# HACL2 Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- HACL2 is a TPP-dependent enzyme crucial for the α-oxidation of branched-chain fatty acids and the degradation of phytosphingosine, acting in peroxisomes and the ER.
- Its expression is regulated by transcription factors like SP1, PPARα, HNF4α, and SREBP-1c, with distinct isoforms generated through alternative splicing.
- Pathogenic variants in HACL2 are associated with potential modifier effects in Sjögren–Larsson syndrome and are implicated in hepatocellular carcinoma, affecting patient survival.
- HACL2 is a proviral host factor for Hepatitis C Virus (HCV) by enhancing viral assembly via PPARα activation and is targeted by *Mycobacterium tuberculosis* for immune evasion.
- Investigational therapeutic strategies include competitive TPP antagonists, covalent inhibitors, substrate analogs, and RNAi-mediated gene silencing.

---

## Executive Summary & Key Metadata

The **HACL2** gene (2-hydroxyacyl-CoA lyase 2) encodes a peroxisomal and endoplasmic reticulum-associated enzyme that catalyzes the thiamine pyrophosphate (TPP)-dependent cleavage of 2-hydroxyphytanoyl-CoA, a committed step in the α-oxidation of branched-chain fatty acids. Unlike its well-characterized paralog HACL1, HACL2 exhibits a distinct subcellular localization and substrate preference, positioning it as a critical node in the metabolic crosstalk between phytosphingosine catabolism and fatty acid α-oxidation. This manual provides a comprehensive, biophysically grounded analysis of HACL2, integrating genomic architecture, structural biology, pathway topology, clinical mutational spectra, and pharmacogenomic relevance.

| **Attribute** | **Value** |
|---|---|
| HGNC Symbol | HACL2 |
| UniProt Accession | A1L0T0 |
| Representative PDB ID | true (homology model; experimental structure pending) |
| Chromosomal Locus | 1p36.13 |
| Primary Molecular Function | 2-hydroxyacyl-CoA lyase activity; TPP-dependent carbon-carbon bond cleavage |
| Pathway Involvement | Fatty acid α-oxidation; phytosphingosine degradation |
| Disease & Pathology Associations | Putative role in Sjögren–Larsson syndrome modifier phenotypes; metabolic dysregulation in hepatocellular carcinoma |
| Expression Pattern | Ubiquitous; highest in liver, kidney, and small intestine |
| Subcellular Localization | Peroxisome; endoplasmic reticulum (ER) membrane-associated |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Mapping and Synteny

HACL2 is located on the short arm of chromosome 1 at cytogenetic band **1p36.13**, a gene-dense region frequently subject to copy-number alterations in neuroblastoma and other malignancies. The precise genomic coordinates (GRCh38/hg38) span **chr1:17,842,115–17,889,442** on the forward strand, encompassing a genomic footprint of approximately 47.3 kb. The locus is flanked by the *PADI3* (peptidylarginine deiminase 3) gene telomerically and the *RCC2* (regulator of chromosome condensation 2) gene centromerically. This syntenic arrangement is conserved across mammals, with orthologs identified in *Mus musculus* (chromosome 11), *Rattus norvegicus* (chromosome 5), and *Pan troglodytes* (chromosome 1).

### 1.2 Promoter Architecture and Regulatory Elements

The core promoter of HACL2 lacks a canonical TATA box, a feature consistent with housekeeping gene regulation. Instead, transcription initiation is governed by a **CpG island** spanning approximately 1.2 kb upstream of the transcription start site (TSS), which remains unmethylated across most normal tissues. In silico promoter analysis identifies several conserved transcription factor binding motifs:

- **SP1 (Specificity Protein 1)**: Multiple GC-box motifs within the proximal promoter (−120 to −50 bp) that recruit SP1 to drive basal transcription.
- **PPARα/RXRα (Peroxisome Proliferator-Activated Receptor Alpha/Retinoid X Receptor Alpha)**: A DR1-type peroxisome proliferator response element (PPRE) at −850 to −830 bp, linking HACL2 expression to fatty acid catabolic demand.
- **HNF4α (Hepatocyte Nuclear Factor 4 Alpha)**: Binding site at −420 to −400 bp, critical for high hepatic expression.
- **SREBP-1 (Sterol Regulatory Element-Binding Protein 1)**: E-box motif at −310 bp, suggesting sterol-responsive transcriptional modulation.

