Alternative titles; symbols
HGNC Approved Gene Symbol: HADHB
Cytogenetic location: 2p23.3 Genomic coordinates (GRCh38): 2:26,244,939-26,290,465 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 2p23.3 | Mitochondrial trifunctional protein deficiency 2 | 620300 | 3 |
The HADHA (600890) and HADHB genes encode the alpha and beta subunits of the mitochondrial trifunctional protein, respectively. The heterocomplex contains 4 alpha and 4 beta subunits and catalyzes 3 steps in the beta-oxidation of fatty acids, including the long-chain 3-hydroxyacyl-CoA dehydrogenase step. The beta subunit harbors the 3-ketoacyl-CoA thiolase activity (EC 2.3.1.16) (Kamijo et al., 1994).
Kamijo et al. (1994) identified cDNAs for the genes encoding the alpha and beta subunits of the holoenzyme. The beta subunit cDNA encodes a 51.293-kD precursor, which ultimately becomes the 47.484-kD mature subunit.
Orii et al. (1999) determined that the HADHA and HADHB genes are linked in a head-to-head arrangement on opposite strands and that they have in common a 350-bp 5-prime flanking region. This region has bidirectional promoter activity with 2 critical cis elements that are activated by transcription factor SP1 (189906) binding. The authors concluded that expression of trifunctional protein subunits is probably coordinately regulated by a common promoter and by SP1.
Uchida et al. (1992) and Carpenter et al. (1992) demonstrated that long-chain 3-hydroxyacyl-CoA dehydrogenase is in fact a trifunctional protein which also has enoyl-CoA hydratase and 3-ketoacyl-CoA thiolase activity. Carpenter et al. (1992) and Uchida et al. (1992) worked with human and rat liver, respectively.
Kamijo et al. (1994) examined the biosynthesis of the human trifunctional protein in cultured skin fibroblasts from 2 patients with LCHAD deficiency. The cells from 1 patient indicated a content of the protein that was less than 10% of that in control cells, due to very rapid degradation of protein newly synthesized in the mitochondria. Diminution of the protein was associated with a decrease in all 3 enzymatic activities. In cells from the second patient, the rate of degradation of newly synthesized protein was faster than in control cells, giving rise to a trifunctional protein 60% of control levels. The 3-hydroxyacyl-CoA dehydrogenase activity with medium-chain to long-chain substrates was decreased drastically, with minor changes in activities of the 2 other enzymes.
Orii et al. (1997) determined that the HADHB gene has 16 exons.
By somatic cell hybrid studies, Craig et al. (1976) tentatively assigned the structural gene for this enzyme to chromosome 7. However, studies by fluorescence in situ hybridization reported 20 years later indicated that both the HADHB and HADHA genes are located on 2p23 (Yang et al., 1996). By fluorescence in situ hybridization, Orii et al. (1997) confirmed the mapping of the HADHB gene to 2p23.
Studies on the defects in the mitochondrial trifunctional enzyme in patients with trifunctional protein deficiency (MTPD1, 609015; MTPD2, 620300) suggested to Ushikubo et al. (1996) that there are 2 types of defects: patients in group 1 have normal amounts of crossreacting material by immunoblot and lack only long-chain 3-hydroxyacyl-CoA dehydrogenase activity, whereas patients in group 2 have a trace amount of crossreacting material, with all 3 activities being low. Ushikubo et al. (1996) identified 3 patients in group 2 and made analyses at the cDNA level. In 1 patient, there was a heterozygous 71-bp deletion at position 110-180 in the alpha subunit. In the other 2 patients, there was an abnormal beta subunit; 1 patient was homozygous for an A-to-G transition at nucleotide 788 (143450.0001), whereas the other patient was compound heterozygous for a 182G-A (143450.0002) and a 740G-A (143450.0003) mutation. This was the first demonstration of disease-causing mutations in the beta subunit. Using a vaccinia virus system and gel filtration analysis for cDNA expression experiments in patients' fibroblasts, Ushikubo et al. (1996) found that both normal alpha and beta subunits, and possibly their association, are important for stabilizing the trifunctional protein. They commented that the 788A-G mutation on the beta subunit seemed to interfere with the association, the result being rapid decomposition of the trifunctional protein.
Orii et al. (1997) identified 2 Japanese patients in whom the 3 enzyme activities of the trifunctional protein were undetectable in fibroblasts. One of the patients had compound heterozygous mutations in the HADHB gene, including an exonic single T insertion that created a new cryptic 5-prime splice site (143450.0005) and a 1331G-A transition that resulted in an arg411-to-lys amino acid substitution (143450.0004). The second patient was homozygous for the 1331G-A transition.
