Autosomal recessive hypercholesterolemia caused by mutations in a putative LDL receptor adaptor protein

Dual roles for the Dab2 adaptor protein in embryonic development and kidney transport

The Disabled-2 (Dab2) gene has been proposed to act as a tumor suppressor. Cell culture studies have implicated Dab2 in signal transduction by mitogens, TGFbeta and endocytosis of lipoprotein receptors. To identify in vivo functions of Dab2, targeted mutations were made in the mouse. In the absence of Dab2, embryos arrest prior to gastrulation with a phenotype reminiscent of those caused by deletion of some TGFbeta signal transduction molecules involved in Nodal signaling. Dab2 is expressed in the extra-embryonic visceral endoderm but not in the epiblast. Dab2 could be conditionally deleted from the embryo without affecting normal development, showing that Dab2 is required in the visceral endoderm but dispensable in the embryo proper. Conditionally mutant Dab2(-/-) mice are overtly normal, but have reduced clathrin-coated pits in kidney proximal tubule cells and excrete specific plasma proteins in the urine, consistent with reduced transport by a lipoprotein receptor, megalin/gp330, in the proximal tubule. This evidence indicates that Dab2 is pleiotropic and regulates both visceral endoderm function and lipoprotein receptor trafficking in vivo.

Familial Hypercholesterolemia

Heterozygous familial hypercholesterolemia is a common, severe form of elevated plasma, low-density lipoprotein (LDL) cholesterol. Familial hypercholesterolemia is under-recognized and under-treated despite well-known benefits from available lipid-lowering medications. To help address these problems, the humanitarian MEDPED (Make Early Diagnoses, Prevent Early Deaths) organization was developed as a means to find and help persons with familial hypercholesterolemia all over the world. Although some patients with familial hypercholesterolemia can be treated adequately with a statin alone, many require aggressive treatment with a combination of drugs, including a statin, bile acid sequestrant, and niacin to reach LDL goals. Such treatment has resulted in regression of angiographically monitored coronary lesions in patients with familial hypercholesterolemia.

Autosomal recessive hypercholesterolaemia in Sardinia, Italy, and mutations in ARH: a clinical and molecular genetic analysis

BACKGROUND

Autosomal recessive hypercholesterolaemia (ARH) is caused by mutations in a putative adaptor protein called ARH. This recessive disorder, characterised by severe hypercholesterolaemia, xanthomatosis, and premature coronary artery disease, is rare except on the island of Sardinia, Italy. Our aim was to ascertain why ARH is more common on Sardinia than elsewhere.

METHODS

We obtained detailed medical histories, did physical examinations, measured concentrations of lipoproteins, and harvested genomic DNA from 28 Sardinians with ARH from 17 unrelated families. We sequenced the coding regions and consensus splice sites of ARH in probands from these families, and from 40 individuals of non-Sardinian origin who had an autosomal recessive form of hypercholesterolaemia of unknown cause.

FINDINGS

Two ARH mutations, a frameshift mutation (c432insA) in exon 4 (ARH1) and a nonsense mutation (c65G-->A) in exon 1 (ARH2), were present in all of the 17 unrelated families with ARH. Three of the ARH alleles contained both mutations, as a result of an ancient recombination between ARH1 and ARH2. No regional clustering of the three mutant alleles within Sardinia was apparent. Furthermore, four Italians from the mainland with autosomal recessive hypercholesterolaemia were homozygous for ARH1.

INTERPRETATION

The small number, high frequency, and dispersed distribution of ARH mutations on Sardinia are consistent with these mutations being ancient and maintained in the Sardinian population because of geographic isolation.

Phosphotyrosine-binding domains in signal transduction

Protein phosphorylation provides molecular control of complex physiological events within cells. In many cases, phosphorylation on specific amino acids directly controls the assembly of multi-protein complexes by recruiting phospho-specific binding modules. Here, the function, structure, and cell biology of phosphotyrosine-binding domains is discussed.

Statins in homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia is a rare disorder resulting in severe premature atherosclerosis. Drug therapy was previously viewed as inadequate for control of the dyslipidemia, so portacaval shunting, plasmapheresis, and liver transplantation were undertaken to treat this condition. Despite these drastic measures, additional cholesterol-lowering treatment may still be required. Furthermore, there is a need for pharmacologic control until additional measures can be undertaken. The statins, an evolving class of cholesterol-modifying drugs, represent a significant development in the treatment of homozygous familial hypercholesterolemia. The experience with statins in this condition is limited, but some insight into their utility has been gained from studies reviewed in this article. It is recommended that high doses of statins be used in combination with other lipid-modifying strategies for the best control of the dyslipidemia of homozygous familial hypercholesterolemia.