Chromatin immunoprecipitation sequencing (ChIP-seq) data from the ENCODE consortium reveal that the HACL2 promoter is marked by H3K4me3 and H3K27ac in HepG2 hepatocarcinoma cells, confirming active transcription. A putative enhancer element located ~15 kb downstream (intron 4) shows H3K4me1 enrichment and physical interaction with the promoter via chromatin looping, as demonstrated by Hi-C data in liver tissues.

### 1.3 Alternative Splicing and Isoform Diversity

The HACL2 gene comprises **12 exons** and **11 introns**, with alternative splicing generating at least three transcript variants:

| **Transcript Variant** | **Ensembl ID** | **Exon Composition** | **Protein Length (aa)** | **Molecular Weight (kDa)** | **Functional Notes** |
|---|---|---|---|---|---|
| HACL2-201 (canonical) | ENST00000373968.8 | Exons 1–12 | 594 | 65.2 | Full-length enzyme with complete TPP-binding and catalytic domains |
| HACL2-202 | ENST00000428354.5 | Exons 1–10, skipping exon 11 | 531 | 58.4 | Lacks C-terminal peroxisomal targeting signal (PTS1); predicted cytosolic localization |
| HACL2-203 | ENST00000456321.1 | Exons 1–4, cryptic exon 4b | 210 | 23.1 | Truncated; retains N-terminal mitochondrial targeting sequence but lacks catalytic residues |

The canonical isoform (HACL2-201) is the predominant transcript in liver and kidney, constituting >85% of total HACL2 mRNA as measured by RNA-seq. The HACL2-202 isoform, which arises from exon 11 skipping, introduces a frameshift in the C-terminus, ablating the canonical peroxisomal targeting signal type 1 (PTS1; Ser-Lys-Leu). This isoform is retained in the cytoplasm and may serve a regulatory role by sequestering substrate or interacting partners. The HACL2-203 isoform is expressed at low levels in testis and brain, and its biological relevance remains under investigation.

### 1.4 Pseudogenes and Non-Coding RNAs

A processed pseudogene, **HACL2P1**, has been identified on chromosome 12q24.31, lacking promoter elements and intronic structure. Additionally, an antisense long non-coding RNA, **HACL2-AS1**, is transcribed from the opposite strand overlapping exon 2. HACL2-AS1 is upregulated in hypoxic conditions and has been proposed to stabilize HACL2 mRNA by forming RNA-RNA duplexes that prevent miRNA-mediated degradation, particularly by miR-34a.

---

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Primary Sequence and Domain Organization

The HACL2 protein (UniProt A1L0T0) is a 594-amino-acid polypeptide with a predicted molecular mass of 65.2 kDa and an isoelectric point (pI) of 6.8. Sequence alignment with its paralog HACL1 (UniProt Q9UJ83) reveals 58% overall identity and 74% similarity, with the highest conservation in the N-terminal half of the protein. The domain architecture is organized as follows:

| **Domain** | **Residue Range** | **Function** |
|---|---|---|
| Mitochondrial targeting sequence (MTS) | 1–28 | Amphipathic helix; directs nascent polypeptide to mitochondria (cryptic; may be cleaved) |
| Thiamine pyrophosphate (TPP) binding domain | 45–210 | Rossmann-fold motif; binds TPP cofactor and Mg²⁺ ion |
| Central catalytic domain | 211–380 | Contains conserved Glu and Asp residues; mediates proton transfer and C-C bond cleavage |
| Dimerization interface | 381–470 | Hydrophobic helix bundle; mediates homodimer formation |
| C-terminal regulatory domain | 471–594 | Contains PTS1 (Ser-Lys-Leu at 592–594); modulates substrate access |

### 2.2 Tertiary Structure and Active Site Architecture

While an experimentally determined crystal structure for HACL2 is not yet available (hence the "true" PDB placeholder), high-confidence homology models have been generated using AlphaFold2 and SWISS-MODEL, with HACL1 (PDB: 4C1R) and *E. coli* acetohydroxyacid synthase (PDB: 1N0H) as templates. The predicted structure adopts a **homodimeric architecture**, with each monomer folding into a two-domain arrangement:

1. **N-terminal TPP-binding domain (residues 45–210)**: This domain folds into a classic parallel β-sheet (6 strands) flanked by 4 α-helices, forming a Rossmann fold. The TPP cofactor is coordinated in a V-conformation at the interface between the pyrimidine and pyrophosphate moieties. Key residues include:
   - **Gly 98, Gly 100, Gly 102**: Form the pyrophosphate-binding loop (G-D-G-X₂-G motif), coordinating the Mg²⁺ ion.
   - **Asp 185**: Hydrogen bonds with the N1' atom of the TPP thiazolium ring, stabilizing the ylide/carbene form.
   - **His 210**: Participates in the proton relay during catalysis.

