D-Bifunctional Protein Deficiency

A number sign (#) is used with this entry because D-bifunctional protein deficiency can be caused by homozygous or compound heterozygous mutation in the HSD17B4 gene (601860) on chromosome 5q2, which encodes the D-bifunctional protein (DBP).

Description

D-bifunctional protein deficiency is a disorder of peroxisomal fatty acid beta-oxidation. See also peroxisomal acyl-CoA oxidase deficiency (264470), caused by mutation in the ACOX1 gene (609751) on chromosome 17q25. The clinical manifestations of these 2 deficiencies are similar to those of disorders of peroxisomal assembly, including X-linked adrenoleukodystrophy (ALD; 300100), Zellweger cerebrohepatorenal syndrome (see 214100) and neonatal adrenoleukodystrophy (NALD; see 601539) (Watkins et al., 1995).

DBP deficiency has been classified into 3 subtypes depending upon the deficient enzyme activity. Type I is a deficiency of both 2-enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase; type II is a deficiency of hydratase activity alone; and type III is a deficiency of dehydrogenase activity alone. Virtually all patients with types I, II, and III have a severe phenotype characterized by infantile-onset of hypotonia, seizures, and abnormal facial features, and most die before age 2 years. McMillan et al. (2012) proposed a type IV deficiency on the basis of less severe features; these patients have a phenotype reminiscent of Perrault syndrome (PRLTS1; 233400). Pierce et al. (2010) noted that Perrault syndrome and DBP deficiency overlap clinically and suggested that DBP deficiency may be underdiagnosed.

Clinical Features

Watkins et al. (1989) reported a black male infant with neonatal hypotonia and macrocephaly who developed seizures and required ventilatory support for the first 4 days of life. By 6 weeks of age, he had made no developmental progress, seizures continued, and the fontanels were large. Brain biopsy at 6 weeks of age showed polymicrogyria. He also had generalized osteopenia with delayed bone maturation. He died at 5.5 months as a result of an acute necrotizing enterocolitis. Postmortem examination showed small adrenal glands with a normal medulla and replacement of the entire cortex with a single type of lipid-containing 'balloon' cell. The changes were identical to those of seen in X-linked and autosomal recessive neonatal forms of adrenoleukodystrophy. Very long-chain fatty acids were increased in the patient's plasma and fibroblasts, and beta-oxidation was impaired. However, biochemical analysis distinguished the disorder from the other forms of ALD: cultured fibroblasts had normal levels of serum phytanic acid and L-pipecolic acid and normal plasmalogen synthesis. In addition, electron microscopy and catalase subcellular distribution studies showed that peroxisomes were present in the patient's tissues. Immunoblot studies of peroxisomal beta-oxidation enzymes showed deficiency of the L-bifunctional enzyme (LBP; enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase; 607037), whereas acyl-CoA oxidase and the mature form of peroxisomal 3-oxoacyl-CoA thiolase (ACAA1; 604054) were present.

Wanders et al. (1990, 1992) also described peroxisomal bifunctional enzyme deficiency, which was associated with a more severe phenotype than that of peroxisomal acyl-CoA oxidase deficiency. Wanders et al. (1992) used complementation analysis to identify the primary defect as residing in the bifunctional enzyme. The patient presented by Watkins et al. (1989) lacked the L-bifunctional enzyme protein, whereas the patient reported by Wanders et al. (1990) had an inactive form of the enzyme.

Using complementation analysis, Suzuki et al. (1994) identified 2 unrelated Japanese girls with presumed L-bifunctional protein deficiency. One child showed profound hypotonia, feeding difficulty, and intractable convulsions soon after delivery. Craniofacial dysmorphism included large fontanel, frontal bossing, low nasal bridge and upward-slanting of palpebral fissures. Other features included hepatomegaly with normal transaminases and bilirubin, funnel chest, talipes equinovarus, and calcific stippling of the patella. She suffered a subdural hematoma due to vitamin K deficiency on the forty-ninth postnatal day and died of pneumonia at 12 months of age (Nakada et al., 1993). The second child, born of consanguineous parents, showed scaphocephaly, frontal bossing, micrognathia, high-arched palate, delayed closure of the anterior fontanel, and calcific stippling at the shoulder and knee joints. She could smile and follow a person at 3 months of age, but regressed thereafter. She manifested adrenocortical insufficiency from the age of 11 months and died of airway obstruction at 21 months of age.

