Mutation in the ARH gene and a chromosome 13q locus influence cholesterol levels in a new form of digenic-recessive familial hypercholesterolemia

Mapping the Diverse Genetic Disorders and Rare Diseases Among the Syrian Population: Implications on Refugee Health and Health Services in Host Countries

The aim of this systematic review is to provide physicians and researchers with a comprehensive list of reported genetic disorders in patients of Syrian origin-those who have become part of the largest displaced population globally-and to highlight the need to consider migrant population-based risk for the development of genetic disease control and prevention programs. This review was performed based on the 2015 PRISMA and the international prospective register of systematic reviews. The present review reports on a total of 166 genetic disorders (only 128 reported on OMIM) identified in the Syrian population. Of these disorders, 27% are endocrine-, nutritional- and metabolic-related diseases. Second to metabolic disorders are congenital malformations, deformations and chromosomal abnormalities. Diseases of the blood and the blood-forming organs accounted for 13% of the total genetic disorders. The majority of the genetic disorders reported in Syrian patients followed an autosomal recessive mode of inheritance. These findings are a reflection of the high rates of consanguineous marriages that favor the increase in incidence of these diseases. From the diseases that followed an autosomal recessive mode of inheritance, 22% are reported to be only present in Syria and other regional countries. Twelve of these genetic diseases were identified to be strictly diagnosed in individuals of Syrian origin. The present systematic review highlights the need to develop programs that target genetic disorders affecting Syrian migrants in host countries. These programs would have potential financial and economic benefits, as well as a positive impact on the physical and mental health of members of the Syrian refugee community and those of their host societies. In turn, this would decrease the burden on the health systems in host countries.

Autosomal recessive hypercholesterolemia in a kindred of Syrian ancestry

Autosomal recessive hypercholesterolemia is a rare genetic disorder due to homozygosity or compound heterozygosity for mutations in the low-density lipoprotein receptor adapter protein 1 gene (LDLRAP1), resulting in elevated low-density lipoprotein cholesterol (LDL-C) levels, large xanthomas, and increased cardiovascular risk. Here, we describe a Danish family of Syrian ancestry carrying a frameshift mutation in LDLRAP1, previously only described in Sardinia and Sicily in Italy and in Spain. In 2 children homozygous for this mutation, we evaluate the effect of long-term lipid-lowering treatment with atorvastatin as monotherapy or in combination with ezetimibe. At referral to the lipid clinic at Viborg Regional Hospital, 3 of 4 children had LDL-C levels of 468, 538, and 371 mg/dL, respectively, with 1 child already showing cutaneous xanthomas at 10 years of age. For comparison, the fourth child and the parents had LDL-C levels of 85, 116, and 124 mg/dL. Genetic testing revealed that all 3 children with severely elevated LDL-C were homozygous for a rare frameshift mutation in LDLRAP1, p.His144GlnfsTer27 (c.431dupA), whereas the fourth child and both parents were heterozygous for this mutation. Lipid-lowering treatment was started in the 2 oldest children (at 10 and 7 years of age). Atorvastatin (40 mg/d) combined with ezetimibe (10 mg/d) reduced LDL-C by 75% in the first child and resulted in near-complete regression of xanthomas. In the second child, atorvastatin (40 mg/d) as monotherapy reduced LDL-C by 61%. Both regimens were superior to treatment with pravastatin as monotherapy (20 mg/d) and to pravastatin in combination with cholestyramine (2 g twice daily). High-intensity statin therapy alone or in combination with ezetimibe resulted in marked reductions in LDL-C in 2 children homozygous for a rare frameshift mutation in LDLRAP1 and lead to regression of large xanthomas.

Association test using Copy Number Profile Curves (CONCUR) enhances power in rare copy number variant analysis

Copy number variants (CNVs) are the gain or loss of DNA segments in the genome that can vary in dosage and length. CNVs comprise a large proportion of variation in human genomes and impact health conditions. To detect rare CNV associations, kernel-based methods have been shown to be a powerful tool due to their flexibility in modeling the aggregate CNV effects, their ability to capture effects from different CNV features, and their accommodation of effect heterogeneity. To perform a kernel association test, a CNV locus needs to be defined so that locus-specific effects can be retained during aggregation. However, CNV loci are arbitrarily defined and different locus definitions can lead to different performance depending on the underlying effect patterns. In this work, we develop a new kernel-based test called CONCUR (i.e., copy number profile curve-based association test) that is free from a definition of locus and evaluates CNV-phenotype associations by comparing individuals' copy number profiles across the genomic regions. CONCUR is built on the proposed concepts of "copy number profile curves" to describe the CNV profile of an individual, and the "common area under the curve (cAUC) kernel" to model the multi-feature CNV effects. The proposed method captures the effects of CNV dosage and length, accounts for the numerical nature of copy numbers, and accommodates between- and within-locus etiological heterogeneity without the need to define artificial CNV loci as required in current kernel methods. In a variety of simulation settings, CONCUR shows comparable or improved power over existing approaches. Real data analyses suggest that CONCUR is well powered to detect CNV effects in the Swedish Schizophrenia Study and the Taiwan Biobank.

Spectrum of mutations of familial hypercholesterolemia in the 22 Arab countries

BACKGROUND AND AIMS

Familial hypercholesterolemia (FH) is an inherited genetic disorder of lipid metabolism characterized by a high serum LDL-cholesterol profile and xanthoma formation, and FH increases the risk of premature atherosclerosis and cardiovascular disease (CVD). Mutations in the low-density lipoprotein (LDLR), apolipoprotein B (APOB), proprotein convertase subtilisin/kexin 9 (PCSK9), and LDLRAP1 genes have been associated with FH. Although FH is a major risk for CVD, the disease prevalence and its underlying molecular basis in the 22 Arab countries are still understudied. This study aimed to analyze all peer-reviewed studies related to the prevalence of FH and its causative mutations in the 22 Arab countries.

METHODS

We searched five literature databases (Scopus, Science Direct, Web of Science, PubMed, and Google Scholar) from inception until June 2018, using all possible search terms to capture all of the genetic and prevalence data related to Arab patients with FH.