Purevsuren et al. (2009) reported 5 Japanese patients, including 3 who had previously been reported, with mitochondrial trifunctional protein deficiency due to homozygous or compound heterozygous mutations in the HADHB gene (see, e.g., 143450.0004 and 143450.0006).
Associations Pending Confirmation
For discussion of a possible association between variation in the HADHB gene and early-onset peripheral neuropathy resembling axonal Charcot-Marie-Tooth disease (see, e.g., CMT2A1, 118210), see 143450.0010.
Spiekerkoetter et al. (2003) characterized 15 patients from 13 families with beta-subunit mutations of the mitochondrial trifunctional protein. Three clinical phenotypes were apparent: 4 patients had a severe neonatal presentation with cardiomyopathy, Reye-like symptoms, and early death; 2 patients had a hepatic form with recurrent hypoketotic hypoglycemia; and 9 patients had a milder, later-onset neuromyopathic phenotype with episodic myoglobinuria. Maternal HELLP syndrome (hemolysis, elevated liver enzymes, low platelets) occurred in 2 mothers independently of the fetal phenotype. Mutation analysis revealed 16 different mutations, 12 of which were missense mutations. Based on homology to yeast thiolase, which had been characterized structurally, Spiekerkoetter et al. (2003) found that the location of the mutation within the protein correlated with the clinical phenotype. Outer loop mutations that were expected to alter protein stability were present only in milder forms. The degree of reduction in thiolase antigen also correlated with the severity of clinical presentation. Thus, although TFP deficiency is highly heterogeneous, some genotype-phenotype correlation could be established.
In vitro functional expression studies of HADHB gene mutations in 5 Japanese patients with MTPD2 by Purevsuren et al. (2009) indicated a genotype/phenotype correlation: patients whose mutations resulted in no residual protein activity had a more severe phenotype than those whose mutations had residual activity.
Dagher et al. (2021) reviewed pathophysiology, clinical phenotype, and molecular findings in MTP deficiency caused by biallelic mutations in the HADHB gene. The authors noted that mutations in the HADHB gene often lead to deficiency of all 3 enzymatic functions of the mitochondrial trifunctional protein because the mutations frequently affect amino acid residues located at the interface of the dimerization domains between HADHB and HADHA.
In a patient with mitochondrial trifunctional protein deficiency (MTPD2; 620300), Ushikubo et al. (1996) found probable homozygosity for an A-to-G transition at nucleotide 788 of the HADHB gene, predicted to cause a substitution of glycine for aspartic acid-263 (D263G). The patient was previously reported by Kamijo et al. (1994).
In a male Caucasian patient who presented with hypoglycemia, hyperammonemia, mild liver dysfunction, and 3-hydroxydicarboxylic aciduria (MTPD2; 620300) at 4 months of age, Ushikubo et al. (1996) described compound heterozygosity for an arg61-to-his (R61H) mutation and an arg247-to-his (R247H; 143450.0003) mutation in the HADHB gene. These were due to nucleotide substitutions 182G-A and 740G-A, respectively.
For discussion of the arg247-to-his (R247H) mutation in the HADHB gene that was found in compound heterozygous state in a patient with hypoglycemia, hyperammonemia, mild liver dysfunction, and 3-hydroxydicarboxylic aciduria (MTPD2; 620300) by Ushikubo et al. (1996), see 143450.0002.
In a Japanese patient with mitochondrial trifunctional protein deficiency (MTPD2; 620300), Orii et al. (1997) found homozygosity for a 1331G-A transition, resulting in an arg411-to-lys (R411K) amino acid substitution. The patient had onset at age 15 years of muscle pain and weakness associated with rhabdomyolysis. Development was normal. In another Japanese patient, the R411K mutation was present in compound heterozygous state with an exonic single T insertion, creating a new cryptic 5-prime splice site (143450.0005). This patient had a more severe phenotype, with onset at 13 months of age of lethargy, hypotonia, and recurrent respiratory infections. He had cardiac arrest, liver dysfunction, and delayed psychomotor development.
By in vitro functional expression studies, Purevsuren et al. (2009) demonstrated that the R411K mutant had 14% residual enzyme activity at 37 degrees C and faint expression of alpha- and beta-subunit proteins. However, residual activity at 30 degrees C increased significantly to more than 50% of wildtype.
In a Japanese patient with mitochondrial trifunctional protein deficiency (MTPD2; 620300), Orii et al. (1997) found a 36-bp deletion at position 776-811 of the HADHB gene. The deletion was caused by a single T insertion at nucleotide position 777, which is 36 bp upstream from the 5-prime splice of intron 9. The insertion caused the deletion by creating a new cryptic 5-prime splice site. The patient was compound heterozygous for an R411K substitution (143450.0004).