Goldfischer et al. (1986) described an infant, 11 months old at the time of her sudden death, who showed clinical, biochemical, and pathologic features similar in many respects to those seen in the Zellweger syndrome. She had increased serum levels of very long chain fatty acids, an accumulation of trihydroxycoprostanoic acid in duodenal aspirate, and slightly increased levels of pipecolic acid concentrations in serum and urine. However, liver biopsy showed an abundance of peroxisomes, which are profoundly deficient in Zellweger syndrome. Furthermore, the activity of the peroxisomal enzyme acyl-CoA:dihydroxyacetone-phosphate acyltransferase was normal in the patient; this membrane-associated enzyme is deficient in Zellweger syndrome. Peroxisomal oxidation of palmitoyl-CoA was reduced about 15% of the control values, suggesting a defect in peroxisomal beta-oxidation. In this patient, Schram et al. (1987) found deficiency of peroxisomal 3-oxoacyl-CoA thiolase in postmortem liver. Clayton et al. (1990) demonstrated that unconjugated varanic acid (3-alpha,7-alpha,12-alpha,24-tetrahydroxycholestanoic acid), an intermediate in the formation of cholic acid from THCA (3-alpha,7-alpha,12-alpha-trihydroxycholestanoic acid), was present in the body fluids of the patient reported by Goldfischer et al. (1986). In a reinvestigation of the patient reported by Goldfischer et al. (1986), Ferdinandusse et al. (2002) found absence of the D-bifunctional protein postmortem brain, whereas thiolase was normally present. In this patient, Ferdinandusse et al. (2002) identified a homozygous mutation in the HSD17B4 gene (601860.0006), confirming a diagnosis of D-bifunctional protein deficiency. There was no longer evidence for the existence of thiolase deficiency as a distinct clinical entity.

Van Grunsven et al. (1998) reported a boy with D-bifunctional protein deficiency confirmed by the demonstration of a homozygous mutation in the HSD17B4 gene (601860.0003). The boy was born of nonconsanguineous Caucasian parents at 36 weeks' gestation after an uncomplicated pregnancy. Head circumference was at the 50th percentile at birth, but macrocephaly developed during the first year of life. Examination showed high forehead with frontal bossing, low-set ears, and a large fontanel. The liver was palpable 2.5 cm below the costal margin. Other features included a long, small thorax, hypospadias, limb-girdle muscle wasting, and generalized hypotonia. Neurologic examination showed negative traction and Moro response, with maximal headlag at 4 weeks. At the age of 2 months, the patient became cyanotic and developed epileptic seizures leading to aspiration. MRI of the brain showed white matter abnormalities consistent with dysmyelination. The patient died at the age of 16 months from aspiration pneumonia. Laboratory studies showed increased plasma very long-chain fatty acids increased levels of several bile acid intermediates, and normal levels of plasmalogen. The 3-hydroxyacyl-CoA dehydrogenase activity of the D-bifunctional protein was completely inactive, whereas the enoyl-CoA hydratase component was active.

Nakano et al. (2001) reported a patient with D-bifunctional protein deficiency who was compound heterozygous for 2 mutations in the HSD17B4 gene (601860.0001 and 601860.0005). Polyhydramnios and fetal ascites were detected at 30 weeks' gestation. At birth, the child had claw hands, hammertoes, abdominal distention, generalized hypotonia, and craniofacial dysmorphism with frontal bossing, low nasal bridge, and large fontanel. Chylous ascites was aspirated on the eighth day of life. Postnatally she developed psychomotor retardation. She died of pneumonia and heart failure at 7 months of age. Autopsy showed polymicrogyria of the cerebrum and cerebellum, a single neuron heterotopia in the white matter, hypoplastic corpus callosum, excessive convolutions of the inferior olivary nucleus, fibrosis around the Glisson capsule in the liver, renal cortical microcysts involving the Bowman capsule, and lamellar inclusions in the adrenal cortex.

Ferdinandusse et al. (2006) reported the clinical features of 126 patients with D-bifunctional protein deficiency. Some of the patients had been previously reported. Most patients presented with neonatal hypotonia and seizures. Other common features included visual impairment, severe psychomotor retardation, and a characteristic facies with high forehead, high-arched palate, enlarged fontanel, long philtrum, epicanthal folds, hypertelorism, macrocephaly, retrognathia, and low-set ears. Brain imaging showed gross ventricular dilatation (29%), neocortical dysplasia (27%), cerebral demyelination (17%), and cerebellar atrophy (17%). Liver disease was present in 26% of patients and 43% had hepatomegaly. Postmortem examination of 11 patients showed polymicrogyria in 64%. Renal cysts and adrenal cortex atrophy were seen in 33% and 42% of autopsied cases, respectively, and uncommon features included delay of bone maturation, skeletal malformations, and calcific stippling. Although most patients died before age 2 years, 12 patients survived beyond 2 years, 5 of whom survived beyond 7.5 years. Biochemical analysis showed a clear correlation between peroxisomal beta-oxidation activity and survival.

