Mohammad Athar

Evidence of Trem2 Variant Associated with Triple Risk of Alzheimer’s Disease

Alzheimer’s disease is one of the main causes of dementia among elderly individuals and leads to the neurodegeneration of different areas of the brain, resulting in memory impairments and loss of cognitive functions. Recently, a rare variant that is associated with 3-fold higher risk of Alzheimer’s disease onset has been found. The rare variant discovered is a missense mutation in the loop region of exon 2 of Trem2 (rs75932628-T, Arg47His). The aim of this study was to investigate the evidence for potential structural and functional significance of Trem2 gene variant (Arg47His) through molecular dynamics simulations. Our results showed the alteration caused due to the variant in TREM2 protein has significant effect on the ligand binding affinity as well as structural configuration. Based on molecular dynamics (MD) simulation under salvation, the results confirmed that native form of the variant (Arg47His) might be responsible for improved compactness, hence thereby improved protein folding. Protein simulation was carried out at different temperatures. At 300K, the deviation of the theoretical model of TREM2 protein increased from 2.0 Å at 10 ns. In contrast, the deviation of the Arg47His mutation was maintained at 1.2 Å until the end of the simulation (t = 10 ns), which indicated that Arg47His had reached its folded state. The mutant residue was a highly conserved region and was similar to “immunoglobulin V-set” and “immunoglobulin-like folds”. Taken together, the result from this study provides a biophysical insight on how the studied variant could contribute to the genetic susceptibility to Alzheimer’s disease.

Next generation sequencing to identify novel genetic variants causative of autosomal dominant familial hypercholesterolemia associated with increased risk of coronary heart disease.

Familial hypercholesterolemia (FH) is an autosomal dominant inherited disease characterized by elevated plasma low-density lipoprotein cholesterol (LDL-C). It is an autosomal dominant disease, caused by variants in Ldlr, ApoB or Pcsk9, which results in high levels of LDL-cholesterol (LDL-C) leading to early coronary heart disease. Sequencing whole genome for screening variants for FH are not suitable due to high cost. Hence, in this study we performed targeted customized sequencing of FH 12 genes (Ldlr, ApoB, Pcsk9, Abca1, Apoa2, Apoc3, Apon2, Arh, Ldlrap1, Apoc2, ApoE, and Lpl) that have been implicated in the homozygous phenotype of a proband pedigree to identify candidate variants by NGS Ion torrent PGM. Only three genes (Ldlr, ApoB, and Pcsk9) were found to be highly associated with FH based on the variant rate. The results showed that seven deleterious variants in Ldlr, ApoB, and Pcsk9 genes were pathological and were clinically significant based on predictions identified by SIFT and PolyPhen. Targeted customized sequencing is an efficient technique for screening variants among targeted FH genes. Final validation of seven deleterious variants conducted by capillary resulted to only one novel variant in Ldlr gene that was found in exon 14 (c.2026delG, p. Gly676fs). The variant found in Ldlr gene was a novel heterozygous variant derived from a male in the proband.

Identification of a novel nonsense variant c.1332dup, p.(D445*) in the LDLR gene that causes familial hypercholesterolemia.

Familial hypercholesterolemia (FH) is an autosomal dominant disease predominantly caused by a mutation in the low-density lipoprotein receptor (LDLR) gene. Here, we describe two severely affected FH patients who were resistant to statin therapy and were managed on an apheresis program. We identified a novel duplication variant c.1332dup, p.(D445*) at exon 9 and a known silent variant c.1413A>G, p.(=), rs5930, NM_001195798.1 at exon 10 of the LDLR gene in both patients.

The Spectrum of Familial Hypercholesterolemia (FH) in Saudi Arabia: Prime Time for Patient FH Registry

Background: Familial hypercholesterolemia (FH) is a life-threatening inherited condition. Untreated patients have the risk to develop raised plasma levels of cholesterol, atherosclerosis and cardiovascular disease (CVD). If diagnosed and treated early in life, the pathological consequences due to atherosclerosis could be avoided and patients with FH can have an anticipated normal life. Mounting evidence suggests that FH is underdiagnosed and undertreated in all populations. The underlying molecular basis of FH is the presence of mutations in one or more genes in the low-density lipoprotein receptor (LDLR), apolipoprotein B (APOB) or proprotein convertase subtilisin/kexin 9 (PCSK9). However, their prevalence is largely unknown in Saudi Arabia but given the high rates of consanguinity, the prevalence appears to be higher. Furthermore, the high prevalence of obesity and diabetes mellitus in Saudi Arabia increases the vascular disease burden in FH cases by adding additional CVD risk factors. Objective: This article explores the spectrum of FH-causing mutations in the highly consanguineous Saudi community, the need for establishing the Saudi FH registry, the challenges in creating gene databases, and cascade screening. Conclusion: The establishment of FH registry and genetic testing should raise awareness not only among healthcare professionals, but the general population as well. It also helps to provide the best treatment regimen in a cost effective manner to this under-recognised population of FH patients.

