Mugen Liu

Mutations in SLC20A2 link familial idiopathic basal ganglia calcification with phosphate homeostasis

Familial idiopathic basal ganglia calcification (IBGC) is a genetic condition with a wide spectrum of neuropsychiatric symptoms, including parkinsonism and dementia. Here, we identified mutations in SLC20A2, encoding the type III sodium-dependent phosphate transporter 2 (PiT2), in IBGC-affected families of varied ancestry, and we observed significantly impaired phosphate transport activity for all assayed PiT2 mutants in Xenopus laevis oocytes. Our results implicate altered phosphate homeostasis in the etiology of IBGC.

Human intracellular ISG15 prevents interferon-α/β over-amplification and auto-inflammation

Intracellular ISG15 is an interferon (IFN)-α/β-inducible ubiquitin-like modifier which can covalently bind other proteins in a process called ISGylation; it is an effector of IFN-α/β-dependent antiviral immunity in mice. We previously published a study describing humans with inherited ISG15 deficiency but without unusually severe viral diseases. We showed that these patients were prone to mycobacterial disease and that human ISG15 was non-redundant as an extracellular IFN-γ-inducing molecule. We show here that ISG15-deficient patients also display unanticipated cellular, immunological and clinical signs of enhanced IFN-α/β immunity, reminiscent of the Mendelian autoinflammatory interferonopathies Aicardi–Goutières syndrome and spondyloenchondrodysplasia. We further show that an absence of intracellular ISG15 in the patients’ cells prevents the accumulation of USP18, a potent negative regulator of IFN-α/β signalling, resulting in the enhancement and amplification of IFN-α/β responses. Human ISG15, therefore, is not only redundant for antiviral immunity, but is a key negative regulator of IFN-α/β immunity. In humans, intracellular ISG15 is IFN-α/β-inducible not to serve as a substrate for ISGylation-dependent antiviral immunity, but to ensure USP18-dependent regulation of IFN-α/β and prevention of IFN-α/β-dependent autoinflammation.

D-peptide inhibitors of the p53–MDM2 interaction for targeted molecular therapy of malignant neoplasms

The oncoproteins MDM2 and MDMX negatively regulate the activity and stability of the tumor suppressor protein p53, conferring tumor development and survival. Antagonists targeting the p53-binding domains of MDM2 and MDMX kill tumor cells both in vitro and in vivo by reactivating the p53 pathway, promising a class of antitumor agents for cancer therapy. Aided by native chemical ligation and mirror image phage display, we recently identified a D-peptide inhibitor of the p53-MDM2 interaction termed DPMI-α (TNWYANLEKLLR) that competes with p53 for MDM2 binding at an affinity of 219 nM. Increased selection stringency resulted in a distinct D-peptide inhibitor termed DPMI-γ (DWWPLAFEALLR) that binds MDM2 at an affinity of 53 nM. Structural studies coupled with mutational analysis verified the mode of action of these D-peptides as MDM2-dependent p53 activators. Despite being resistant to proteolysis, both DPMI-α and DPMI-γ failed to actively traverse the cell membrane and, when conjugated to a cationic cell-penetrating peptide, were indiscriminately cytotoxic independently of p53 status. When encapsulated in liposomes decorated with an integrin-targeting cyclic-RGD peptide, however, DPMI-α exerted potent p53-dependent growth inhibitory activity against human glioblastoma in cell cultures and nude mouse xenograft models. Our findings validate D-peptide antagonists of MDM2 as a class of p53 activators for targeted molecular therapy of malignant neoplasms harboring WT p53 and elevated levels of MDM2.

Identification of a novel genetic locus on chromosome 8p21.1-q11.23 for idiopathic basal ganglia calcification

Idiopathic basal ganglia calcification (IBGC) is a neurodegenerative disorder that is characterized by basal ganglia and extrabasal ganglia calcification, and usually inherited in an autosomal dominant pattern. To date, two genetic loci for IBGC were identified on chromosomes 14q and 2q, but further genetic heterogeneity clearly exists. In this study, a large Chinese family with autosomal dominant IBGC was characterized. Linkage analysis excluded the 14q13 and 2q37 loci. The large family was then characterized by genome‐wide linkage analysis to identify a novel genetic locus for IBGC. Significant linkage was identified with markers on chromosome 8p21.1–q11.23 with a maximum LOD score of 4.10. Fine mapping defined the new genetic locus within a 25 Mb region between markers D8S1809 and D8S1833. Future studies of the candidate genes at the 8p21.1–q11.23 locus may lead to identification of a disease‐causing gene with IBGC.