RESULTS

A total of 5,484 titles and abstracts were identified; 51 studies met our inclusion criteria for the final systematic review. Fifty-one mutations in Arab patients with FH were identified in only eight Arab countries; 47 were identified in the LDLR gene, two in the PCSK9 gene, and two in LDLRAP1 gene. Twenty mutations in the LDLR gene were distinctive to Arab patients. A few studies reported prevalence estimates, ranging from 0.4% to 6.8%.

CONCLUSIONS

This is the first systematic review to dissect the up-to-date status of the genetic epidemiology of Arab patients with FH. It seems that FH is underdiagnosed and that its prevalence is understudied due to the dearth of published information about Arab patients with FH. Therefore, there is a need for well-controlled genetic epidemiological studies on Arab patients with FH.

Autosomal recessive hypercholesterolemia: a mild phenotype of familial hypercholesterolemia: insight from the kinetic study using stable isotope and animal studies

Autosomal recessive hypercholesterolemia (ARH) is an extremely rare inherited disorder, the cause of which is mutations in the low-density lipoprotein (LDL) receptor adaptor protein 1 (LDLRAP1) gene. Only 36 families with 14 different mutations have been reported in the literature to date. The clinical phenotype of ARH is milder than that of homozygous familial hypercholesterolemia (FH) caused by LDL receptor gene mutations. Recently, the lipoprotein metabolism of ARH was investigated in both humans and mice by several investigators, including ourselves. Based on these findings the preserved clearance of LDL receptor-dependent very-LDL (VLDL) may be a possible mechanism underlying the responsiveness to statins and the milder phenotype of ARH. Although ARH has been described as being "recessive," several studies, including ours, have indicated that a heterozygous carrier status of the LDLRAP1 gene is associated with mild hypercholesterolemia and exacerbates the phenotype of FH resulting from LDL receptor gene mutations. This review summarizes current understanding regarding ARH and its causative gene, LDLRAP1, and attempts to provide new insight into novel pharmacological targets for treating dyslipidemic patients.

Lysosomal acid lipase A and the hypercholesterolaemic phenotype

PURPOSE OF REVIEW

Mutations in lysosomal acid lipase A (LIPA) result in two phenotypes depending on the extent of lysosomal acid lipase (LAL) deficiency: the severe, early-onset Wolman disease or the less severe cholesteryl ester storage disease (CESD). In CESD, the severity of the symptoms, hepatomegaly and hypercholesterolaemia, can be highly variable, presenting in childhood or adulthood. Therefore, it is likely that many patients are undiagnosed or misdiagnosed. Nevertheless, LAL deficiency has been recognized for more than 25 years, but adequate therapeutic strategies are limited.

RECENT FINDINGS

CESD has an estimated prevalence of one in 90,000 to 170,000 individuals in the general population, confirming the likelihood that this disease is currently underdiagnosed. A number of studies have shown that in LIPA deficient patients the hypercholesterolaemic phenotype can be attenuated using statin therapy, and favourable effects on reduction of lipid accumulation in lysosomes have been reported. Targeting lysosomal exocytosis with LAL replacement therapy was shown to be successful in animal models and recently a phase I/II study demonstrated its safety and its potential metabolic efficacy on transaminase levels.

SUMMARY

The hypercholesterolaemic phenotype in CESD can be difficult to distinguish from other known hypercholesterolaemic disorders. In the majority of CESD cases with hypercholesterolaemia favourable responses on statin treatment are observed, but the effect on reduction of lipid accumulation in lysosomes needs to be further evaluated. Combining statins with LAL replacement therapy may provide a promising approach for optimal treatment of LIPA deficiencies in the future.

Digenic inheritance in medical genetics

Digenic inheritance (DI) is the simplest form of inheritance for genetically complex diseases. By contrast with the thousands of reports that mutations in single genes cause human diseases, there are only dozens of human disease phenotypes with evidence for DI in some pedigrees. The advent of high-throughput sequencing (HTS) has made it simpler to identify monogenic disease causes and could similarly simplify proving DI because one can simultaneously find mutations in two genes in the same sample. However, through 2012, I could find only one example of human DI in which HTS was used; in that example, HTS found only the second of the two genes. To explore the gap between expectation and reality, I tried to collect all examples of human DI with a narrow definition and characterise them according to the types of evidence collected, and whether there has been replication. Two strong trends are that knowledge of candidate genes and knowledge of protein–protein interactions (PPIs) have been helpful in most published examples of human DI. By contrast, the positional method of genetic linkage analysis, has been mostly unsuccessful in identifying genes underlying human DI. Based on the empirical data, I suggest that combining HTS with growing networks of established PPIs may expedite future discoveries of human DI and strengthen the evidence for them.

HealthTwiSt: the Berlin Twin Registry for health research

The Berlin Twin Registry has its focus on health research. It is operated as a private company, making twin studies available to academic institutions as well as commercial partners in the area of biotechnology and nutrition. Recruitment is based on invitation in the context of mass media coverage of scientific results. Phenotyping in the unselected twin subjects is directed toward intermediate phenotypes that can bear on common diseases. These phenotypes include proteomic approaches and gene expression. Some results are briefly described to give an impression of the range of research topics and related opportunities for retrospective and prospective collaborative research.

A case of del(13)(q14.2)(q31.3) associated with hypothyroidism, hypertriglyceridemia, hypercholesterolemia and total ophthalmoplegia

13q deletion syndrome is caused by the absence of a portion of the long arm of chromosome 13. This syndrome is a rare condition characterized by a wide range of clinical findings. Phenotype varies with the location and size of the deletion. We report a female dizygotic twin with a proximal deletion of 13q and failure to thrive, hypotonia, and multiple anomalies included pytosis and total ophthalmology at right side, strabismus at left, bilateral iris heterochromia and telecantus. She had a broad nasal bridge with flat philtrum, micrognathia and antevert ear lobes. Her umbilicus had vanished. Her left coxa was dislocated and left toes were overlapped. She was also found to have hypertriglyceridemia, hypercholesterolemia, and hypothyroidism. Chromosome analysis showed a proximal deletion of chromosome 13 [karyotype 46,XX,del(13) (q14.2q31.3)] which was confirmed by high-resolution microarray based comparative genomic hybridization. The described patient is unique among similar rare cases with different deletion breakpoints. It is the first case of 13q14.2q31.3 deletion where the breakpoints are clearly defined, indicating the importance of detailed clinical description and high-resolution genomic analysis for characterization of rare genetic syndromes.