Biochemical Features

DBP deficiency can be divided into 3 types, depending on which enzymatic activity is deficient: (1) type I-deficient patients have a deficiency of both the hydratase and dehydrogenase units of DBP (in fibroblasts of almost all type I-deficient patients, no DBP protein can be detected by immunoblotting with an antibody against human DBP); (2) type II-deficient patients have an isolated deficiency of the hydratase unit; and (3) type III-deficient patients have an isolated deficiency of the dehydrogenase unit (Wanders et al., 2001). This classification can be made on the basis of enzyme activity measurements in combination with mutation analysis, as described by Gloerich et al. (2003).

Diagnosis

The diagnosis of DBP deficiency is commonly made based on the accumulation of very long chain fatty acids (VLCFA), dihydroxy- and trihydroxycholestanoic acid (DHCA and THCA), and pristanic and phytanic acid in plasma. However, some patients with residual enzyme activity may not have abnormal plasma values, making the diagnosis difficult. Gronborg et al. (2010) reported 2 sibs with the disorder who were initially found to have normal VLCFA plasma values when studied based on clinical features. Features included neonatal hypotonia, early-onset seizures, and severe developmental delay. Both also had hearing loss and showed developmental regression at about age 4 years. The older sib died at age 10 years, 9 months, without correct diagnosis. Brain MRI in the 2 sibs at ages 28 and 30 months, respectively, showed typical findings of a peroxisomal disorder, prompting repeated plasma examination in the younger sib and studies of patient fibroblasts, which led to proper molecular diagnosis. The MRI findings included leukoencephalopathy of cerebral and cerebellar white matter, polymicrogyria, and pachygyria. Gronborg et al. (2010) concluded that brain MRI may aid in the diagnosis of patients with DBP deficiency, particularly in cases with no or mild plasma abnormalities.

Prenatal Diagnosis

Suzuki et al. (1999) reported the first successful prenatal diagnosis of D-bifunctional protein deficiency using cultured amniocytes obtained from a fetus at 16 weeks' gestation. These authors used several methods, including an assay for lignoceric acid oxidation activity, indirect immunofluorescence staining and immunoblot analysis for the presence of DBP, and genetic analysis using RT-PCR. After pregnancy termination, immunohistochemical and biochemical studies of the fetus confirmed their prenatal diagnosis.

Molecular Genetics

In 2 Japanese patients reported by Suzuki et al. (1994) as having L-bifunctional protein deficiency, Suzuki et al. (1997) identified 2 different homozygous deletions in the HSD17B4 gene (601860.0001; 601860.0002), confirming D-bifunctional protein deficiency.

In 9 patients who carried the diagnosis of L-bifunctional protein deficiency on the basis of complementation analysis, van Grunsven et al. (1999) identified mutations in the HSD17B4 gene, confirming a diagnosis of D-bifunctional protein deficiency. Four of the 9 patients had the same mutation (601860.0003).

In a patient with D-bifunctional protein deficiency originally reported by Watkins et al. (1989), van Grunsven et al. (1999) identified a homozygous 2-bp deletion in the HSD17B4 gene (601860.0007). The patient was originally thought to have L-bifunctional protein deficiency based on immunoblot analysis of postmortem liver tissue. However, reanalysis showed accumulation of both very long chain fatty acids and bile acid intermediates, which was hard to reconcile with an isolated deficiency of the L-bifunctional protein. The results suggested that most, if not all, patients whose peroxisomal disorder had been diagnosed as L-bifunctional protein deficiency were in fact cases of D-bifunctional protein deficiency.

Ferdinandusse et al. (2006) reported the mutational spectrum of DBP deficiency on the basis of molecular analysis in 110 patients. They identified 61 different mutations by DBP cDNA analysis, 48 of which had not been previously reported. The predicted effects of the different disease-causing amino acid changes in protein structure were determined using the crystal structures. The effects ranged from the replacement of catalytic amino acid residues or residues in direct contact with the substrate or cofactor to disturbances of protein folding or dimerization of the subunits. To study whether there is a genotype-phenotype correlation for DBP deficiency, these structure-based analyses were combined with extensive biochemical analyses of patient material (cultured skin fibroblasts and plasma) and available clinical information on the patients. They found that the effect of the mutations identified in patients with a relatively mild clinical and biochemical presentation was less detrimental to the protein structure than the effect of mutations identified in those with a very severe presentation. These results suggested that the amount of residual DBP activity correlates with the severity of the phenotype. Thus the data indicated that on the basis of the predicted effect of mutations on protein structure, a genotype-phenotype correlation exists for DBP deficiency.

Population Genetics

DBP deficiency has an estimated prevalence of 1 in 100,000 (Ferdinandusse et al., 2006).