Molecular genetics in 1991 arrhythmia probands and 2782 relatives in Norway: Results from 17 years of genetic testing in a national laboratory.

The aim of this study was to explore the prevalence of likely pathogenic or pathogenic variants and assess the diagnostic yield from genetic testing for cardiac arrhythmias in Norway since 2003. Data from 1991 probands and 2782 relatives were retrospectively collected from the laboratory information management system at Unit for Cardiac and Cardiovascular Genetics, Oslo University hospital. Of 1991 probands, 57.4% were females, age at genetic testing was 33.1 (±22.7) years, and 32.5% were under the age of 18. A likely pathogenic or pathogenic variant (including 14 novel) was detected in 15.4% in total. Of the 2782 relatives, 53.7% were females, age at genetic testing was 35.6 (±22.5) years, 27.3% were under the age of 18, and 45.3% carried the family variant. Probands and relatives combined, 1/3356 persons in the Norwegian population were heterozygous for an arrhythmia-causing variant. The founder variant p.Q530X (NM_000218.2: c.1588C>T) in KCNQ1 accounted for 34% of all variants in Norway. In conclusion, genetic testing provided a genetic basis of the arrhythmia in 15.4% of the probands. Familial cascade screening identified four times as many variant-positive relatives, allowing early detection and prompt stratification of arrhythmic risk of those variant carriers.

Relationship between physical and psychological functioning and health-related quality of life in congenital Aniridia.

PURPOSE: Congenital aniridia is a serious eye disease characterized by absence of iris to various degrees. The aims of this study were to investigate health-related quality of life (HRQoL) in adults with aniridia and assess the relationships between HRQoL, psychological status, ocular health and obesity. METHODS: Twenty-nine adults with congenital aniridia (48% male, aged 18-79 years) participated. HRQoL was measured with SF-36 and the EQ visual analogue scale (VAS). The physical (PCS) and mental (MCS) component summaries of the SF-36 were calculated with higher scores indicating better HRQoL. Symptoms of anxiety and depression were measured using the Hospital Anxiety and Depression Scale (HADS). Obesity was assessed with the Patient-Reported Outcomes in Obesity (PROS). Sociodemographic characteristics, genetic variants and ocular and medical health variables were also analysed. RESULTS: The participants scored significantly lower in the general health domain of the SF-36 than the general population (65.2 vs. 75.3, p = 0.017). The EQ VAS score was also lower in the aniridia group (64.9 vs. 77.9, p = 0.021). Low PCS score was correlated with presence of ocular pain (p = 0.019), high HADS score (p = 0.017) and high PROS score (p = 0.009). Low MCS score was related to higher educational level (p = 0.038) and high HADS score (p < 0.001). High HADS and PROS scores were both related to low EQ VAS scores. CONCLUSION: Adults with congenital aniridia scored worse on certain measures of HRQoL than the general population. Poorer HRQoL was associated with increased symptoms of anxiety, depression and obesity and with presence of ocular pain.

Vigorous exercise in patients with hypertrophic cardiomyopathy.

BACKGROUND: We aimed to investigate if history of vigorous exercise was associated with changes in left ventricular morphology, left ventricular function and ventricular arrhythmias (VAs) in hypertrophic cardiomyopathy genotype positive, phenotype negative (Genotype+ LVH-) and in phenotype positive (HCM LVH+). METHODS: In this cross sectional study we included 187 subjects (age 49±16years, 89(48%) female, 121(65%) HCM LVH+ and 66 (35%) Genotype+ LVH-) who answered a questionnaire on physical activity history. Exercise ≥6 metabolic equivalents was defined as vigorous. Subjects with a history of vigorous exercise ≥4h/week during ≥6years were defined as athletes. All underwent echocardiography and Holter monitoring. VAs were defined as aborted cardiac arrest, sustained or non-sustained ventricular tachycardia. RESULTS: In both Genotype+ LVH- and HCM LVH+, lifetime vigorous exercise correlated with larger left ventricular end-diastolic volume (rho 0.44 and 0.38 respectively, both p<0.001). Lifetime vigorous exercise correlated with increased left ventricular mass in Genotype+ LVH- (rho 0.28, p=0.03), but not in HCM LVH+ (p=0.53). Left ventricular systolic function was similar between athletes and non-athletes in Genotype+ LVH- and HCM LVH+. HCM LVH+ athletes had lower E/e' (p=0.03) and higher e' (p=0.02) compared to non-athletes, while this difference was not observed in Genotype+ LVH-. Lifetime vigorous exercise was similar among HCM LVH+ with and without VAs (p=0.89). CONCLUSIONS: Increased lifetime vigorous exercise was associated with larger left ventricular volumes in hypertrophic cardiomyopathy, but correlated to left ventricular mass only in Genotype+ LVH-. Vigorous exercise was associated with favorable diastolic function in HCM LVH+, and was not associated with VAs.