HealthTwiSt: The Berlin Twin Registry

The Berlin Twin Registry began as a short-term local project and developed into a resource that now serves partners throughout Germany and Europe. A twin registry as a private company is a different approach with pros and cons. Compared to academic institutions, there are greater flexibilities in collaborations, as well as acquisition and use of research funds. Recruitment is based on invitation in the context of the mass media coverage of scientific results. Phenotyping in normal twin subjects is concentrated on intermediate phenotypes that can bear on common diseases. These phenotypes include proteomic approaches and gene expression. Some results are briefly described to give an impression of the range of research topics and related opportunities for retrospective and prospective collaborative research.

Genome-wide linkage scan of a pedigree with familial hypercholesterolemia suggests susceptibility loci on chromosomes 3q25-26 and 21q22

Background

Familial hypercholesterolemia (FH) is a heritable disorder that can increase the risk of premature coronary heart disease. Studies suggest there are substantial genetic heterogeneities for different populations. Here we tried to identify novel susceptibility loci for FH in a Chinese pedigree.

Methodology/Principal Findings

We performed a SNP-based genome-wide linkage scan with the Chinese FH pedigree. Two suggestive linkage loci not previously reported were identified on chromosomes 3q25.1-26.1 (NPL = 9.01, nominal P<0.00001, and simulated occurrence per genome scan = 1.08) and 21q22.3 (NPL = 8.95, nominal P<0.00001, and simulated occurrence per genome scan = 1.26). In the interaction analysis with a trimmed version of the pedigree, we obtained a significantly increased joint LOD score (2.70) compared with that obtained when assuming the two loci uncorrelated, suggesting that more than one locus was involved in this pedigree. Exon screening of two candidate genes ABCG1 and LSS from one of the suggestive region 21q22 didn't report any causative mutations.

Conclusions/Significances

These results confirm complex etiologies and suggest new genetic casual factors for the FH disorder. Further study of the two candidate regions is advocated.

Genome-wide and interaction linkage scan for nonsyndromic cleft lip with or without cleft palate in two multiplex families in Shenyang, China

OBJECTIVES

To identify the loci involved in nonsyndromic cleft lip with or without cleft palate (NSCL/P) in Northern Chinese people in Shenyang by using genomewide and interaction linkage scan.

METHODS

Two multiplex families in Shenyang from North China were ascertained through probands with NSCL/P. Blood of every member was drawn for DNA extraction and analysis. Genotypes were available for 382 autosomal short tandem repeat (STR) markers from the ABI Prism Linkage Mapping Set version 2.5. Linkage between markers and NSCL/P was assessed by 2-point parametric LOD scores, multipoint-heterogeneity parametric LOD scores (HLODs), and multipoint nonparametric linkage score (NPL).

RESULTS

The initial scan suggested linkage on Chromosomes 1, 2, and 15. In subsequent fine mapping, 1q32-q42 showed a maximum multipoint LOD score of 1.9(empirical P=0.013) and an NPL score of 2.35 (empirical P=0.053). For 2p24-p25, the multipoint NPL increased to 2.94 (empirical P=0.007). 2-locus interaction analysis obtained a maximum NPL score of 3.73 (P=0.00078) and a maximum LOD score of 3 for Chromosome 1 (at 221 cM) and Chromosome 2 (at 29 cM).

CONCLUSION

Both parametric and nonparametric linkage scores greatly increased over the initial linkage scores on 1q32-q42, suggesting a susceptibility locus in this region. Nonparametric linkage gave a strong evidence for a candidate region on chromosome 2p24-p25. The superiority of 2-locus linkage scores compared to single-locus scores gave additional evidence for linkage on 1q32-q42 and 2p24-p25, and suggested that certain genes in the two regions may contribute to NCSL/P risks with interaction.

A genome-wide linkage scan identifies multiple quantitative trait loci for HDL-cholesterol levels in families with premature CAD and MI

Plasma HDL cholesterol levels (HDL-C) are an independent predictor of coronary artery disease (CAD). We have completed a genome-wide linkage scan for HDL-C in a US cohort consisting of 388 multiplex families with premature CAD (GeneQuest). The heritability of HDL-C in GeneQuest was 0.37 with gender and age as covariates (P = 5.1 x 10(-4)). Two major quantitative trait loci (QTL) for log-transformed HDL-C adjusted for age and gender were identified onto chromosomes 7p22 and 15q25 with maximum multipoint logarithm of odds (LOD) scores of 3.76 and 6.69, respectively. Fine mapping decreased the 7p22 LOD score to a nonsignificant level of 3.09 and split the 15q25 QTL into two loci, one minor QTL on 15q22 (LOD = 2.73) that spanned the LIPC gene, and the other at 15q25 (LOD = 5.63). A family-based quantitative transmission disequilibrium test (QTDT) revealed significant association between variant rs1800588 in LIPC and HDL-C in the GeneQuest population (P = 0.0067), which may account for the minor QTL on 15q22. The 15q25 QTL is the most significant locus identified for HDL-C to date, and these results provide a framework for the ultimate identification of the underlying HDL-C variant and gene on chromosomes 15q25, which will provide insights into novel regulatory mechanisms of HDL-C metabolism.