Compound heterozygous LDLR variant in severely affected familial hypercholesterolemia patient.

Familial hypercholesterolemia (FH) is most commonly caused by mutations in the LDL receptor (LDLR), which is responsible for hepatic clearance of LDL from the blood circulation. We described a severely affected FH proband and their first-degree blood relatives; the proband was resistant to statin therapy and was managed on an LDL apheresis program. In order to find the causative genetic variant in this family, direct exon sequencing of the LDLR, APOB and PCSK9 genes was performed. We identified a compound heterozygous mutation in the proband with missense p.(W577C) and frameshift p.(G676Afs33) variants at exons 12 and 14 of the LDLR gene respectively. DNA sequencing of LDLR gene from the parents demonstrated that the missense variant was inherited from the mother and frameshift variant was inherited from the father. The frameshift variant resulted in a stop signal 33 codons downstream of the deletion, which most likely led to a truncated protein that lacks important functional domains, including the trans-membrane domain and the cytoplasmic tail domain. The missense variant is also predicted to be likely pathogenic and affect EGF-precursor homology domain of the LDLR protein. The segregation pattern of the variants was consistent with the lipid profile, suggesting a more severe FH phenotype when the variants are in the compound heterozygous state. The finding of a compound heterozygous mutation causing severe FH phenotype is important for the genotype-phenotype correlation and also enlarges the spectrum of FH-causative LDLR variants in the Arab population, including the Saudi population.

Functional alterations due to amino acid changes and evolutionary comparative analysis of ARPKD and ADPKD genes.

A targeted customized sequencing of genes implicated in autosomal recessive polycystic kidney disease (ARPKD) phenotype was performed to identify candidate variants using the Ion torrent PGM next-generation sequencing. The results identified four potential pathogenic variants in PKHD1 gene [c.4870C > T, p.(Arg1624Trp), c.5725C > T, p.(Arg1909Trp), c.1736C > T, p.(Thr579Met) and c.10628T > G, p.(Leu3543Trp)] among 12 out of 18 samples. However, one variant c.4870C > T, p.(Arg1624Trp) was common among eight patients. Some patient samples also showed few variants in autosomal dominant polycystic kidney disease (ADPKD) disease causing genes PKD1 and PKD2 such as c.12433G > A, p.(Val4145Ile) and c.1445T > G, p.(Phe482Cys), respectively. All causative variants were validated by capillary sequencing and confirmed the presence of a novel homozygous variant c.10628T > G, p.(Leu3543Trp) in a male proband. We have recently published the results of these studies (Edrees et al., 2016). Here we report for the first time the effect of the common mutation p.(Arg1624Trp) found in eight samples on the protein structure and function due to the specific amino acid changes of PKHD1 protein using molecular dynamics simulations. The computational approaches provide tool predict the phenotypic effect of variant on the structure and function of the altered protein. The structural analysis with the common mutation p.(Arg1624Trp) in the native and mutant modeled protein were also studied for solvent accessibility, secondary structure and stabilizing residues to find out the stability of the protein between wild type and mutant forms. Furthermore, comparative genomics and evolutionary analyses of variants observed in PKHD1, PKD1, and PKD2 genes were also performed in some mammalian species including human to understand the complexity of genomes among closely related mammalian species. Taken together, the results revealed that the evolutionary comparative analyses and characterization of PKHD1, PKD1, and PKD2 genes among various related and unrelated mammalian species will provide important insights into their evolutionary process and understanding for further disease characterization and management.

Identification of a recurrent frameshift mutation at the LDLR exon 14 (c.2027delG, p.(G676Afs*33)) causing familial hypercholesterolemia in Saudi Arab homozygous children

Familial hypercholesterolemia (FH) is an autosomal dominant disease, predominantly caused by variants in the low-density lipoprotein (LDL) receptor gene (LDLR). Herein, we describe genetic analysis of severely affected homozygous FH patients who were mostly resistant to statin therapy and were managed on an apheresis program. We identified a recurrent frameshift mutation p.(G676Afs*33) in exon 14 of the LDLR gene in 9 probands and their relatives in an apparently unrelated Saudi families. We also describe a three dimensional homology model of the LDL receptor protein (LDLR) structure and examine the consequence of the frameshift mutation p.(G676Afs*33), as this could affect the LDLR structure in a region involved in dimer formation, and protein stability. This finding of a recurrent mutation causing FH in the Saudi population could serve to develop a rapid genetic screening procedure for FH, and the 3D-structure analysis of the mutant LDLR, may provide tools to develop a mechanistic model of the LDLR function.