Lamin A/C cardiomyopathy: young onset, high penetrance, and frequent need for heart transplantation.

Aims: Lamin A/C (LMNA) mutations cause familial dilated cardiomyopathy (DCM) with frequent conduction blocks and arrhythmias. We explored the prevalence, cardiac penetrance, and expressivity of LMNA mutations among familial DCM in Norway. Furthermore, we explored the risk factors and the outcomes in LMNA patients. Methods and results: During 2003-15, genetic testing was performed in patients referred for familial DCM. LMNA genotype-positive subjects were examined by electrocardiography, Holter monitoring, cardiac magnetic resonance imaging, and echocardiography. A positive cardiac phenotype was defined as the presence of atrioventricular (AV) block, atrial fibrillation/flutter (AF), ventricular tachycardia (VT), and/or echocardiographic DCM. Heart transplantation was recorded and compared with non-ischaemic DCM of other origin. Of 561 unrelated familial DCM probands, 35 (6.2%) had an LMNA mutation. Family screening diagnosed an additional 93 LMNA genotype-positive family members. We clinically followed up 79 LMNA genotype-positive [age 42 ± 16 years, ejection fraction (EF) 45 ± 13%], including 44 (56%) with VT. Asymptomatic LMNA genotype-positive family members (age 31 ± 15 years) had a 9% annual incidence of a newly documented cardiac phenotype and 61% (19/31) of cardiac penetrance during 4.4 ± 2.9 years of follow-up. Ten (32%) had AV block, 7 (23%) AF, and 12 (39%) non-sustained VT. Heart transplantation was performed in 15 of 79 (19%) LMNA patients during 7.8 ± 6.3 years of follow-up. Conclusion: LMNA mutation prevalence was 6.2% of familial DCM in Norway. Cardiac penetrance was high in young asymptomatic LMNA genotype-positive family members with frequent AV block and VT, highlighting the importance of early family screening and cardiological follow-up. Nearly 20% of the LMNA patients required heart transplantation.

Data on exercise and cardiac imaging in a patient cohort with hypertrophic cardiomyopathy.

Data presented in this paper are supplementary material to our study "Vigorous exercise in patients with hypertrophic cardiomyopathy" [1]. The current article presents supplementary data on collection and analyses of exercise parameters and genetic data in the original research article.

The systolic paradox in hypertrophic cardiomyopathy.

