neurodevelopmental disorder in children, showing varying degrees of motor impairments, activity limitations and accompanying disturbances of function. A long-held belief has been that the cause of CP is birth asphyxia. However, the realization that only a small minority of CP cases can be attributed to a perinatal hypoxicischemic event has led to an extensive search for other causes, among them genetic factors. Although a number of genes have been investigated as possible causes of CP, the results have been inconclusive. The APOE gene and its protein product, apolipoprotein E (apoE), have received much attention. apoE is the most important transporter of cholesterol and lipids in the brain, substances that are essential for myelination, cell membrane and synapse formation. Furthermore, apoE has also been shown to have a number of regulatory functions of importance for normal brain development and function. The gene exists in three variants, APOEε2, APOEε3 and APOEε4, encoding three corresponding protein isoforms, apoE2, apoE3 and apoE4, which differ at only one or two positions in the amino acid sequence. Because of differing internal chemical bonds, these small differences will result in fundamental changes in the three-dimensional structure of the proteins, with subsequent differences in receptor affinity and lipid and cholesterol binding capacity. apoE4 has been identified as a risk factor for Alzheimer’s disease and poorer outcome after traumatic brain injuries compared to apoE2 and apoE3, making APOE interesting as a candidate for genetic influence in the development of CP. Several reports have, however, suggested that apoE4 may show properties of “antagonistic pleiotropy” i.e. having protective properties in a developing brain, but being detrimental in older people. The aim of the research for this thesis was to study whether the APOE alleles differentially influenced the severity of CP, assessed by gross and fine motor function, presence of epilepsy and eating and feeding abilities making gastrostomy tube feeding necessary. In accordance with the concept of antagonistic pleiotropy, the hypothesis was that carriers of APOEε4 would have the least severe CP. A further aim of the thesis was to determine whether the severity of CP would be influenced by genetic variation in six gene transcription enhancers regulating the expression of APOE and consequently the level of apoE protein in cerebrospinal fluid. Finally, based on the hypothesis that different allele combinations would give varying degrees of CP severity, the combinations of APOE alleles and genetic variation in a specific transcription enhancer (rs59007384 (TOMM40B)) were studied with regard to cerebral palsy severity. Children registered in the Cerebral Palsy Register of Norway (CPRN) were invited to participate and 281 families returned swabs with buccal cells for genotyping of APOE and six transcription regulators. The results of the genetic analyses were correlated with clinical data held in the CPRN. There were no associations between the genetic variations studied and gross motor function. Carriers of APOEε4 were more likely to have epilepsy, more likely to have poor fine motor function (children with unilateral CP) and more likely to need a gastrostomy feeding tube for adequate nutrition. Presence of a specific allele (a T-allele) in the transcription enhancer TOMM40B increased the risk of fine motor impairment and epilepsy. Compared to carriers of other allele combinations, carriers of no T-allele in TOMM40B and no APOEε4 allele were found to have a favorable outcome regarding fine motor function, epilepsy and gastrostomy tube feeding. No dose-response variation was found for other allele combinations. The findings did not support our main hypothesis of a protective developmental effect of APOEε4. On the contrary, carrying APOEε4 seemed to increase the risk of more severe CP. Thus, not only the structure of the apoE proteins, but also the amount in cerebrospinal fluid seemed to influence the severity of CP. Our hypothesis of expected variations in CP severity with different allele combinations was partly supported. Based on the demonstrated association of clinical manifestations of CP with specific genetic variations and allele combinations, the studies in this thesis support the concept of genetic influence on the severity of cerebral palsy.