Evidence for an interaction of schizophrenia susceptibility loci on chromosome 6q23.3 and 10q24.33-q26.13 in Arab Israeli families

A genome scan for schizophrenia related loci in Arab Israeli families by Lerer et al. [Lerer et al. (2003); Mol Psychiatry 8:488-498] detected significant evidence for linkage at chromosome 6q23. Subsequent fine mapping [Levi et al. (2005); Eur J Hum Genet 13:763-771], association [Amann-Zalcenstein et al. (2006); Eur J Hum Genet 14:1111-1119] and replication studies [Ingason et al. (2007); Eur J Hum Genet 15:988-991] identified AHI1 as a putative susceptibility gene. The same genome scan revealed suggestive evidence for a schizophrenia susceptibility locus in the 10q23-26 region. Genes at these two loci may act independently in the pathogenesis of the disease in our homogeneous sample of Arab Israeli families or may interact with each other and with other factors in a common biological pathway. The purpose of our current study was to test the hypothesis of genetic interaction between these two loci and to identify the type of interaction between them. The initial stage of our study focused on the 10q23-q26 region which has not been explored further in our sample. The second stage of the study included a test for possible genetic interaction between the 6q23.3 locus and the refined 10q24.33-q26.13 locus. A final candidate region of 19.9 Mb between markers D10S222 (105.3 Mb) and D10S587 (125.2 Mb) was found on chromosome 10 by non-parametric and parametric linkage analyses. These linkage findings are consistent with previous reports in the same chromosomal region. Two-locus multipoint linkage analysis under three complex disease inheritance models (heterogeneity, multiplicative, and additive models) yielded a best maximum LOD score of 7.45 under the multiplicative model suggesting overlapping function of the 6q23.3 and 10q24.33-q26.13 loci.

A genome scan for loci influencing levels and trends of lipoprotein lipid-related traits since childhood: The Bogalusa Heart Study

Coronary heart disease is the result of life-long processes. Previous genetic linkage analyses of lipid and lipoprotein variables that can be measured throughout life have focused on a single measure at one point in time. Genome-wide linkage analyses were performed in the present study to identify loci influencing the long-term levels and trends of high-density lipoprotein cholesterol (HDLC) and low-density lipoprotein cholesterol (LDLC) and triglycerides in a longitudinal cohort. Microsatellite markers (n=357) were typed on 779 white and 444 black siblings, ages 14-43 years. Subjects had been examined serially 2-13 times with 6963 serial observations over an average of 22 years from childhood to adulthood. Total and incremental area under the growth curves of lipid traits was calculated and used as measures for long-term levels and trends. After adjusting for age, sex and body mass index, heritability estimates of total area values for all lipid variables were higher than those of a single measurement in either childhood or adulthood. In blacks, significant linkage to LDLC incremental area (peak LOD=3.6 at 50 cM) was observed on chromosome 1; and suggestive linkage for total area of LDLC (LOD=2.9 at 21 cM) on chromosome 19. Only one suggestive linkage (LOD=2.2 at 161 cM) on chromosome 2 was identified in whites for LDLC incremental area. Other suggestive linkage (LOD> or =2.0) was noted for LDLC and HDLC in terms of either total or incremental area on chromosomes 2, 5, 7 and 15 for blacks and whites. Several lipid-related candidate genes such as low-density lipoprotein receptor (LDLR), LDL receptor-related proteins 3 and 8, ApoE, ApoAII and ApoCII are located in these regions. Linkage evidence found in this community-based study indicates that regions on these chromosomes harbor genetic loci that affect the propensity to develop dyslipidemia from childhood.

A genomewide search finds major susceptibility loci for gallbladder disease on chromosome 1 in Mexican Americans

Gallbladder disease (GBD) is one of the major digestive diseases. Its risk factors include age, sex, obesity, type 2 diabetes, and metabolic syndrome (MS). The prevalence of GBD is high in minority populations, such as Native and Mexican Americans. Ethnic differences, familial aggregation of GBD, and the identification of susceptibility loci for gallstone disease by use of animal models suggest genetic influences on GBD. However, the major susceptibility loci for GBD in human populations have not been identified. Using ultrasound-based information on GBD occurrence and a 10-cM gene map, we performed multipoint variance-components analysis to localize susceptibility loci for GBD. Phenotypic and genotypic data from 715 individuals in 39 low-income Mexican American families participating in the San Antonio Family Diabetes/Gallbladder Study were used. Two GBD phenotypes were defined for the analyses: (1) clinical or symptomatic GBD, the cases of cholecystectomies due to stones confirmed by ultrasound, and (2) total GBD, the clinical GBD cases plus the stone carriers newly diagnosed by ultrasound. With use of the National Cholesterol Education Program/Adult Treatment Panel III criteria, five MS risk factors were defined: increased waist circumference, hypertriglyceredemia, low high-density lipoprotein cholesterol, hypertension, and high fasting glucose. The MS risk-factor score (range 0-5) for a given individual was used as a single, composite covariate in the genetic analyses. After accounting for the effects of age, sex, and MS risk-factor score, we found stronger linkage signals for the symptomatic GBD phenotype. The highest LOD scores (3.7 and 3.5) occurred on chromosome 1p between markers D1S1597 and D1S407 (1p36.21) and near marker D1S255 (1p34.3), respectively. Other genetic locations (chromosomes 2p, 3q, 4p, 8p, 9p, 10p, and 16q) across the genome exhibited some evidence of linkage (LOD >or=1.2) to symptomatic GBD. Some of these chromosomal regions corresponded with the genetic locations of Lith loci, which influence gallstone formation in mouse models. In conclusion, we found significant evidence of major genetic determinants of symptomatic GBD on chromosome 1p in Mexican Americans.

Novel pheochromocytoma susceptibility loci identified by integrative genomics

Pheochromocytomas are catecholamine-secreting tumors that result from mutations of at least six different genes as components of distinct autosomal dominant disorders. However, there remain familial occurrences of pheochromocytoma without a known genetic defect. We describe here a familial pheochromocytoma syndrome consistent with digenic inheritance identified through a combination of global genomics strategies. Multipoint parametric linkage analysis revealed identical LOD scores of 2.97 for chromosome 2cen and 16p13 loci. A two-locus parametric linkage analysis produced maximum LOD score of 5.16 under a double recessive multiplicative model, suggesting that both loci are required to develop the disease. Allele-specific loss of heterozygosity (LOH) was detected only at the chromosome 2 locus in all tumors from this family, consistent with a tumor suppressor gene. Four additional pheochromocytomas with a similar genetic pattern were identified through transcription profiling and helped refine the chromosome 2 locus. High-density LOH mapping with single nucleotide polymorphism-based array identified a total of 18 of 62 pheochromocytomas with LOH within the chromosome 2 region, which further narrowed down the locus to <2 cM. This finding provides evidence for two novel susceptibility loci for pheochromocytoma and adds a recessive digenic trait to the increasingly broad genetic heterogeneity of these tumors. Similarly, complex traits may also be involved in other familial cancer syndromes.