Novel combined variants of LDLR and LDLRAP1 genes causing severe familial hypercholesterolemia

BACKGROUND AND AIMS Familial hypercholesterolemia (FH) is a predominantly autosomal dominant hereditary disorder with significant potential for expansion of coronary artery disease. METHODS To identify candidate variant/s in FH phenotype implicated genes, next-generation sequencing was performed using a targeted customized gene panel. RESULTS We recognized a 45-year-old Saudi female FH patient with double variants in the LDLR [c.1255 T > G, p.(Y419D)] and LDLRAP1 genes [c.604_605delTCinsA, p.(S202Tfs*2)]. The proband was found to be homozygous for the LDLR variant and heterozygous for the LDLRAP1 variant. Three of the probands children were found to be double heterozygous for the LDLR/LDLRAP1 gene variant. While her other three children were heterozygous for the same single LDLR variant. Both variants were not previously reported. The variants segregation pattern correlated with the clinical picture and with the patients lipid profile. FH severity was greater in the proband while her children did not show any clinical manifestations. The missense variant p.(Y419D) was found to be deleterious and clinically significant based on prediction identified by PolyPhen-2 and Proven. Molecular dynamics simulation was used to further analyze the effect of the variant p.(Y419D) on the structure and function of the LDLR protein. The secondary structure was investigated, as well as the solvent accessibility and stabilizing residues. The frameshift variant of the LDLRAP1 gene results in a truncated peptide that could affect the cellular internalization of LDLR/LDL complex. CONCLUSIONS The finding of the combined variants in LDLR/LDLRAP1 genes triggering a severe FH phenotype is essential to elaborate the spectrum of variants causing FH and to understand the genotype-phenotype correlation.

4. Identification of a novel nonsense variant C.1332DUP, P. (D445*) in the LDLR gene that causes familial hypercholesterolemia

Abstract type Basic Science Research. Presentation type Digital Poster. Introduction Familial hypercholesterolemia (FH) is an autosomal dominant disease predominantly caused by a mutation in the low-density lipoprotein receptor (LDLR) gene. Here, we describe two severely affected FH patients from the same Saudi family, who were resistant to statin therapy and were managed on an apheresis program. Methodology Two proband samples were collected from the patients. Direct sequencing of the LDLR gene was performed by using the Sanger sequencing method. Results We identified a novel duplication variant c.1332dup, p. (D445*) at exon 9 and a known silent variant c.1413A > G, p. (=), rs5930, NM_001195798.1 at exon 10 of the LDLR gene in both patients. Both probands were homozygous for the mutation, which is located in the EGF-precursor homology domain of the LDLR protein, and show severe FH phenotype. Conclusion The duplication variant results in the production of a defective LDL receptor containing the p. (D445*) variant. This variant results in a premature stop codon at position 445 in exon 9 of the LDLR gene, which results in truncation of the protein. The segregation pattern of the variant is consistent with the lipid profile, suggesting a more severe FH phenotype when the variant is in the homozygous state. Finding of this study could be very useful in developing critical genetic screen for potential FH patients. In addition, these data contribute to the understanding of the molecular basis of FH in Saudis.

Identification of Four Novel Factor VIII Gene Mutations and Protein Structure Analysis using Molecular Dynamic Simulation

Hemophilia A is an X-linked recessive hemorrhagic disorder caused by mutations in the factor VIII gene. To find out known and novel causative mutations in Hemophilia A, we carried out genetic analysis among Saudi patients. Twenty six Patients who were negative for inv-1/inv-22 were selected for analysis with Multiplex Ligation-dependent Probe Amplification (MLPA) and Sanger sequencing. Furthermore the functional and structural effects of the novel mutations were predicted using Molecular dynamic simulation. The results showed three known large deletions in 6 samples (delE8,9,10,11,12,13,14; delE7,8,9,10; and delE4) and twelve mutations in 18 samples; four of them were novel. The novel mutations detected were two substitutions in exon 8 at position c.1021G>C, p.(Ala341Pro) and position c.1183A>C, p.(Lys395Gln), one substitution in exon13 at position c.1930T>A, p.(Leu644Met), and one substitution in exon22 at position c.6322G>C, p.(Ala2108Pro). Clinical data of Patients with novel mutations showed <1% Factor VIII levels (severe hemophilia) with episodic bleeding and were on a routine treatment of plasma derived clotting factor concentrate. This data is in line with MD simulation showed significant changes of the different mutations on the protein structure compared to native protein. These results should enrich the spectrum of mutations and enlarge the factor VIII protein’s database in Saudi Arabian population; furthermore it showed that the in silico MD simulation for functional and structural studies could be a reasonable approach for investigating the advance genotype-phenotype correlation.