OBJECTIVE: We explored cardiac volumes and the effects on systolic function in hypertrophic cardiomyopathy (HCM) patients with left ventricular hypertrophy (HCM LVH+) and genotype-positive patients without left ventricular hypertrophy (HCM LVH-). METHODS: We included 180 HCM LVH+, 100 HCM LVH- patients and 80 healthy individuals. End-Diastolic Volume Index (EDVI), End-Systolic Volume Index (ESVI) and ejection fraction (EF) were assessed by echocardiography. Left ventricular (LV) global longitudinal strain (GLS) was measured by speckle tracking echocardiography. RESULTS: EDVI and ESVI were significantly smaller in HCM LVH+ compared with HCM LVH- patients (41±14 mL/m2 vs 49±13 mL/m2 and 16±7 mL/m2 vs 19±6 mL/m2, respectively, both p<0.001) and in healthy individuals (41±14 mL/m2 vs 57±14 mL/m2 and 16±7 mL/m2 vs 23±9 mL/m2, respectively, both p<0.001). HCM LVH- patients had significantly lower EDVI and ESVI compared with healthy individuals (49±13 mL/m2 vs 57±14 mL/m2 and 19±6 mL/m2 vs 23±9 mL/m2, both p<0.001). EF was similar (61%±7% vs 60%±8% vs 61%±6%, p=0.43) in the HCM LVH+, HCM LVH- and healthy individuals, despite significantly worse GLS in the HCM LVH+ (-16.4%±3.7% vs -21.3%±2.4% vs -22.3%±3.7%, p<0.001). GLS was worse in the HCM LVH- compared with healthy individuals in pairwise comparison (p=0.001). Decrease in ESVI was closely related to EF in HCM LVH+ and HCM LVH- (R=0.45, p<0.001 and R=0.43, p<0.001) as expected, but there was no relationship with GLS (R=0.02, p=0.77 and R=0.11, p=0.31). Increased maximal wall thickness (MWT) correlated significantly with worse GLS (R=0.58, p<0.001), but not with EF (R=0.018, p=0.30) in the HCM LVH+ patients. CONCLUSION: HCM LVH+ had smaller cardiac volumes that could explain the preserved EF, despite worse GLS that was closely related to MWT. HCM LVH- had reduced cardiac volumes and subtle changes in GLS compared with healthy individuals, indicating a continuum of both volumetric and systolic changes present before increased MWT.

Novel mutation in the SLC12A3 gene in a Sri Lankan family with Gitelman syndrome & coexistent diabetes: a case report.

BACKGROUND: Gitelman syndrome (GS) is a rare autosomal recessively inherited salt-wasting tubulopathy associated with mutations in the SLC12A3 gene, which encodes for NaCl cotransporter (NCC) in the kidney. CASE PRESENTATION: In this report, we describe two siblings from a Sri Lankan non-consanguineous family presenting with hypokalaemia associated with renal potassium wasting, hypomagnesemia, hypocalciuria and hypereninemic hyperaldosteronism with normal blood pressure. Genetic testing showed that both were homozygotes for a novel missense mutation in exon 10 of the SLC12A3 gene [NM_000339.2, c.1276A > T; p.N426Y], which has not previously been reported in the literature in association with GS. Their mother was a heterozygous carrier for the same mutation. The father was not alive at the time of testing. This novel mutation extends the spectrum of known SLC12A3 gene mutations and further supports the allelic heterogeneity of GS. Interestingly both siblings had young onset Diabetes with strong family history. CONCLUSION: These findings have implications in providing appropriate genetic counseling to the family with regard to the risk associated with inbreeding, the detection of carrier/presymptomatic relatives. It further expands the known spectrum of genotypic and phenotypic characteristics of Gitelman syndrome.

Variable phenotypic expression of nonsense mutation p.Thr5* in the APOE gene.

Subjects with hypercholesterolemia who do not carry a mutation in the low density lipoprotein receptor gene, in the apolipoprotein B gene or in the proprotein convertase subtilisin/kexin type 9 gene, could possible carry a mutation in the apolipoprotein E (APOE) gene. DNA from 844 unrelated hypercholesterolemic subjects who did not carry a mutation in any of the three above mentioned genes, was subjected to DNA sequencing of the APOE gene. Two subjects were found to be heterozygous for mutation p.Thr5*. This mutation which generates a stop codon in the signal peptide, is assumed to prevent the synthesis of APOE. Family studies revealed that the mutation was carried on an APOE4 allele in both families. In one of the families only those who had an APOE2 allele as the second allele, had hypercholesterolemia. These were functionally hemizygous for APOE2 and presented with a Type III hyperlipoproteinemia phenotype. However, in the second family, hypercholesterolemia was observed in the index patient who had APOE3 as the second allele, but not in four heterozygous family members who also had APOE3 as the second allele. These findings underscore that the phenotypic expression of mutations in the APOE gene is variable and that the trait exhibits reduced penetrance.