Autosomal recessive hypercholesterolemia (ARH) and homozygous familial hypercholesterolemia (FH): a phenotypic comparison

Autosomal recessive hypercholesterolemia (ARH) is a rare disorder, due to complete loss of function of an adaptor protein (ARH protein) required for receptor-mediated hepatic uptake of LDL. ARH is a phenocopy of homozygous familial hypercholesterolemia (HoFH) due to mutations in LDL receptor (LDLR) gene; however, previous studies suggested that ARH phenotype is less severe than that of HoFH. To test this hypothesis we compared 42 HoFH and 42 ARH patients. LDLR and ARH genes were analysed by Southern blotting and sequencing. LDLR activity was measured in cultured fibroblasts. In ARH plasma LDL cholestrol (LDL-C) level (14.25+/-2.29 mmol/L) was lower than in receptor-negative HoFH (21.38+/-3.56 mmol/L) but similar to that found in receptor-defective HoFH (15.52+/-2.39 mmol/L). The risk of coronary artery disease (CAD) was 9-fold lower in ARH patients. No ARH patients </=20 years of age were found to have CAD as opposed to 43% of HoFH. The CAD prevalence was or tended to be lower in ARH also in the 21-40 (45% versus 86%) and 41-60 (78% versus 100%) age groups. Heterozygous ARH carriers showed higher level of LDL-C (+17%) than non-carrier family members. In conclusion the clinical phenotype of ARH is milder than that of receptor-negative HoFH and resembles that observed in receptor-defective HoFH.

Genome scan for quantitative trait loci influencing HDL levels: evidence for multilocus inheritance in familial combined hyperlipidemia

Several genome scans in search of high-density lipoprotein (HDL) quantitative trait loci (QTLs) have been performed. However, to date the actual identification of genes implicated in the regulation of common forms of HDL abnormalities remains unsuccessful. This may be due, in part, to the oligogenic and multivariate nature of HDL regulation, and potentially, pleiotropy affecting HDL and other lipid-related traits. Using a Bayesian Markov Chain Monte Carlo (MCMC) approach, we recently provided evidence of linkage of HDL level variation to the APOA1-C3-A4-A5 gene complex, in familial combined hyperlipidemia pedigrees, with an estimated number of two to three large QTLs remaining to be identified. We also presented results consistent with pleiotropy affecting HDL and triglycerides at the APOA1-C3-A4-A5 gene complex. Here we use the same MCMC analytic strategy, which allows for oligogenic trait models, as well as simultaneous incorporation of covariates, in the context of multipoint analysis. We now present results from a genome scan in search for the additional HDL QTLs in these pedigrees. We provide evidence of linkage for additional HDL QTLs on chromosomes 3p14 and 13q32, with results on chromosome 3 further supported by maximum parametric and variance component LOD scores of 3.0 and 2.6, respectively. Weaker evidence of linkage was also obtained for 7q32, 12q12, 14q31-32 and 16q23-24.

Genome-wide search for new genes controlling plasma lipid concentrations in mice and humans

PURPOSE OF REVIEW

Quantitative trait locus analysis has been used in both humans and mice for the purpose of finding new genes regulating plasma lipid levels. We review these methods and discuss new approaches that can help find quantitative trait locus genes.

RECENT FINDINGS

Many quantitative trait loci have been found that regulate plasma levels for HDL cholesterol (37 in mice and 30 in humans), LDL cholesterol (25 in mice and 20 in humans) and triglycerides (19 in mice and 30 in humans). Most of the human quantitative trait loci have concordant mouse quantitative trait loci mapping to homologous regions (93% for HDL cholesterol, 100% for LDL cholesterol and 80% for triglycerides), suggesting that many genes identified in mice may also regulate the same traits in humans. New approaches based on recently developed genomic and bioinformatic technologies and resources should greatly facilitate finding these genes.

SUMMARY

New genes regulating plasma lipid levels can be found in mice and then tested in humans. Some of these genes could be potential therapeutic targets for human atherosclerosis.

Pathogenesis, detection and treatment of Achilles tendon xanthomas

Tendon xanthomatosis often accompanies familial hypercholesterolaemia, but it can also occur in other pathologic states. Achilles tendons are the most common sites of tendon xanthomas. Low-density lipoprotein (LDL) derived from the circulation accumulates into tendons. The next steps leading to the formation of Achilles tendon xanthomas (ATX) are the transformation of LDL into oxidized LDL (oxLDL) and the active uptake of oxLDL by macrophages within the tendons. Although physical examination may reveal Achilles tendon xanthomas (ATX), there are several imaging methods for their detection. It is worth mentioning that ultrasonography is the method of choice in everyday clinical practice. Although several treatments for Achilles tendon xanthomas (ATX) have been proposed (LDL apheresis, statins, etc.), they target mostly in the treatment of the basic metabolic disorder of lipid metabolism, which is the main cause of these lesions. In this review we describe the formation, detection, differential diagnosis and treatment of ATX as well as the relationship between tendon xanthomas and atheroma.

