Last Updated: November 23, 2022
Introduction to D-Bifunctional Protein Deficiency
D-bifunctional protein deficiency (DBPD) is an autosomal recessive disorder that is a member of a family of disorders that result from defects in the biogenesis and/or functioning of the peroxisomes and, as such, are referred to as peroxisome biogenesis disorders, PBD. The PBD are caused either by peroxisomal assembly defects or by deficiencies of single peroxisomal proteins. D-bifunctional protein deficiency results from mutations in a single gene, HSD17B4, that encodes an enzyme involved in peroxisomal β-oxidation of certain fatty acids and is also involved in the synthesis of bile acids.
Overview of Peroxisomal Biogenesis
The peroxisomes are a single membrane organelle, similar to lysosomes, present in virtually all eukaryotic cells. The peroxisome is a specialized enzyme “factory” that contains in excess of 50 different enzymes involved in a variety of metabolic processes including, but not limited to, β-oxidation of very long chain fatty acids (VLCFA), α-oxidation of branched-chain fatty acids, and synthesis of ether lipids.
Proteins that are involved in, and necessary for correct, peroxisome biogenesis are called peroxins (PEX). At least 15 PEX genes have been identified in humans.
Enzymes that are targeted to the peroxisomes contain either of two amino acid consensus elements called peroxisome targeting sequences (PTS). The PTS1 is a C-terminal consensus sequence of –[S/A/C][K/R/H][L/M] referred to as the SKL motif. This sequence element is recognized by a cytosolic PTS1 receptor encoded by the PEX5 gene. There are two isoforms of PEX5 encoded proteins in humans identified as Pex5pS and Pex5pL (for short and long forms, respectively). The Pex5pL protein has an internal 37 amino acid insertion, hence the “long” designation.
The PTS2 is an N-terminal consensus sequence of –[R/K][L/V/I/Q]XX[L/V/I/H/Q][L/S/G/A/K]X[H/Q][L/A/F]–, (where X represents any amino acid). The PTS2 receptor is encoded by the PEX7 gene and the encoded protein is referred to as Pex7p. Proteins that are targeted to the membrane of the peroxisome (called peroxisome membrane proteins, PMPs) contain a consensus sequence identified as the PEX19 binding site (PEX19BS) and this site is recognized by the membrane protein receptor encoded by the PEX19 gene.
Pex5pS, Pex5L, and Pex7p interact with newly synthesized target proteins in the cytosol and direct them to the peroxisome. On the membrane of the peroxisome is a component of the protein import machinery encoded by the PEX14 gene called Pex14p. Following interaction of Pex5pS or Pex5pL, bound to a protein containing a PTS1 sequence, with Pex14p, the PTS1 containing protein is transferred into the peroxisome. The activity of Pex7p in peroxisome protein import actually requires Pex5pL as well. PTS2 containing proteins interact with Pex7p and then, in conjunction with Pex5pL, the complex interacts with Pex14p and the PTS2 containing protein is transferred into the peroxisome. Very few proteins contain a PTS2 sequence but one enzyme of note is phytanoyl-CoA hydroxylase (PHYH) which is defective in classic Refsum disease.
Molecular Biology of D-Bifunctional Protein Deficiency
The D-bifunctional protein belongs to the large family of 17β-hydroxysteroid dehydrogenases (17β-HSD or HSD17B) that includes 15 genes. The D-bifunctional protein is encoded by the HSD17B4 (hydroxysteroid 17-beta dehydrogenase 4) gene. The HSD17B4 gene is located on chromosome 5q23.1 and is composed of 27 exons that generate 12 alternatively spliced mRNAs. Collectively these 12 transcripts encode a total of 10 enzyme isoforms. The C-terminus of the D-bifunctional protein possesses a PTS1 domain.
D-bifunctional protein is unique among the HSD17B family of enzymes for two reasons. First it possesses three catalytically active domains and second it is the only HSD17B enzyme localized to the peroxisomes. The three functional activities of D-bifunctional protein are the N-terminal domain that harbors the short-chain alcohol dehydrogenase reductase (SDR) activity, the central domain that harbors the hydratase activity, and the C-terminal domain that harbors a sterol carrier protein 2 (SCP2) like domain.
D-bifunctional protein is involved in the peroxisomal β-oxidation of very long-chain fatty acids (VLCFA) such as C24:0 (lignoceric acid or tetracosanoic acid) and C26:0 (cerotic acid or hexacosanoic acid), peroxisomal β-oxidation of branched fatty acids such as pristanic acid, and in the synthesis of the bile acids, di- and trihydroxycholestanoic acid (DHCA and THCA).
Mutations in the HSD17B4 gene that result in D-bifunctional protein deficiency include frameshift, in-frame deletions, and missense mutations. Most of the mutations identified in patients with D-bifunctional protein deficiency are private mutations. The frequency of D-bifunctional protein deficiency is around 1 in 100,000 live births.
Clinical Features of D-Bifunctional Protein Deficiency
As a result of the two primary enzymatic functions of D-bifunctional protein (hydratase and dehydrogenase) there are three different types of D-bifunctional protein deficiency. Type 1 D-bifunctional protein deficiency results from defects in both the hydratase and dehydrogenase activities of the enzyme. Type 2 D-bifunctional protein deficiency results from defects in the hydratase activity of the enzyme. Type 3 D-bifunctional protein deficiency results from defects in the dehydrogenase
activity of the enzyme.
D-bifunctional protein deficiency is associated with neurodegeneration beginning in infancy. Newborns with D-bifunctional protein deficiency exhibit facial dysmorphism, hypotonia, seizures, and psychomotor delay. Biochemical analysis of blood will show highly elevated levels of very long-chain fatty acids (VLCFA), particularly pristanic acid (C26:0) as well as the bile acid intermediates, DHCA and THCA.
Many of the pathological features of D-bifunctional protein deficiency resemble those of Zellweger syndrome. Differential diagnosis of D-bifunctional protein deficiency from Zellweger syndrome can be made by the fact that D-bifunctional protein deficiency is associate with normal synthesis and levels of plasmalogens. As the symptoms of the disorder get worse, affected children develop hyperreflexia (exaggerated reflexes), hypertonia (increased muscle tone), loss of vision and hearing, and more severe and recurrent seizures. Most children with D-bifunctional protein deficiency do not survive past the age of 2.