ASPP2 deficiency causes features of 1q41q42 microdeletion syndrome

Abstract

Chromosomal abnormalities are implicated in a substantial number of human developmental syndromes, but for many such disorders little is known about the causative genes. The recently described 1q41q42 microdeletion syndrome is characterized by characteristic dysmorphic features, intellectual disability and brain morphological abnormalities, but the precise genetic basis for these abnormalities remains unknown. Here, our detailed analysis of the genetic abnormalities of 1q41q42 microdeletion cases identified TP53BP2, which encodes apoptosis-stimulating protein of p53 2 (ASPP2), as a candidate gene for brain abnormalities. Consistent with this, Trp53bp2-deficient mice show dilation of lateral ventricles resembling the phenotype of 1q41q42 microdeletion patients. Trp53bp2 deficiency causes 100% neonatal lethality in the C57BL/6 background associated with a high incidence of neural tube defects and a range of developmental abnormalities such as congenital heart defects, coloboma, microphthalmia, urogenital and craniofacial abnormalities. Interestingly, abnormalities show a high degree of overlap with 1q41q42 microdeletion-associated abnormalities. These findings identify TP53BP2 as a strong candidate causative gene for central nervous system (CNS) defects in 1q41q42 microdeletion syndrome, and open new avenues for investigation of the mechanisms underlying CNS abnormalities.

Figure 1

TP53BP2 is deleted in 1q41q42 microdeletion patients with brain abnormalities. (a) Genomic locations of 1q41q42 deletions from new and published patients. Red indicates brain morphological abnormalities were reported for the patient, green indicates absence of reported brain abnormalities. The positions of the CAPN8, CAPN2, TP53BP2, and FBXO28 genes are marked with black arrows; black line marks the position of TP53BP2. *FBXO28 is not contained in the smallest region of overlap (SRO) of patient deletions with brain abnormalities. Panel generated using UCSC genome browser (http://genome.ucsc.edu), hg19 assembly. (b) The proportions of cases with and without TP53BP2 deletion with and without brain abnormalities. (c) Focus on the SRO of patient deletions with brain morphological abnormalities. Patients with abnormal ventricles are marked in dark purple

Figure 2

Enlarged ventricles and other abnormalities in patients with TP53BP2 deletions. (a) Summary of the patient records analysed in this study. (b) Quantification of lateral ventricle volume by probabilistic morphometry. Volumes were computed by ALVIN on all suitable scans in each MRI series, from 1q41q42 microdeletion patients with and without TP53BP2 deletion. Left panel – T1 and T2 sagittal, axial and coronal scans were used for computations of LV volumes, bars showing mean±S.D.; right panel – T1 axial scans used only, each data point showing the mean from T1 axial scans for each patient. MRI scans of 12 patients and a group of 28 healthy paediatric individuals (NIH controls) were used. Median age of microdeletion cohort=19 months, median of NIH controls=15 months. For statistical comparison of LV volumes in patient groups, values were first tested for normality by the D'Agostino and Pearson omnibus test, then compared to NIH controls using the two-tailed Student's t-test and corrected for multiple hypothesis testing by multiplying the P-value by the number of tests. Mean±S.D. are shown. (c) Axial and sagittal MRI images of brains of 1q41q42 microdeletion patients. T1 non-contrast images are shown, except for cases where only T2 scans were available (Rosenfeld Subject 14, Spreiz et al. and Case 7). Double white arrowheads: hypoplasia of corpus callosum; single white arrowheads: asymmetry in lateral ventricle size; asterisk: mega cisterna magna

Figure 3

Trp53bp2^Δ3/Δ3 and Trp53bp2^Δ3/+ mice have CNS defects. (a) Gross dilation of lateral ventricles is visible in Trp53bp2^Δ3/Δ3 embryos, resulting in bulging foreheads. Intraventricular haemorrhage is frequently seen (white arrows). (b) Present and absent pineal gland (red arrows) in wild-type and Trp53bp2^Δ3/Δ3 embryos, respectively, as seen in a single HREM section. (c) Surface rendering of lateral ventricles in 1q41q42 microdeletion patients (left, constructed from MRI scans) and Trp53bp2^Δ3/Δ3 vs wild-type embryos (right, constructed from microCT scans). (d) Brain abnormalities in heterozygous Trp53bp2^Δ3/+ embryos. Left: Asymmetry of lateral ventricles in a heterozygous Trp53bp2^Δ3/+ embryo caused by a structural abnormality (red arrow). MicroCT image is shown. Right: Asymmetry in neuroepithelial tissue distribution in the brain (red arrow) and the associated subcranial haemorrhage (white and red arrows) in a BALB/c Trp53bp2^Δ3/+ embryo

Figure 4

Trp53bp2^Δ3/Δ3 mice have neural tube defects. Strain dependence of NTD penetrance in Trp53bp2^Δ3/Δ3 mice: Embryo phenotypes in pure C57BL/6 (left), mixed 129 Sv-C57BL/6 (middle), and BALB/c (right) background embryos range in severity from small exencephaly (right, white arrow) to severe craniorachischisis (left upper panel, white arrows). Spina bifida is also observed in the mixed (middle panels, white arrows) and B6 (left lower panel, white arrow) backgrounds

Figure 5

ASPP2-deficient mice have non-CNS phenotypes. (a) Craniofacial abnormalities in Trp53bp2^Δ3/Δ3 E14.5 embryos include dolichocephaly and hypoplasia or agenesis of the lower jaw (white arrows). (b) Eye abnormalities observed in Trp53bp2^Δ3/Δ3 mice feature coloboma (top, missing tissue indicated by red arrow) and microphthalmia (bottom) in E14.5 embryos. (c) Abnormal heart position is seen in all B6 embryos examined. The microCT-based 3D reconstruction shows the heart is twisted approximately 45° along the coronal axis (top panel), sometimes resulting in an altered direction of the ductus arteriosus with respect to the aorta (axial view, white arrow). Colour coding: yellow – right ventricle; pink – left ventricle; blue – trachea; red – aorta; light green – ductus arteriosus. (d) Ventricular septal defect in Trp53bp2^Δ3/Δ3 E14.5 embryonic heart (red arrow), shown on a representative HREM section. (e) Palate (red arrows) showing cleft palate in Trp53bp2^Δ3/Δ3 E14.5 embryo. (f) Largest-area HREM section of gonads (red arrows) showing abnormalities and unclear gender in a Trp53bp2^Δ3/Δ3 E14.5 embryo versus male E14.5 wild-type embryo. (g) Single versus double ureter (red arrows) in wild-type versus Trp53bp2^Δ3/Δ3 E14.5 embryos, respectively, as visible by HREM

Figure 6

Summary of phenotypes in Trp53bp2^Δ3/Δ3 and Trp53bp2^Δ3/+ embryos. Abnormalities detected by naked eye (visual inspection), microCT, and HREM in E13.5 and E14.5 ASPP2-deficient embryos. Frequency and the number of embryos evaluated for each phenotype are given. Wild-type littermate embryos were used as reference controls. In the case of BALB/c embryos, eye abnormalities were difficult to determine visually and were therefore quantified for a small number of embryos