Efficient two-trait-locus linkage analysis through program optimization and parallelization: application to hypercholesterolemia

We have optimized and parallelized the GENEHUNTER-TWOLOCUS program that allows to perform linkage analysis with two trait loci in the multimarker context. The optimization of the serial program, before parallelization, results in a speedup of a factor of more than 10. The parallelization affects the two-locus-score calculation, which is predominant in terms of computation time. We obtain perfect speedup, that is, the computation time decreases exactly by a factor of the number of processors. In addition, two-locus LOD and NPL scores are now calculated for varying genetic positions of both disease loci, not just one locus varied and the position of the other disease locus fixed, as before. This results in easily interpretable 3-D plots. We have reanalyzed a pedigree with hypercholesterolemia using our new version of GENEHUNTER-TWOLOCUS. Whereas originally, two individuals had to be discarded due to excessive computation-time demands, the entire 17-bit pedigree could now be analyzed as a whole. We obtain a two-trait-locus LOD score of 5.49 under a multiplicative model, compared to LOD scores of 3.08 and 2.87 under a heterogeneity and additive model, respectively. This further increases evidence for linkage to both 1p36.1-p35 and 13q22-q32 regions, and corroborates the hypothesis that the two genes act in a multiplicative way on LDL cholesterol level. Furthermore, we compare the computation times for two-trait-locus analysis needed by the programs GENEHUNTER-TWOLOCUS, TLINKAGE, and SUPERLINK. Altogether, our algorithmic improvements of GENEHUNTER-TWOLOCUS allow researchers to analyze complex diseases under realistic two-trait-locus models with pedigrees of reasonable size and using many markers.

No genetic linkage or molecular evidence for involvement of the PCSK9, ARH or CYP7A1 genes in the Familial Hypercholesterolemia phenotype in a sample of Danish families without pathogenic mutations in the LDL receptor and apoB genes

A locus on chromosome 1p34.1-p32 has been linked to autosomal dominant Familial Hypercholesterolemia (FH) and is termed the third FH locus. We tested whether this third FH locus is linked to the FH phenotype in 20 Danish families, with 158 members, without pathogenic mutations in the genes, encoding the low-density lipoprotein (LDL) receptor or apolipoprotein B (apoB). We could exclude the third FH locus as a cause of FH by genetic linkage analysis in the families taken together. Since haplotype analysis of each family nevertheless suggested that the FH phenotype co-segregated in a manner consistent with linkage to the third FH locus in three small pedigrees, we performed sequencing analysis without being able to demonstrate mutations in the proprotein convertase subtilisin/kexin type 9 (PCSK9) gene, the main candidate gene in the third FH locus. By the same combination of genetic linkage and molecular analysis we could also exclude mutations in the gene for the LDL receptor adaptor protein and in the gene for cholesterol-7-alpha-hydroxylase as causes of FH in our sample. Although not indicating linkage to any known loci, our data still indicate that another dominant gene may be involved in causing a FH phenotype.

A novel ARH splice site mutation in a Mexican kindred with autosomal recessive hypercholesterolemia

Autosomal recessive hypercholesterolemia (ARH) is characterized by elevated LDL serum levels, xanthomatosis, and premature coronary artery disease. Three loci have been described for this condition (1p35, 15q25-q26 and 13q). Recently, the responsible gene at the 1p35 locus, encoding an LDL receptor adaptor protein (ARH) has been identified. We studied a Mexican ARH family with two affected siblings. Sequence analysis of the ARH gene (1p35 locus) revealed that the affected siblings are homozygous for a novel mutation (IVS4+2T>G) affecting the donor splice site in intron 4, whereas both the parents and an unaffected sister are heterozygous for this mutation. The IVS4+2T>G mutation results in a major alternative transcript derived from a cryptic splice site, which carries an in-frame deletion of 78 nucleotides in the mature mRNA. The translation of this mRNA yields a mutant protein product (ARH-26) lacking 26 amino acids, resulting in the loss of beta-strands beta6 and beta7 from the PTB domain. This is the first case where a naturally occurring mutant with an altered PTB domain has been identified.

Compendium of genome-wide scans of lipid-related phenotypes: adding a new genome-wide search of apolipoprotein levels

The genetic dissection of complex inherited diseases is a major challenge. Despite limited success in finding genes, substantial data based on genome-wide scan strategies is now available for a variety of diseases and related phenotypes. This can perhaps best be appreciated in the field of lipid and lipoprotein levels, where the amount of information generated is becoming overwhelming. We have created a database containing the results from whole-genome scans of lipid-related phenotypes undertaken to date. The usefulness of this database is demonstrated by performing a new autosomal genomic scan on apolipoprotein B (apoB), LDL-apoB, and apoA-I levels, measured in 679 subjects of 243 nuclear families. Linkage was tested using both allele-sharing and variance-component methods. Only two loci provided support for linkage with both methods: a LDL-apoB locus on 18q21.32 and an apoA-I locus on 3p25.2. Adding those findings to the database highlighted the fact that the former is reported as a lipid-related locus for the first time, whereas the latter has been observed before. However, concerns arise when displaying all data on the same map, because a large portion of the genome is now covered with loci supported by at least suggestive evidence of linkage.

Current management of severe homozygous hypercholesterolaemias

PURPOSE OF REVIEW

This review focuses on recent advances in the management of patients with homozygous familial hypercholesterolaemia, autosomal recessive hypercholesterolaemia and familial defective apolipoprotein B.

RECENT FINDINGS

Autosomal recessive hypercholesterolaemia has been described as a 'phenocopy' of homozygous familial hypercholesterolaemia. Although the clinical phenotypes are similar, autosomal recessive hypercholesterolaemia seems to be less severe, more variable within a single family, and more responsive to lipid-lowering drug therapy. The cardiovascular complications of premature atherosclerosis are delayed in some individuals and involvement of the aortic root and valve is less common than in homozygous familial hypercholesterolaemia. Apheresis is still the treatment of choice in homozygous familial hypercholesterolaemia and in autosomal recessive hypercholesterolaemia patients in whom maximal drug therapy does not achieve adequate control. In addition to the profound cholesterol-lowering effects of apheresis, other potentially beneficial phenomena have been documented: improved vascular endothelial function and haemorheology, reduction in lipoprotein (a) and procoagulatory status, and a decrease in adhesion molecules and C-reactive protein.