In a Japanese male infant, born of consanguineous parents, with trifunctional protein deficiency (MTPD2; 620300), Purevsuren et al. (2009) identified a homozygous 1364T-G transversion in the HADHB gene, resulting in a val422-to-gly (V422G) substitution. He presented on the fifth day of life with dyspnea, cyanosis, lactic acidosis, hyperammonemia, and liver dysfunction. He died of cardiac arrest at age 3 months. In vitro functional expression assays showed no apparent residual enzyme activity at 37 degrees C, and Western blot showed no alpha or beta subunits. However, there was some residual enzyme activity at 30 degrees C.
In a pair of dizygotic Japanese twins, born of consanguineous parents, with mitochondrial trifunctional protein deficiency (see 620300), Naiki et al. (2014) identified a homozygous c.1175C-T transition (c.1175C-T, NM_000183) in exon 14 of the HADHB gene, resulting in an ala392-to-val (A392V) substitution near the active site of the protein. Each unaffected parent was heterozygous for the mutation, which was not found in 200 normal alleles. Western blot analysis of patient cells showed decreased or absent levels of the alpha and beta subunits of the mitochondrial trifunctional protein. The mutant HADHB protein failed to form an active hetero-octamer with the wildtype alpha subunit (HADHA; 600890). The patients had infantile onset of hypoparathyroidism, episodic rhabdomyolysis, and a progressive polyneuropathy, consistent with a neuromyopathic phenotype.
In a Korean infant with fatal mitochondrial trifunctional protein deficiency (MTPD2; 620300), Park et al. (2009) identified compound heterozygous mutations in the HADHB gene: a 1-bp duplication (c.358dupT, NM_000183.2) in exon 7, resulting in a frameshift and premature termination (Ala120CysfsTer8), and a c.1364T-G transversion in exon 15, resulting in a val455-to-gly (V455G; 143450.0009) substitution. Each unaffected parent was heterozygous for 1 of the mutations, which were not found in 100 Korean control alleles. Functional studies of the variants were not performed.
For discussion of the c.1364T-G transversion (c.1364T-G, NM_000183.2) in exon 15 of the HADHB gene, resulting in a val455-to-gly (V455G) substitution, that was found in compound heterozygous state in a Korean infant with fatal mitochondrial trifunctional protein deficiency (MTPD2; 620300) by Park et al. (2009), see 143450.0008.
This variant is classified as a variant of unknown significance because its contribution to an early-onset peripheral neuropathy resembling axonal Charcot-Marie-Tooth disease (see, e.g., CMT2A1, 118210) has not been confirmed.
In 2 sibs, born of unrelated Korean parents, with early-onset axonal sensorimotor neuropathy, Hong et al. (2013) identified compound heterozygous variants in the HADHB gene: a G-to-C transversion (c.210-1G-C, NM_000183) in intron 4, resulting in the deletion of exon 5, and a c.686G-T transversion, resulting in an arg229-to-leu (R229L; 143450.0011) substitution at a highly conserved residue in the thiolase N domain. The variants, which were found by whole-exome sequencing, were filtered against the dbSNP (build 135) and 1000 Genomes Project databases, and segregated with the disorder in the family. Western blot analysis of patient fibroblasts showed decreased levels of HADHB compared to controls, and expression of the mutations in HEK293 cells confirmed that the mutations resulted in unstable mutant proteins. However, additional functional studies of the variants were not performed, and enzyme activity in patient cells was not determined. The patients developed distal limb weakness resulting in gait difficulties and foot drop at about 5 years of age. The disorder was progressive, and both required a walker in their teens. In their mid-thirties, both patients had distal muscle weakness and atrophy affecting the upper and lower limbs, associated with distal sensory impairment and loss of deep tendon reflexes. Electrophysiologic studies showed slightly decreased motor nerve conduction velocities and absence of most of the sensory nerve action potentials. Sural nerve biopsy of 1 patient showed absence of large myelinated fibers, occasional regenerating axonal clusters, pseudo-onion bulb formation, regenerating clusters, axons with swelling and vacuolization of the axoplasm, and abnormal mitochondria. The findings were consistent with a length-dependent axonal neuropathy. Neither patient had additional findings of mitochondrial trifunctional protein deficiency, such as myopathy, rhabdomyolysis, cardiomyopathy, or liver dysfunction.
This variant is classified as a variant of unknown significance because its contribution to an early-onset peripheral neuropathy resembling axonal Charcot-Marie-Tooth disease (see, e.g., CMT2A1, 118210) has not been confirmed.
For discussion of the c.686G-T transversion (c.686G-T, NM_000183) in the HADHB gene, resulting in an arg229-to-leu (R229L) substitution, that was found in compound heterozygous state in Korean sibs with early-onset axonal neuropathy by Hong et al. (2013), see 143450.0010.
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