Novel CACNA1S mutation causes autosomal dominant hypokalemic periodic paralysis in a Chinese family

Hypokalemic periodic paralysis (HypoPP) is an autosomal dominant disorder which is characterized by periodic attacks of muscle weakness associated with a decrease in the serum potassium level. The skeletal muscle calcium channel α-subunit gene CACNA1S is a major disease-causing gene for HypoPP, however, only three specific HypoPP-causing mutations, Arg528His, Arg1,239His and Arg1,239Gly, have been identified in CACNA1S to date. In this study, we studied a four-generation Chinese family with HypoPP with 43 living members and 19 affected individuals. Linkage analysis showed that the causative mutation in the family is linked to the CACNA1S gene with a LOD score of 6.7. DNA sequence analysis revealed a heterozygous C to G transition at nucleotide 1,582, resulting in a novel 1,582C→G (Arg528Gly) mutation. The Arg528Gly mutation co-segregated with all affected individuals in the family, and was not present in 200 matched normal controls. The penetrance of the Arg528Gly mutation was complete in male mutation carriers, however, a reduced penetrance of 83% (10/12) was observed in female carriers. No differences were detected for age-at-onset and severity of the disease (frequency of symptomatic attacks per year) between male and female patients. Oral intake of KCl is effective in blocking the symptomatic attacks. This study identifies a novel Arg528Gly mutation in the CACNA1S gene that causes HypoPP in a Chinese family, expands the spectrum of mutations causing HypoPP, and demonstrates a gender difference in the penetrance of the disease.

Mutations in ABCB6 Cause Dyschromatosis Universalis Hereditaria

Dyschromatosis universalis hereditaria (DUH) is a pigmentary genodermatosis characterized by a mixture of hyperpigmented and hypopigmented macules distributed randomly over the body. No causative genes have been reported to date. In this study, we investigated a large five-generation Chinese family with DUH. After excluding the two known DUH loci, we performed genome-wide linkage analysis and identified a DUH locus on chromosome 2q33.3-q36.1 with a maximum LOD score of 3.49 with marker D2S2382. Exome sequencing identified a c.1067T>C (p.Leu356Pro) mutation in exon 3 of ABCB6 (ATP-binding cassette subfamily B, member 6) in the DUH family. Two additional missense mutations, c.508A>G (p.Ser170Gly) in exon 1 and c.1736G>A (p.Gly579Glu) in exon 12 of ABCB6, were found in two out of six patients by mutational screening using sporadic DUH patients. Immunohistologic examination in biopsy specimens showed that ABCB6 is expressed in the epidermis and had a diffuse cytoplasmic distribution. Examination of subcellular localization of wild-type ABCB6 in a B16 mouse melanoma cell line revealed that it is localized to the endosome-like compartment and dendrite tips, whereas disease-causing mutations of ABCB6 resulted in its retention in the Golgi apparatus. Our studies identified ABCB6 as the first pathogenic gene associated with DUH. These findings suggest that ABCB6 may be a physiological factor for skin pigmentation.

αA- and αB-Crystallins Interact with Caspase-3 and Bax to Guard Mouse Lens Development

The small heat shock protein, α-crystallin, exists in two isoforms, αA and αB, and displays strong ability against stress-induced apoptosis. Regarding their functional mechanisms, we and others have demonstrated that they are able to regulate members in both caspase and Bcl-2 families. In addition, we have also shown that αA and αB may display differential anti-apoptotic mechanisms under certain stress conditions. While αA-crystallin regulates activation of the AKT signaling pathway, αB negatively regulates the MAPK pathway to suppress apoptosis induced by UV and oxidative stress. Although previous studies revealed that αA and αB could regulate members in both caspase and Bcl-2 families, the molecular mechanism, especially the in vivo regulation still waits to be elucidated. In the present communication, we present both in vitro and in vivo evidence to further demonstrate the regulation of caspase-3 and Bax by αA and αB. First, Surface Plasmon Resonance (SPR) and yeast two-hybrid selection analysis demonstrate that αA and αB directly bind to caspase-3 and Bax with differential affinities. Second, immunohistochemistry reveals that αA and αB regulate caspase-3 and Bax at different developmental stages of mouse embryo. Third, coimmunoprecipitation shows that αA and αB form in vivo interacting complexes with caspase-3 and Bax. Together, our results further confirm that αA and αB regulate caspase-3 and Bax in vitro and in vivo to regulate lens differentiation.