Identification and characterization of two novel mutations in the LPL gene causing type I hyperlipoproteinemia.

BACKGROUND: Type 1 hyperlipoproteinemia is a rare autosomal recessive disorder most often caused by mutations in the lipoprotein lipase (LPL) gene resulting in severe hypertriglyceridemia and pancreatitis. OBJECTIVES: The aim of this study was to identify novel mutations in the LPL gene causing type 1 hyperlipoproteinemia and to understand the molecular mechanisms underlying the severe hypertriglyceridemia. METHODS: Three patients presenting classical features of type 1 hyperlipoproteinemia were recruited for DNA sequencing of the LPL gene. Pre-heparin and post-heparin plasma of patients were used for protein detection analysis and functional test. Furthermore, in vitro experiments were performed in HEK293 cells. Protein synthesis and secretion were analyzed in lysate and medium fraction, respectively, whereas medium fraction was used for functional assay. RESULTS: We identified two novel mutations in the LPL gene causing type 1 hyperlipoproteinemia: a two base pair deletion (c.765_766delAG) resulting in a frameshift at position 256 of the protein (p.G256TfsX26) and a nucleotide substitution (c.1211 T > G) resulting in a methionine to arginine substitution (p.M404 R). LPL protein and activity were not detected in pre-heparin or post-heparin plasma of the patient with p.G256TfsX26 mutation or in the medium of HEK293 cells over-expressing recombinant p.G256TfsX26 LPL. A relatively small amount of LPL p.M404 R was detected in both pre-heparin and post-heparin plasma and in the medium of the cells, whereas no LPL activity was detected. CONCLUSIONS: We conclude that these two novel mutations cause type 1 hyperlipoproteinemia by inducing a loss or reduction in LPL secretion accompanied by a loss of LPL enzymatic activity.

Rapid molecular diagnostics of severe primary immunodeficiency determined by using targeted next-generation sequencing.

BACKGROUND: Primary immunodeficiency diseases (PIDDs) are inherited disorders of the immune system. The most severe form, severe combined immunodeficiency (SCID), presents with profound deficiencies of T cells, B cells, or both at birth. If not treated promptly, affected patients usually do not live beyond infancy because of infections. Genetic heterogeneity of SCID frequently delays the diagnosis; a specific diagnosis is crucial for life-saving treatment and optimal management. OBJECTIVE: We developed a next-generation sequencing (NGS)-based multigene-targeted panel for SCID and other severe PIDDs requiring rapid therapeutic actions in a clinical laboratory setting. METHODS: The target gene capture/NGS assay provides an average read depth of approximately 1000×. The deep coverage facilitates simultaneous detection of single nucleotide variants and exonic copy number variants in one comprehensive assessment. Exons with insufficient coverage (<20× read depth) or high sequence homology (pseudogenes) are complemented by amplicon-based sequencing with specific primers to ensure 100% coverage of all targeted regions. RESULTS: Analysis of 20 patient samples with low T-cell receptor excision circle numbers on newborn screening or a positive family history or clinical suspicion of SCID or other severe PIDD identified deleterious mutations in 14 of them. Identified pathogenic variants included both single nucleotide variants and exonic copy number variants, such as hemizygous nonsense, frameshift, and missense changes in IL2RG; compound heterozygous changes in ATM, RAG1, and CIITA; homozygous changes in DCLRE1C and IL7R; and a heterozygous nonsense mutation in CHD7. CONCLUSION: High-throughput deep sequencing analysis with complete clinical validation greatly increases the diagnostic yield of severe primary immunodeficiency. Establishing a molecular diagnosis enables early immune reconstitution through prompt therapeutic intervention and guides management for improved long-term quality of life.

Type 1 hyperlipoproteinemia due to a novel deletion of exons 3 and 4 in the GPIHBP1 gene.