SUMMARY

Patients with severe homozygous hypercholesterolaemia illustrate the natural history of atherosclerosis within a condensed timeframe. Effective cholesterol-lowering treatment started in early childhood is essential to prevent onset of life-threatening atherosclerotic involvement of the aortic root and valve, and the coronary arteries. Noninvasive methods for regular monitoring of the major sites involved in the atherosclerotic process are necessary in patients with no symptoms or signs of ischaemia. Management of patients with severe homozygous hypercholesterolaemia continues to be a major challenge.

Autosomal recessive hypercholesterolaemia: long-term follow up and response to treatment

Autosomal recessive hypercholesterolaemia (ARH) is caused by mutations in ARH on chromosome 1p35-36, encoding a putative adaptor protein. Mutations in the gene prevent normal internalisation of the low density lipoprotein (LDL) receptor by cultured lymphocytes and monocyte-derived macrophages, but not skin fibroblasts. This newly identified disorder is characterised by severe hypercholesterolaemia, large tendon, tuberous and planar xanthomas and premature atherosclerosis. We describe long-term (9-23 years) follow up and response to treatment of eight subjects with ARH from four families (Turkish/Lebanese, Indian-Asian, English and Italian). The clinical phenotype of ARH is similar to that of classical homozygous familial hypercholesterolaemia (FH) caused by mutations in the LDL-receptor gene but is more variable, less severe and is more responsive to lipid-lowering therapy with bile acid sequestrants and/or HMG-CoA reductase inhibitors. The latter reduced total serum cholesterol by up to 60% and the former by 20-35%. The cardiovascular complications of premature atherosclerosis seem to be delayed in some individuals and the involvement of the aortic root and valve are rarer in comparison with homozygous FH.

LDL-receptor mutations in Europe

Familial hypercholesterolemia (FH) is a clinical definition for a remarkable increase of cholesterol serum concentration, presence of xanthomas, and an autosomal dominant trait of either increased serum cholesterol or premature coronary artery disease (CAD). The identification of the low-density lipoprotein (LDL)-receptor (LDLR) as the underlying cause and its genetic characterization in FH patients revealed more insights in the trafficking of LDL, which primarily transports cholesterol to hepatic and peripheral cells. Mutations within LDLR result in hypercholesterolemia and, subsequently, cholesterol deposition in humans to a variable degree. This confirms the pathogenetic role of LDLR and also highlights the existence of additional factors in determining the phenotype. Autosomal dominant FH is caused by LDLR deficiency and defective apolipoprotein B-100 (APOB), respectively. Heterozygosity of the LDLR is relatively common (1:500). Clinical diagnosis is highly important and genetic diagnosis may be helpful, since treatment is usually effective for this otherwise fatal disease. Very recently, mutations in PCSK9 have been also shown to cause autosomal dominant hypercholesterolemia. For autosomal recessive hypercholesterolemia, mutations within the so-called ARH gene encoding a cellular adaptor protein required for LDL transport have been identified. These insights emphasize the crucial importance of LDL metabolism intra- and extracellularly in determining LDL-cholesterol serum concentration. Herein, we focus on the published European LDLR mutation data that reflect its heterogeneity and phenotypic penetrance.

Signals for sorting of transmembrane proteins to endosomes and lysosomes

Sorting of transmembrane proteins to endosomes and lysosomes is mediated by signals present within the cytosolic domains of the proteins. Most signals consist of short, linear sequences of amino acid residues. Some signals are referred to as tyrosine-based sorting signals and conform to the NPXY or YXXO consensus motifs. Other signals known as dileucine-based signals fit [DE]XXXL[LI] or DXXLL consensus motifs. All of these signals are recognized by components of protein coats peripherally associated with the cytosolic face of membranes. YXXO and [DE]XXXL[LI] signals are recognized with characteristic fine specificity by the adaptor protein (AP) complexes AP-1, AP-2, AP-3, and AP-4, whereas DXXLL signals are recognized by another family of adaptors known as GGAs. Several proteins, including clathrin, AP-2, and Dab2, have been proposed to function as recognition proteins for NPXY signals. YXXO and DXXLL signals bind in an extended conformation to the mu2 subunit of AP-2 and the VHS domain of the GGAs, respectively. Phosphorylation events regulate signal recognition. In addition to peptide motifs, ubiquitination of cytosolic lysine residues also serves as a signal for sorting at various stages of the endosomal-lysosomal system. Conjugated ubiquitin is recognized by UIM, UBA, or UBC domains present within many components of the internalization and lysosomal targeting machinery. This complex array of signals and recognition proteins ensures the dynamic but accurate distribution of transmembrane proteins to different compartments of the endosomal-lysosomal system.

Genetics, clinical phenotype, and molecular cell biology of autosomal recessive hypercholesterolemia

The recent characterization of a rare genetic defect causing autosomal recessive hypercholesterolemia (ARH) has provided new insights into the underlying mechanism of clathrin-mediated internalization of the LDL receptor. Mutations in ARH on chromosome 1p35-36.1 prevent normal internalization of the LDL receptor by cultured lymphocytes and monocyte-derived macrophages but not by skin fibroblasts. In affected cells, LDL receptor protein accumulates at the cell surface; this also occurs in the livers of recombinant mice lacking ARH, thereby providing an explanation for the failure of clearance of LDL from the plasma in subjects lacking ARH. The approximately 50 known affected individuals are mostly of Sardinian or Middle Eastern origin. The clinical phenotype of ARH is similar to that of classic homozygous familial hypercholesterolemia caused by defects in the LDL receptor gene, but it is more variable, generally less severe, and more responsive to lipid-lowering therapy. Structural features of the ARH protein and its capacity to interact simultaneously with the internalization sequence of the LDL receptor, plasma membrane phospholipids, and the clathrin endocytic machinery suggest how ARH can play a pivotal role in gathering the LDL receptor into forming endocytic carrier vesicles.