Identification of a novel GPR143 mutation in a large Chinese family with congenital nystagmus as the most prominent and consistent manifestation

AbstractCongenital nystagmus is characterized by involuntary, rhythmical, repeated oscillations of one or both eyes. We studied a large Chinese family with nystagmus as a prominent and consistent manifestation phenotype in nine patients to map and identify a disease-causing gene for nystagmus. X-linked recessive inheritance was observed in the family, and foveal hypoplasia was detected in some of the nine patients. The disease gene was mapped to an approximately 10.6 Mb region flanked by DXS996 and DXS7593 on Xp22 with a significant peak multipoint LOD score. Analysis of 21 candidate genes in the region revealed a novel p.S89F mutation in the second transmembrane domain of GPR143, a G protein-coupled receptor which causes ocular albinism when mutated. All male patients in the family were hemizygous for the mutation; the female carriers were heterozygous for the mutation. The p.S89F mutation was not identified in 100 normal females or 100 normal males. Our results indicate that a mutation in the GPR143 gene can cause a variant form of ocular albinism, with congenital nystagmus as the most prominent and only consistent finding in all patients in this Chinese family. These results expand the spectrum of clinical phenotypes associated with GPR143 mutations.

A novel DSPP mutation is associated with type II dentinogenesis Imperfecta in a chinese family

BackgroundHereditary defects of tooth dentin are classified into two main groups: dentin dysplasia (DD) (types I and II) and dentinogenesis imperfecta (DGI) (types I, II, and III). Type II DGI is one of the most common tooth defects with an autosomal dominant mode of inheritance. One disease-causing gene, the dentin sialophosphoprotein (DSPP) gene, has been reported for type II DGI.MethodsIn this study, we characterized a four-generation Chinese family with type II DGI that consists of 18 living family members, including 8 affected individuals. Linkage analysis with polymorphic markers D4S1534 and D4S414 that span the DSPP gene showed that the family is linked to DSPP. All five exons and exon-intron boundaries of DSPP were sequenced in members of type II DGI family.ResultsDirect DNA sequence analysis identified a novel mutation (c.49C→T, p.Pro17Ser) in exon 1 of the DSPP gene. The mutation spot, the Pro17 residue, is the second amino acid of the mature DSP protein, and highly conserved during evolution. The mutation was identified in all affected individuals, but not in normal family members and 100 controls.ConclusionThese results suggest that mutation p.Pro17Ser causes type II DGI in the Chinese family. This study identifies a novel mutation in the DSPP gene, and expands the spectrum of mutations that cause DGI.

HSF4 regulates DLAD expression and promotes lens de-nucleation

HSF4 mutations lead to both congenital and age-related cataract. The purpose of this study was to explore the mechanism of cataract formation caused by HSF4 mutations. The degradation of nuclear DNA is essential for the lens fiber differentiation. DNase 2β (DLAD) is highly expressed in lens cells, and mice with deficiencies in the DLAD gene develop nuclear cataracts. In this study, we found that HSF4 promoted the expression and DNase activity of DLAD by directly binding to the DLAD promoter. In contrast, HSF4 cataract causative mutations failed to bind to the DLAD promoter, abrogating the expression and DNase activity of DLAD. These results were confirmed by HSF4 knockdown in zebrafish, which led to incomplete de-nucleation of the lens and decreased expression and activity of DLAD. Together, our results suggest that HSF4 exerts its function on lens differentiation via positive regulation of DLAD expression and activity, thus facilitating de-nucleation of lens fiber cells. Our demonstration that HSF4 cataract causative mutations abrogate the induction of DLAD expression reveals a novel molecular mechanism regarding how HSF4 mutations cause cataractogenesis.