Research Article: A Point Mutation in Translation Initiation Factor eIF2B Leads to Function- and Time-Specific Changes in Brain Gene Expression

Date Published: October 31, 2011

Publisher: Public Library of Science

Author(s): Liraz Marom, Igor Ulitsky, Yuval Cabilly, Ron Shamir, Orna Elroy-Stein, Bob Lightowlers.

Abstract: Mutations in eukaryotic translation initiation factor 2B (eIF2B) cause Childhood Ataxia with CNS Hypomyelination (CACH), also known as Vanishing White Matter disease (VWM), which is associated with a clinical pathology of brain myelin loss upon physiological stress. eIF2B is the guanine nucleotide exchange factor (GEF) of eIF2, which delivers the initiator tRNAMet to the ribosome. We recently reported that a R132H mutation in the catalytic subunit of this GEF, causing a 20% reduction in its activity, leads under normal conditions to delayed brain development in a mouse model for CACH/VWM. To further explore the effect of the mutation on global gene expression in the brain, we conducted a wide-scale transcriptome analysis of the first three critical postnatal weeks.

Genome-wide mRNA expression of wild-type and mutant mice was profiled at postnatal (P) days 1, 18 and 21 to reflect the early proliferative stage prior to white matter establishment (P1) and the peak of oligodendrocye differentiation and myelin synthesis (P18 and P21). At each developmental stage, between 441 and 818 genes were differentially expressed in the mutant brain with minimal overlap, generating unique time point-specific gene expression signatures.

The current study demonstrates that a point mutation in eIF2B, a key translation initiation factor, has a massive effect on global gene expression in the brain. The overall changes in expression patterns reflect multiple layers of indirect effects that accumulate as the brain develops and matures. The differentially expressed genes seem to reflect delayed waves of gene expression as well as an adaptation process to cope with hypersensitivity to cellular stress.

Partial Text: Childhood Ataxia with Central nervous system Hypomyelination (CACH), also known as Vanishing White Matter disease (VWM), is an autosomal recessive genetic leukodystrophy associated with mutations in any one of the five subunits of eukaryotic translation initiation factor 2B (eIF2B) [1], [2]. The classical form of CACH/VWM is manifested during early childhood as progressive motor and cognitive impairments that ultimately lead to death by adolescence. Onset of signs and symptoms usually follows exposure to various environmental stressors, such as febrile illness, minor head trauma and acute fright, which also lead to exacerbation of symptoms during the course of disease progression. The diagnosis of CACH/VWM is based on MRI scans showing decreased brain white matter signals. The disease predominantly affects oligodendrocytes and astrocytes, while neurons are relatively preserved [3]–[9]. An R136H mutation in the human EIF2B5 gene, encoding the catalytic subunit of eIF2B, is known to cause the classical form of CACH/VWM when present in a homozygous state. We recently generated a mutant mouse model for CACH/VWM disease by introducing an R132H mutation into the mouse EIf2b5 gene locus, which corresponds to the R136H mutation in the human gene. The mutant mice exhibit delayed development of brain white matter, higher proportion of small-caliber nerve fibers, abnormal abundance of oligodendrocytes and astrocytes, specifically in young animals, and abnormal levels of major myelin proteins. Moreover, the mutant mice failed to recover from cuprizone-induced demyelination, reflecting an increased sensitivity to brain insults and difficulty in repairing damaged myelin [10].

Brain development is the result of coordinated interactions between molecular networks and systematic cross-talk between certain cell types influencing the development of others via waves of transcription/translation of specific genes. During development, the brain undergoes typical stages involving proliferation, differentiation, myelination, synapse formation, and maturation of synaptic function [18], [23]–[25]. Research into the underlying molecular mechanisms led to a series of gene profiling studies revealing that during development, gene expression profiles undergo spatial and temporal changes that correlate with phenotypic functions [19], [26]–[29]. The study of Wen et al. [30] demonstrated that fundamental patterns of temporal fluctuations in gene expression during development can be determined even without the dissection of whole tissues into distinct anatomical sub-regions. Here we studied a mutant mouse homozygous for a point mutation in the catalytic subunit of the translation initiation factor eIF2B, which leads to a ∼20% decrease in its enzymatic activity. This mutant mouse is the first animal model for eIF2B-related leukodystrophy (also termed CACH/VWM disease). The disease in humans is manifested as impaired white matter functions, which deteriorate upon stress conditions to loss of axonal function, followed by death [5]–[7]. We recently demonstrated that the Eif2b5-mutated mice suffered from delayed brain development under normal conditions, a circumstance not associated before with human patients probably since it can only be detected in the pre-symptomatic stage [10]. The current study further demonstrates the profound effect of the seemingly-superficial impairment in eIF2B activity on the entire gene expression profile throughout early postnatal brain development. Both wild-type and mutant mice underwent similar changes in global gene expression, which underlie CNS transition from a primarily proliferative state during the first postnatal week to a highly differentiated state later on. However, the expression of cell-cycle associated genes at P1 and white matter-specific genes at P21 were significantly dysregulated in mutant compared to wild-type mice (Figures 2B, 3C). The unique time-point-specific differential gene expression signature (Figure 1, Table 1) together with the delayed brain development observed in Eif2b5-mutated mice [10], suggests that the mutation in eIf2b5 is responsible for delayed waves of gene expression, resulting in prolonged development. The delay is not uniform across the transcriptome, but seems to affect specific subset of genes. This can be the result of a slight decrease in translation efficiency of mRNAs that encode key regulators e.g. specific transcription factors, RNA processing machinery components and/or RNA binding proteins that affect the steady-state level of a subset of gene products.