Mutation in the ARH Gene and a Chromosome 13q Locus Influence Cholesterol Levels in a New Form of Digenic-Recessive Familial Hypercholesterolemia

Abstract

Abstract-We studied a Syrian family with 3 children who had low-density lipoprotein cholesterol (LDL) concentrations of 13.3, 12.2, and 8.6 mmol/L, respectively. Three other siblings and the parents all had LDL values Ͻ4.52 mmol/L, suggesting an autosomal-recessive mode of inheritance. The extended pedigree had 66 additional persons with normal LDL values. A genome-wide scan in the core family with 427 markers showed support for linkage on both chromosomes 1 and 13. Markers on chromosome 1 revealed a 3.07 multipoint LOD score between 1p36.1-p35, an 18-cM interval. Surprisingly, we also found linkage to 13q22-q32, a 14-cM interval, with a 3.08 LOD score. We had identified this locus earlier as containing a gene strongly influencing LDL in another Arab family with autosomal-dominant familial hypercholesterolemia and in normal dizygotic twins. We found evidence for an interaction between these loci. We next genotyped our twin panel and confirmed linkage of the 1p36.1-p35 locus to LDL (PϽ0.002) in this normal population. Elucidation of ARH, the LDL receptor adaptor protein at chromosome 1p35, caused us to sequence that gene. We first identified the genomic structure of ARH gene and then sequenced the gene in our family. We found an intron 1 acceptor splice-site mutation. This mutation was not found in any other family members, in 31 nonrelated Syrian persons, or in 30 Germans. Our results underscore the importance of ARH on chromosome 1 and the chromosome 13q locus to LDL, not only in families with unusual illnesses, but also to the general population. Key Words: cholesterol Ⅲ LDL Ⅲ hyperlipidemia Ⅲ genetics Ⅲ familial hypercholesterolemia E levated plasma low-density lipoprotein cholesterol (LDL) concentrations, an important risk factor for atherosclerosis, are influenced by genetic factors. Elucidation of monogenic lipid disturbances has contributed greatly to our understanding of these disorders specifically and to lipid metabolism in general. [1][2][3][4] Ciccarese et al 5 described families from Sardinia in whom hypercholesterolemia is inherited in an autosomal-recessive fashion (ARH). They studied 5 families and were successful in mapping an ARH locus to chromosome 15q25-q26, which they termed ARH-1. Eden et al 6 and this study independently show that the autosomalrecessive familial hypercholesterolemia gene maps to chromosome 1p36.1-p35 (ARH). Recently, Garcia et al 7 succeeded in cloning the gene residing at the chromosome 1p35 locus. The gene codes for a putative LDL receptor adaptor protein.In an earlier study, we found linkage of unexpectedly low LDL values in a family with autosomal-dominant familial hypercholesterolemia (FH) to a locus on chromosome 13q. 8 We now report novel findings in an Arab kindred with ARH. We describe linkage to both the chromosome 1p35 and chromosome 13q loci, as well as evidence for an interaction between these loci. We identified the responsible mutation in the gene on 1p35 encoding for the LDL receptor adaptor protein and described the intron-exon structure of the gene. Fina...

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

“…4. With standard single-disease-locus linkage analysis under a recessive model, two regions were identified; 1p35 (LOD ¼ 3.07) and 13q (LOD ¼ 3.08).…”

“…In addition, by comparing the results obtained with different two-locus disease models, information on the biological mechanism leading to the disease was gained. In the original study as described by Al-Kateb et al, 4 due to the large size of the pedigree, the family could not be analyzed as a whole with the previous version of GENEHUNTER-TWOLOCUS. Two informative individuals therefore had to be discarded.…”

“…It gives researchers the opportunity to gain information not only about evidence for linkage and the positions of both disease loci, but also on how the two loci act, and interact, on the trait. As an example, we have reanalyzed the hypercholesterolemia data, as a follow-up of the study published by Al-Kateb et al, 4 with the complete 17-bit pedigree. With a two-trait-locus analysis jointly taking into account the two significantly linked loci at 1p and 13q in that family, we were able to rule out the heterogeneity and additive two-locus models, and clearly confirmed the multiplicative model as the correct two-locus mode of inheritance, with an associated LOD score of 5.49.…”

“…With a two-trait-locus analysis jointly taking into account the two significantly linked loci at 1p and 13q in that family, we were able to rule out the heterogeneity and additive two-locus models, and clearly confirmed the multiplicative model as the correct two-locus mode of inheritance, with an associated LOD score of 5.49. The significant statistical evidence for interaction of these two loci in the pathogenesis of hypercholesterolemia is underscored by (1) identification of the causative mutations for hypercholesterolemia at 1p36.1 -p35 that reside in the ARH gene encoding a putative LDL receptor adaptor, 19,4 and (2) by evidence that the second region at 13q22 -q32 harbors a locus modifying LDL cholesterol levels in both normal individuals and in a family with autosomal dominant familial hypercholesterolemia. 20 Our timing and memory comparisons between GENE-HUNTER-TWOLOCUS, TLINKAGE, and SUPERLINK have shown that only GENEHUNTER-TWOLOCUS can handle the large number of markers available with the hypercholesterolemia data set.…”

“…1 This program is capable to calculate LOD and NPL scores with two trait loci. It has already been successfully applied in several projects, for example, house dust mite allergy, 1 autosomal recessive familial hypercholesterolemia, 4 and asthma. 5 However, due to otherwise excessive computation-time demands, the two-locus calculations as implemented in GENEHUNTER-TWOLOCUS are only feasible for moderate pedigree sizes.…”