Microcephaly with simplified gyration, epilepsy, and infantile diabetes linked to inappropriate apoptosis of neural progenitors

Abstract

We describe a syndrome of primary microcephaly with simplified gyral pattern in combination with severe infantile epileptic encephalopathy and early-onset permanent diabetes in two unrelated consanguineous families with at least three affected children. Linkage analysis revealed a region on chromosome 18 with a significant LOD score of 4.3. In this area, two homozygous nonconserved missense mutations in immediate early response 3 interacting protein 1 (IER3IP1) were found in patients from both families. IER3IP1 is highly expressed in the fetal brain cortex and fetal pancreas and is thought to be involved in endoplasmic reticulum stress response. We reported one of these families previously in a paper on Wolcott-Rallison syndrome (WRS). WRS is characterized by increased apoptotic cell death as part of an uncontrolled unfolded protein response. Increased apoptosis has been shown to be a cause of microcephaly in animal models. An autopsy specimen from one patient showed increased apoptosis in the cerebral cortex and pancreas beta cells, implicating premature cell death as the pathogenetic mechanism. Both patient fibroblasts and control fibroblasts treated with siRNA specific for IER3IP1 showed an increased susceptibility to apoptotic cell death under stress conditions in comparison to controls. This directly implicates IER3IP1 in the regulation of cell survival. Identification of IER3IP1 mutations sheds light on the mechanisms of brain development and on the pathogenesis of infantile epilepsy and early-onset permanent diabetes.

Figure 1

Family Pedigrees and Linkage Data (A) Simplified genealogical trees of two unrelated families with MSG, epilepsy, and PND. Filled symbols represent affected patients. A double line represents consanguinity. An asterisk represents subjects analyzed by SNP arrays and included in the linkage analysis. The results of microsatellite marker analysis confirming homozygosity are also shown. The shaded areas represent the alleles containing the mutation, and the box represents the area found by linkage analysis. (B) Output from easyLINKAGE showing an HLOD score of 4.3 for the region on chromosome 18q.

Figure 2

Patient Brain MRIs (A–C) MRIs from patient 1 performed at 1 year of age. (A) T1-weighted image showing a simplified gyral pattern and increased intracranial space with near-normal ventricular size. (B and C) T2-weighted coronal and axial view showing equal involvement of parietal lobes, normal cerebellum, and basal ganglia with delayed myelination. (D–F) Low-resolution MRIs from patient 2 at approximately 7 months of age. (D) Sagittal T1-weighted view showing a thin but present corpus callosum and normal brain stem. (E) T1-weighted coronal view through the hippocampus showing simplified gyration and hippocampal hypoplasia. (F) T2- weighted view, demonstrating the simplified gyral pattern and normal ventricular size.

Figure 3

Brain Macroscopy and Histopathology (A and B) Macroscopic view in real proportions of normal age-matched brain (upper specimen) compared with index patient 3 postmortem brain (lower specimen); note the extreme microcephaly and simplified gyration looking almost lissencephalic in its medial surface. (C and D) Macroscopic photographs of patient 3 brain, showing shallow and insufficient-in-number sulci over the whole brain surface, but more apparent in the temporal and parietal lobes. Note that photos in (C) and (D) are not in scale with those in (A) and (B). (E–J) Histopathology. (E) Histopathology of normal cerebral frontal cortex (age-matched to patient 3). (F) Patient 3 histopathology of the cerebral cortex showing numerous apoptotic neurons (arrows) and some apoptotic glial cells (H&E × 20). (G) Cerebellar folia: Purkinje neurons are apoptotic (arrows) and internal granule cell layer is diminished (H&E × 20). (H) Nearly all neurons and the inferior olivary nuclei are apoptotic. (I and J) Patient 3 pancreas stained for insulin (J), showing depletion of insulin-producing islet cells in comparison to a normal pancreas (I).

Figure 4

IER3IP1 Sequence and Expression in Developing Brain (A) Schematic representation of human IER3IP1 showing the sequence, the predicted protein domains, and the location of the p.Val21Gly and p.Leu78Pro mutations. Cross-species conservation of IER3IP1 in the areas of the mutations is shown at the bottom of the diagram. Both of the predicted amino acid changes are in a highly conserved area of the gene and in the hydrophobic/transmembrane domains, and they are depicted in red. (B) Expression of Ier3ip1 at E14.5 days in the whole-mouse embryo (left); zoomed in, in the right panel is the mouse brain, with arrows pointing to increased expression in the ventricular and subventricular zone at the site of neurogenesis.

Figure 5

Susceptibility of Cultured Fibroblasts to Apoptosis and TNF-α Stimulation (A) Patient cells are more susceptible to apoptosis, similar to cells from a known WRS patient, when treated with 5 mM DTT, in comparison to five control cell lines (p ≤ 0.001, unpaired t test, SPSS version 17.0). Values represent a mean of three experiments ± SEM. (B) IER3IP1 expression levels in control fibroblasts are decreased approximately 10-fold after the addition of IER3IP1 siRNA, as compared to control siRNA (p ≤ 0.001, unpaired t test, SPSS version 17.0) (left panel). Control fibroblasts treated with IER3IP1 siRNA also demonstrate a significantly increased susceptibility to apoptosis (p ≤ 0.001, unpaired t test, SPSS version 17.0) when treated with 1.25 mM DTT in comparison to control fibroblasts (right panel). Values represent a mean of three experiments ± SEM. (C) IER3 mRNA levels are increased approximately 10-fold in patient and control cells after the addition of TNF-α (left panel) (p < 0.0001, unpaired t test, SPSS version 17.0). IER3IP1 levels are increased approximately 1.5-fold in patient and control fibroblast cell lines after the addition of TNF-α (p < 0.001, unpaired t test, SPSS version 17.0) (right panel). Stars represent a significant p < 0.01 difference.