De Castro-Orós et al
protein: signal peptide, ligand-binding domain, epidermal growth factor precursor (EGFP) like, O-linked sugar, transmembrane, and cytoplasmic domain (Figure 2).42 The prediction of the presence of different domains in the protein was possible because of the gene sequencing, determining that each domain was encoded by separate exons or group of them and suggesting that the LDLr might have evolved through shuffling of exons from other genes, because it has parts similar to unrelated proteins.43,44 After Goldstein and Brown6 identified LDLr dysfunction as cause of FH, multiple mutations were associated with this disease.43,45,46
The LDLr production is tightly regulated by a sophisticated feedback mechanism that controls the transcription of the LDLR in response to variations in the intracellular sterol concentra- tion and the cellular demand for cholesterol.47 DNA motifs are essential for the transcriptional regulation of the LDLR and are located within 280 bp of the proximal promoter (Figure 3).This region contains all the cis-acting elements for basal expres- sion and sterol regulation and includes three imperfect direct repeats of 16 bp each, repeats 1–3. Repeats 1 and 3 contain binding sites for Sp1 transcription factor, and contribute to the basal expression of the gene requiring the contribution of the repeat 2 for a strong expression.48 Repeat 2 contains a sterol regulatory element (SRE) that enhances transcription when the intracellular sterol concentration is low through interaction with a transcriptional factor designated as sterol regulatory element binding protein-1.49 Other two regions, named as FP1 and FP2 and located between −281 to −269, contain important cis-acting elements that have been described as essential for maximal induction of transcription.50 To date, several naturally occurring mutations have been mapped to the transcriptional regulatory elements of the LDLR.51–59
Exon 1 encodes the signal peptide, a hydrophobic sequence of 21 amino acids. This peptide is cleaved from the protein during the translocation into the endoplasmic reticulum. Currently, 79 (4.6% of total variants described)
frameshift, missense, and nonsense sequence variants have been described in this exon (see http;//www.ucl.ac.uk/fh, http://www.umd.necker.fr).
Exons 2–6 encode the ligand-binding domain, which consists of seven tandem repeats of 40 amino acids each. There is a cluster of negatively charged amino acids, Asp-X-Ser-Asp-Glu in each repeat and six cysteine residues that form three disulfide bonds.37 Binding of lipoproteins to the LDLr appears to be mediated by an interaction between acidic residues in the LDLr-binding domain and basic residues ofApo E and Apo B-100.60,61 Deletion of individuals repeats R3–R7 results in a loss of LDL binding (Apo B-100-mediated), but a LDLr fragment consisting of R4 and R5 is sufficient to bind to Apo E-phospholipids vesicles.62 Recently, a new mechanism for the release of LDL particles in the endosome has been proposed. It is based on the instability of R5 at endosomal low pH and low Ca2+. Under this kind of condition, R5 is unable to bind Ca2+ and appears in an unfolded conformation not expected to bind LDL particles.63 To date, in this region, 693 allelic variants (40.7%) have been found.
The second domain of the human LDLr consists of a 411 amino acid sequence, encoded by exons 7–14. This sequence shows a 33% of homology of the human EGFP. Like the ligand-binding domain, this region also contains three repeats of 40–50 amino acids with cysteine-rich sequences. The EGFP-like domain is required for the acid-dependent dissociation of the LDL particles from the LDLr and clathrin- coated pits that takes place in the endosome during receptor recycling. When the EGFP domain is deleted from the LDLr, the receptor can no longer bind LDL but it still binds lipopro- teins that contain Apo E.64 Zhang et al65 showed that PCSK9 bind to EGFP-A repeat (the first one) of LDLr, decreasing receptor recycling and increasing degradation. From all mutations described to date, approximately 788 (46%) have been associated with this domain (see http://www.ucl.ac.uk/ fh, http://www.umd.necker.fr).
Domain Signal peptide
Ligand binding with cys-rich repeats
Figure 2 The LDLR gene. Notes: exons are shown as dark bars numbered underneath. Arrows indicate exons encoding the different domains of the low-density lipoprotein receptor (LDLR) protein: the signal peptide (exon 1), ligand-binding domain (exons 2–6), eGF precursor-like domain (exons 7–14), the domain named as OLS, O-linked carbohydrate chains (exon 15), transmembrane (TM) domain (exons 16 and 5′ part of exon 17), and the cytoplasmic domain (3′ region of exon 17 and 5′ region of exon 18).
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The Application of Clinical Genetics 2010:3