OBJECTIVES: Type 1 hyperlipoproteinemia is an autosomal recessive disorder characterized by severely elevated plasma triglyceride levels, which may lead to abdominal pain and pancreatitis, eruptive xanthomas and failure to thrive. Mutations in the genes encoding lipoprotein lipase (LPL), apolipoprotein CII (APOC2), apolipoprotein AV (APOA5), lipase maturing factor 1 (LMF1) or glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) have been found to cause Type 1 hyperlipoproteinemia. METHODS: Two sibpairs belonging to two different branches of an extended pedigree were referred for molecular elucidation for their increased plasma triglyceride levels, which untreated were >27 mmol/L. The genes LPL, APOC2, APOA5, LMF1 and GPIHBP1 were analyzed by DNA sequencing. RESULTS: No mutations were found in LPL, APOC2, APOA5 or LMF1. No PCR products were obtained for exons 3 and 4 of GPIHBP1 from DNA of the 4 affected subjects. Subsequent long-range PCR revealed that the four affected were homozygous for a deletion comprising exons 3 and 4 of GPIHBP1. No increase in LPL activity was found in post-heparin plasma from the subjects. CONCLUSION: Homozygosity for a deletion of exons 3 and 4 of GPIHBP1 results in Type 1 hyperlipoproteinemia.

PCSK9-mediated degradation of the LDL receptor generates a 17 kDa C-terminal LDL receptor fragment.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the LDL receptor (LDLR) at the cell surface and reroutes the internalized LDLR to intracellular degradation. In this study, we have shown that PCSK9-mediated degradation of the full-length 160 kDa LDLR generates a 17 kDa C-terminal LDLR fragment. This fragment was not generated from mutant LDLRs resistant to PCSK9-mediated degradation or when degradation was prevented by chemicals such as ammonium chloride or the cysteine cathepsin inhibitor E64d. The observation that the 17 kDa fragment was only detected when the cells were cultured in the presence of the γ-secretase inhibitor DAPT indicates that this 17 kDa fragment undergoes γ-secretase cleavage within the transmembrane domain. The failure to detect the complementary 143 kDa ectodomain fragment is likely to be due to its rapid degradation in the endosomal lumen. The 17 kDa C-terminal LDLR fragment was also generated from a Class 5 mutant LDLR undergoing intracellular degradation. Thus, one may speculate that an LDLR with bound PCSK9 and a Class 5 LDLR with bound LDL are degraded by a similar mechanism that could involve ectodomain cleavage in the endosome.

Mutations in the SORT1 gene are unlikely to cause autosomal dominant hypercholesterolemia.

OBJECTIVE: To study whether mutations in the SORT1 gene could be a cause of autosomal dominant hypercholesterolemia and to study the effect of sortilin on the binding and internalization of low density lipoprotein (LDL). METHODS: 842 unrelated hypercholesterolemic subjects without mutations in genes known to cause autosomal dominant hypercholesterolemia, were screened for mutations in the SORT1 gene by DNA sequencing. Transfections of wild-type or mutant SORT1 plasmids in HeLa T-REx cells and the use of siRNA were used to study the effect of sortilin on the number of cell-surface LDL receptors and on the binding and internalization of LDL. RESULTS: A total of 45 mutations in the SORT1 gene were identified of which 15 were missense mutations. Eight of these were selected for in vitro studies, of which none had a major impact on the amount of LDL bound to the cell surface. There was a positive correlation between the amount of sortilin on the cell surface and the amount of LDL bound. The observation that a mutant sortilin which is predominantly found on the cell surface rather than in post-Golgi compartments, bound very high amounts of LDL, indicates that sortilin does not increase the binding of LDL through an intracellular mechanism. Rather, our data indicate that sortilin binds LDL on the cell surface. CONCLUSION: Even though sortilin binds and internalizes LDL by receptor-mediated endocytosis, mutations in the SORT1 gene are unlikely to cause autosomal dominant hypercholesterolemia and may only have a marginal effect on plasma LDL cholesterol levels.

