HSD17B4 Gly16Ser — D-Bifunctional Protein and Peroxisomal Fatty Acid Oxidation

Inside every cell, the peroxisome acts as a specialized metabolic furnace — the only compartment capable of breaking down very-long-chain fatty acids (VLCFAs)¹¹ very-long-chain fatty acids (VLCFAs)
Fatty acids with 22 or more carbon atoms, such as C24:0 (lignoceric acid) and C26:0 (hexacosanoic acid). These cannot enter the mitochondria for standard beta-oxidation and must first be chain-shortened in peroxisomes before mitochondria can complete the process. and branched-chain fatty acids such as pristanic acid. The HSD17B4 gene encodes D-bifunctional protein (DBP)²² D-bifunctional protein (DBP)
Also called MFP-2 (multifunctional protein 2) or MFPE (multifunctional enzyme 2). DBP is one of two peroxisomal multifunctional enzymes; the other is L-bifunctional protein (ЕХНY1). DBP acts on D-configured intermediates and is responsible for the majority of peroxisomal fatty acid beta-oxidation in humans., a peroxisomal enzyme that catalyzes two consecutive steps of fatty acid chain-shortening: 2-enoyl-CoA hydratase activity (hydration) and 3-hydroxyacyl-CoA dehydrogenase activity (oxidation). Without functional DBP, VLCFAs and branched-chain fatty acids accumulate in tissues, particularly in myelin sheaths and adrenal cortex, causing progressive neurological damage.

The rs137853096 variant substitutes a serine for the glycine at position 16 of the DBP protein (p.Gly16Ser, c.46G>A). Glycine-16 sits within the N-terminal dehydrogenase domain of DBP and is conserved across vertebrates, indicating functional importance. Demain et al. 2017³³ Demain et al. 2017
Demain LA et al. Expanding the genotypic spectrum of Perrault syndrome. Clin Genet, 2017 identified p.Gly16Ser as a recurrent pathogenic allele in Perrault syndrome, found compound heterozygous with a novel p.Val82Phe variant in an affected female with sensorineural hearing loss and primary ovarian insufficiency.

The Mechanism

DBP's dehydrogenase domain requires a precise three-dimensional fold to position the NAD+ cofactor and substrate correctly. Glycine is the smallest amino acid with no side chain, and its presence at position 16 is critical for the tight turn geometry of the N-terminal domain. Substituting serine introduces a hydroxyl side chain that is likely to disrupt local secondary structure and destabilize the dehydrogenase fold, reducing or abolishing enzymatic activity.

When both copies of HSD17B4 carry pathogenic variants, the peroxisomal beta-oxidation pathway is blocked. Two distinct clinical presentations emerge depending on the residual enzyme activity:

Severe neonatal DBP deficiency (classical form): Near-complete loss of activity causes accumulation of VLCFAs, pristanic acid, and bile acid intermediates within days of birth. Infants present with hypotonia, neonatal seizures, polymicrogyria, and facial dysmorphism. Biochemically, elevated C26:0, C26:0/C22:0 ratio, and abnormal bile acids confirm the diagnosis. Chen et al. 2021⁴⁴ Chen et al. 2021
Chen S et al. Two Novel HSD17B4 Heterozygous Mutations in Association With D-Bifunctional Protein Deficiency. Front Pediatr, 2021 reported a neonatal case with significant VLCFA elevation and a fatal outcome at 5 months despite supportive care.

Perrault syndrome (attenuated form): Partial residual DBP activity — as expected with a missense variant like p.Gly16Ser — produces a less severe but still significant phenotype that may not manifest until childhood or adolescence. The cardinal features are sensorineural hearing loss (present in both sexes) and primary ovarian insufficiency (POI) in 46,XX females. Some individuals also develop cerebellar ataxia, peripheral neuropathy, or cognitive difficulties. Pierce et al. 2010⁵⁵ Pierce et al. 2010
Pierce SB et al. Mutations in the DBP-deficiency protein HSD17B4 cause ovarian dysgenesis, hearing loss, and ataxia of Perrault Syndrome. Am J Hum Genet, 2010 established HSD17B4 as a causative gene for Perrault syndrome in compound heterozygous sisters who presented with ovarian dysgenesis, sensorineural hearing loss, and progressive ataxia.

A 2025 study further demonstrated that HSD17B4 deficiency disrupts primary ciliogenesis — Bae et al. 2025⁶⁶ Bae et al. 2025
Bae JE et al. HSD17B4 deficiency causes dysregulation of primary cilia and is alleviated by acetyl-CoA. Nat Commun, 2025 showed that cells carrying the G16S patient variant had reduced cilium formation and length, and that acetyl-CoA supplementation (via acetate) rescued cilia function in patient-derived cells and in Hsd17b4-knockout mice.

The Evidence

DBP deficiency is classified as pathogenic by ClinVar with 17 pathogenic and 3 likely pathogenic submissions across 28 laboratories (VCV000007655, aggregate review: criteria provided, multiple submitters). The variant is associated with two phenotype entries: bifunctional peroxisomal enzyme deficiency (OMIM #261515) and Perrault syndrome 1 (OMIM

233400).

The A allele is extremely rare globally. In gnomAD exomes (v4) it is detected at a frequency of approximately 0.035% (490 out of 1,401,272 alleles) — consistent with a pathogenic variant maintained at low frequency by heterozygous carrier transmission. Crucially, no homozygous individuals have been identified in population databases, consistent with the severe phenotype of biallelic loss of function.

Lieber et al. 2014⁷⁷ Lieber et al. 2014
Lieber DS et al. Next generation sequencing with copy number variant detection expands the phenotypic spectrum of HSD17B4-deficiency. BMC Med Genet, 2014 documented an adult male with compound heterozygous HSD17B4 mutations (a missense plus a 12 kb exonic deletion) who presented with cerebellar ataxia, peripheral neuropathy, hearing loss, and infertility — expanding recognized presentations beyond neonatal disease.

Practical Actions

Because this is an autosomal recessive condition, one copy of p.Gly16Ser (heterozygous carrier) does not produce disease. The single functional HSD17B4 allele produces sufficient DBP activity for normal peroxisomal fatty acid metabolism. The clinical significance of carrier status is exclusively reproductive.

For individuals with biallelic HSD17B4 pathogenic variants, management is supportive: monitoring and management of hearing loss (hearing aids, cochlear implant evaluation), hormonal management of primary ovarian insufficiency in females (typically estrogen replacement therapy), fat-soluble vitamin supplementation (vitamins A, D, E, K) to compensate for potential impaired absorption secondary to peroxisomal dysfunction, and neurological monitoring for emerging ataxia or neuropathy. No enzyme replacement or gene therapy is currently approved; ataluren-mediated readthrough has been explored for nonsense variants.

Interactions

HSD17B4 encodes a bifunctional protein with two separate enzymatic activities. Different HSD17B4 variants can impair the hydratase domain, the dehydrogenase domain, or the C-terminal sterol carrier domain to different extents, resulting in three recognized subtypes of DBP deficiency (type I, II, III) with different residual activity profiles. p.Gly16Ser affects the dehydrogenase domain. In compound heterozygous individuals, the combination of Gly16Ser with a second variant in the hydratase or sterol-carrier domain may produce a more severe phenotype than two dehydrogenase-domain variants. The AGPS and AMACR genes function upstream in the peroxisomal lipid metabolism pathway; variants in these genes can produce overlapping phenotypes and should be considered in differential diagnosis when HSD17B4 testing is negative.