Familial predisposition to tufted angioma: identification of blood and lymphatic vascular components

Tufted angioma is a rare benign vascular lesion of unknown etiology which mainly affects children under 5 years. It is characterized by nodules or tufts of capillary-sized vessels in the dermis. Here we report the second familial occurrence of tufted angioma, with a mode of inheritance compatible with a monogenic autosomal dominant trait with reduced penetrance. A preliminary investigation was performed to exclude association between the predisposition and certain candidate genes including KDR (kinase insert domain receptor), TEK (TEK tyrosine kinase endothelial), ACVRL1 (activin receptor-like kinase 1), ENG (endoglin) and FLT4 (fms-like tyrosine kinase 4). KDR, ENG and FLT4 were all compatible with linkage, with haplotypes being shared between three affected individuals and the one obligate carrier available for testing. TEK and ACVRL1 could essentially be excluded. Finally, we provide definitive evidence for the existence of both blood and lymphatic vascular elements in the lesion.

Molecular mechanisms of autosomal recessive hypercholesterolemia

PURPOSE OF REVIEW

Autosomal recessive hypercholesterolemia (ARH) is a rare Mendelian dyslipidemia characterized by markedly elevated plasma LDL levels, xanthomatosis, and premature coronary artery disease. LDL receptor function is normal, or only moderately impaired in fibroblasts from ARH patients, but their cultured lymphocytes show increased cell-surface LDL binding, and impaired LDL degradation, consistent with a defect in LDL receptor internalization. Recently, the disorder was shown to be caused by mutations in a phosphotyrosine binding domain protein, ARH, which is required for internalization of low density lipoproteins in the liver. This review summarizes the findings of new investigations into the pathophysiology and molecular genetics of ARH.

RECENT FINDINGS

All mutations that have been characterized to date preclude the synthesis of a full-length protein. GST-pulldown experiments indicate that the phosphotyrosine binding domain of ARH interacts with the internalization sequence (NPVY) in the cytoplasmic tail of LDLR, and that conserved motifs in the C-terminal portion of the protein bind to clathrin and to the beta2-adaptin subunit of AP-2.

SUMMARY

The available data suggest that ARH functions as an adaptor protein that couples LDLR to the endocytic machinery.

Autosomal recessive hypercholesterolemia in a Sicilian kindred harboring the 432insA mutation of the ARH gene

We describe a Sicilian family presenting a recessive form of hypercholesterolemia harboring a mutation of the autosomal recessive hypercholesterolemia (ARH) gene. In two of the three sibs, a 26-year-old male and a 22-year-old female, a severe hypercholesterolemia was diagnosed with very high levels of plasma cholesterol (15.9 and 12.2 mmol/l, respectively); tendon xanthomatas and xanthelasms were present and in the male proband was documented a diffuse coronary atherosclerotic disease with a rapid and fatal progression. Both the parents had normal or slightly increased levels of plasma cholesterol. All causes of secondary hypercholesterolemia were ruled out as well as an involvement of the LDL receptor or apoB genes. Beta-Sitosterol plasma levels were in the normal range. Cultured fibroblasts from skin biopsy from parents and the two probands displayed a normal ability to bind and degrade 125I-LDL. Direct sequencing of ARH gene demonstrated the presence of a 432insA mutation in homozygosis in the two probands; parents were heterozygotes for the same mutation. This mutation is the first report of a mutation of the ARH gene responsible for recessive forms of hypercholesterolemia in Sicily.

The autosomal recessive hypercholesterolemia (ARH) protein interfaces directly with the clathrin-coat machinery

The low density lipoprotein (LDL) receptor plays a pivotal role in cholesterol metabolism. Inherited mutations that disturb the activity of the receptor lead to elevations in plasma cholesterol levels and early-onset coronary atherosclerosis. Defects in either the LDL receptor or apolipoprotein B, the proteinaceous component of LDL particles that binds the LDL receptor, elevate circulating LDL-cholesterol levels in an autosomal-dominant fashion, with heterozygotes displaying values between homozygous and normal individuals. Rarely, similar clinical phenotypes occur with a recessive pattern of inheritance, and several genetic lesions in the autosomal recessive hypercholesterolemia (ARH) gene on chromosome 1 have been mapped in this class of patients. ARH has an N-terminal phosphotyrosine-binding (PTB) domain evolutionarily related to that found in Disabled-2 and numb, two endocytic proteins. PTB domains bind to the consensus sequence FXNPXY, corresponding to the internalization motif of the LDL receptor. We show here that in addition to the FXNPXY sequence, ARH binds directly to soluble clathrin trimers and to clathrin adaptors by a mode involving the independently folded appendage domain of the beta subunit. At steady state, ARH colocalizes with endocytic proteins in HeLa cells, and the LDL receptor fluxes through peripheral ARH-positive sites before delivery to early endosomes. Because ARH also binds directly to phosphoinositides, which regulate clathrin bud assembly at the cell surface, our data suggest that in ARH patients, defective sorting adaptor function in hepatocytes leads to faulty LDL receptor traffic and hypercholesterolemia.

Restoration of LDL receptor function in cells from patients with autosomal recessive hypercholesterolemia by retroviral expression of ARH1

Familial hypercholesterolemia is an autosomal dominant disorder with a gene-dosage effect that is usually caused by mutations in the LDL receptor gene that disrupt normal clearance of LDL. In the homozygous form, it results in a distinctive clinical phenotype, characterized by inherited hypercholesterolemia, cholesterol deposition in tendons, and severe premature coronary disease. We described previously two families with autosomal recessive hypercholesterolemia that is not due to mutations in the LDL receptor gene but is characterized by defective LDL receptor-dependent internalization and degradation of LDL by transformed lymphocytes from the patients. We mapped the defective gene to chromosome 1p36 and now show that the disorder in these and a third English family is due to novel mutations in ARH1, a newly identified gene encoding an adaptor-like protein. Cultured skin fibroblasts from affected individuals exhibit normal LDL receptor activity, but their monocyte-derived macrophages are similar to transformed lymphocytes, being unable to internalize and degrade LDL. Retroviral expression of normal human ARH1 restores LDL receptor internalization in transformed lymphocytes from an affected individual, as demonstrated by uptake and degradation of (125)I-labeled LDL and confocal microscopy of cells labeled with anti-LDL-receptor Ab.