The prevalence of mutations in KCNQ1, KCNH2, and SCN5A in an unselected national cohort of young sudden unexplained death cases.

INTRODUCTION: Sudden unexplained death account for one-third of all sudden natural deaths in the young (1-35 years). Hitherto, the prevalence of genopositive cases has primarily been based on deceased persons referred for postmortem genetic testing. These deaths potentially may represent the worst of cases, thus possibly overestimating the prevalence of potentially disease causing mutations in the 3 major long-QT syndrome (LQTS) genes in the general population. We therefore wanted to investigate the prevalence of mutations in an unselected population of sudden unexplained deaths in a nationwide setting. METHODS: DNA for genetic testing was available for 44 cases of sudden unexplained death in Denmark in the period 2000-2006 (equaling 33% of all cases of sudden unexplained death in the age group). KCNQ1, KCNH2, and SCN5A were sequenced and in vitro electrophysiological studies were performed on novel mutations. RESULTS: In total, 5 of 44 cases (11%) carried a mutation in 1 of the 3 genes corresponding to 11% of all investigated cases (R190W KCNQ1, F29L KCNH2 (2 cases), P297S KCNH2 and P1177L SCN5A). P1177L SCN5A has not been reported before. In vitro electrophysiological studies of P1177L SCN5A revealed an increased sustained current suggesting a LQTS phenotype. CONCLUSION: In a nationwide setting, the genetic investigation of an unselected population of sudden unexplained death cases aged 1-35 years finds a lower than expected number of mutations compared to referred populations previously reported. We therefore conclude that the prevalence of mutations in the 3 major LQTS associated genes may not be as abundant as previously estimated.

Serum levels of proprotein convertase subtilisin/kexin type 9 in subjects with familial hypercholesterolemia indicate that proprotein convertase subtilisin/kexin type 9 is cleared from plasma by low-density lipoprotein receptor-independent pathways.

Secreted proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the low-density lipoprotein receptor (LDLR) at the cell surface and disrupts the normal recycling of the LDLR. When human PCSK9 is injected into LDLR-deficient mice, PCSK9 is still rapidly cleared by the liver. This finding may suggest that PCSK9 is physiologically also cleared by receptors other than the LDLR. An alternative explanation could be that PCSK9 has undergone modifications during purification and is cleared by scavenger receptors on liver endothelial sinusoidal cells when injected into mice. If the only mechanism for clearing PCSK9 in humans is through the LDLR, one would expect that differences in the number of LDLRs would affect the plasma levels of low-density lipoprotein cholesterol (LDLC) and PCSK9 in a similar fashion. In this study, levels of LDLC and PCSK9 were measured in familial hypercholesterolemia (FH) homozygotes, FH heterozygotes, and normocholesterolemic subjects. The ratio between the levels of LDLC and PCSK9 was 1.7-fold higher in FH heterozygotes and 3-fold higher in FH homozygotes than in the normocholesterolemic subjects. Thus, defective LDLRs have a greater impact on the levels of LDLC than on the levels of PCSK9. By assuming that the rate of PCSK9 synthesis is similar in the 3 groups, this finding suggests that in humans, plasma PCSK9 is also cleared by LDLR-independent mechanisms.

Characterization of a naturally occurring degradation product of the LDL receptor.

In this study we have characterized a naturally occurring truncated form of the low density lipoprotein receptor (LDLR). Western blot analysis of transfected cells indicated that the truncated form (∆N-LDLR) is a degradation product of the full-length LDLR generated by cleavage in the linker region between ligand-binding repeats 4 and 5 of the ligand-binding domain. The cleavage of the linker was not caused by components of the culture media, as heat inactivation of the media did not prevent cleavage. Rather, it is assumed that cleavage was caused by an enzyme secreted from the cells. Biotinylation experiments showed that ∆N-LDLR is located on the cell surface and is detectable approximately 5 h after synthesis of the full-length LDLR. Flow cytometric analysis showed that ∆N-LDLR was not able to bind and internalize low density lipoprotein (LDL). ∆N-LDLR appeared to be equally stable as the full-length LDLR. Thus, generation of ∆N-LDLR does not appear to be the first signal for degradation of the LDLR. The existence of two functionally different populations of LDLRs on the cell surface, of which ∆N-LDLR constitutes 28%, must be taken into account when interpreting results of experiments to study LDLRs on the cell surface. Furthermore, if the cleavage of the linker between ligand-binding repeats 4 and 5 could be prevented by an enzyme inhibitor, this could represent a novel therapeutic strategy to increase the number of functioning LDLRs and thereby decrease the levels of plasma LDL cholesterol.

Interaction between the ligand-binding domain of the LDL receptor and the C-terminal domain of PCSK9 is required for PCSK9 to remain bound to the LDL receptor during endosomal acidification.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the epidermal growth factor homology domain repeat A of the low-density lipoprotein receptor (LDLR) at the cell surface and disrupts recycling of the internalized LDLR. As a consequence, the LDLR is rerouted to the lysosomes for degradation. Although PCSK9 may bind to an LDLR lacking the ligand-binding domain, at least three ligand-binding repeats of the ligand-binding domain are required for PCSK9 to reroute the LDLR to the lysosomes. In this study, we have studied the binding of PCSK9 to an LDLR with or without the ligand-binding domain at increasingly acidic conditions in order to mimic the milieu of the LDLR:PCSK9 complex as it translocates from the cell membrane to the sorting endosomes. These studies have shown that PCSK9 is rapidly released from an LDLR lacking the ligand-binding domain at pH in the range of 6.9-6.1. A similar pattern of release at acidic pH was also observed for the binding to the normal LDLR of mutant PCSK9 lacking the C-terminal domain. Together these data indicate that an interaction between the negatively charged ligand-binding domain of the LDLR and the positively charged C-terminal domain of PCSK9 is required for PCSK9 to remain bound to the LDLR during the early phase of endosomal acidification as the LDLR translocates from the cell membrane to the sorting endosome.

Characterization of residues in the cytoplasmic domain of the LDL receptor required for exit from the endoplasmic reticulum.

Newly synthesized low density lipoprotein receptors (LDLRs) exit the endoplasmic reticulum (ER) as the first step in the secretory pathway. In this study we have generated truncating deletions and substitutions within the 50 amino acid cytoplasmic domain of the LDLR in order to identify residues required for the exit from the ER. Western blot analysis was used to determine the relative amounts of the 120 kDa precursor form of the LDLR located in the ER and the 160 kDa mature form that has exited the ER. These studies have shown that the exit of an LDLR lacking the cytoplasmic domain, is markedly reduced. Moreover, the longer the cytoplasmic domain, the more efficient is the exit from the ER. At least 30 residues were required for the LDLR to efficiently exit the ER. Mutations in the two di-acidic motifs ExE(814) and/or ExD(837) had only a small effect on the exit from the ER. The requirement for a certain length of the cytoplasmic domain for efficient exit from the ER, could reflect the distance needed to interact with the COPII complex of the ER membrane or the requirement for the LDLR to undergo dimerization.

Role of the C-terminal domain of PCSK9 in degradation of the LDL receptors.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the low density lipoprotein receptor (LDLR) at the cell surface and disrupts the normal recycling of the LDLR. In this study, we investigated the role of the C-terminal domain for the activity of PCSK9. Experiments in which conserved residues and histidines on the surface of the C-terminal domain were mutated indicated that no specific residues of the C-terminal domain, apart from those responsible for maintaining the overall structure, are required for the activity of PCSK9. Rather, the net charge of the C-terminal domain is important. The more positively charged the C-terminal domain, the higher the activity toward the LDLR. Moreover, replacement of the C-terminal domain with an unrelated protein of comparable size led to significant activity of the chimeric protein. We conclude that the role of the evolutionary, poorly conserved C-terminal domain for the activity of PCSK9 reflects its overall positive charge and size and not the presence of specific residues involved in protein-protein interactions.