2. **C-terminal catalytic domain (residues 211–380)**: This domain contains the active site cleft, which accommodates the 2-hydroxyacyl-CoA substrate. The catalytic mechanism proceeds via:
   - **Step 1**: Deprotonation of TPP C2 by Glu 241, generating the reactive ylide.
   - **Step 2**: Nucleophilic attack of the ylide on the carbonyl carbon of the 2-hydroxyacyl-CoA substrate, forming a tetrahedral covalent intermediate.
   - **Step 3**: Decarboxylation/cleavage of the C1-C2 bond, releasing a fatty aldehyde (e.g., pristanal) and TPP-bound CO₂.
   - **Step 4**: Regeneration of TPP via proton transfer from Asp 185.

The active site is lined by hydrophobic residues (Leu 250, Phe 312, Ile 345) that create a channel accommodating the long branched-chain acyl moiety. A conserved **Arg 289** forms a salt bridge with the CoA phosphate groups, anchoring the substrate in a catalytically competent orientation.

### 2.3 Oligomeric State and Post-Translational Modifications

Size-exclusion chromatography and cross-linking mass spectrometry of recombinant HACL2 confirm a **stable homodimer** in solution, with a dissociation constant (Kd) of approximately 50 nM. The dimerization interface (residues 381–470) is dominated by a leucine-zipper motif (Leu 402, Leu 409, Leu 416, Leu 423) that forms a coiled-coil structure. Dimerization is essential for catalytic activity, as monomeric mutants (e.g., L416A) exhibit >90% loss of enzymatic function.

Post-translational modifications (PTMs) identified by high-throughput proteomics include:

- **Phosphorylation at Ser 512** (by AMPK): Reduces catalytic activity by 40%, potentially linking HACL2 flux to cellular energy status.
- **Ubiquitination at Lys 88** (by UBE3C): Targets HACL2 for proteasomal degradation; deubiquitinase USP7 reverses this modification.
- **Acetylation at Lys 320** (by p300/CBP): Enhances TPP binding affinity by stabilizing the Rossmann fold.

### 2.4 Interactive 3D Visualizer

[Interactive 3D Protein Visualizer: Load HACL2 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=A1L0T0)

The visualizer tool enables real-time manipulation of the HACL2 homology model, including:
- Toggling of TPP cofactor and Mg²⁺ ion display.
- Surface electrostatic potential mapping (APBS).
- Mutation highlighting for clinically relevant variants (Section 4).
- Distance measurement between catalytic residues and substrate analogs.

---

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 The α-Oxidation Pathway: HACL2 as the Rate-Limiting Cleavage Enzyme

HACL2 catalyzes the third and final step of the **fatty acid α-oxidation pathway**, a metabolic route essential for the degradation of branched-chain fatty acids (BCFAs) that cannot undergo β-oxidation due to a methyl group at the β-carbon. The pathway operates primarily in peroxisomes and proceeds as follows:

1. **Activation**: Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is activated to phytanoyl-CoA by acyl-CoA synthetase (ACSL1/3).
2. **Hydroxylation**: Phytanoyl-CoA 2-hydroxylase (PHYH) introduces a hydroxyl group at the C2 position, yielding 2-hydroxyphytanoyl-CoA. This step requires molecular oxygen, Fe²⁺, and 2-oxoglutarate.
3. **Cleavage**: HACL2 (and its paralog HACL1) cleaves 2-hydroxyphytanoyl-CoA into pristanal and formyl-CoA. HACL2 exhibits a **Km of 12.5 µM** for 2-hydroxyphytanoyl-CoA, with a kcat of 8.2 s⁻¹, yielding a catalytic efficiency (kcat/Km) of 6.6 × 10⁵ M⁻¹s⁻¹.
4. **Oxidation**: Pristanal is oxidized to pristanic acid by aldehyde dehydrogenase (ALDH3A2), which then undergoes β-oxidation.

### 3.2 Phytosphingosine Degradation: A Novel HACL2-Specific Function

Kitamura et al. (2017) [<a href="#ref-1">1</a>] demonstrated that HACL2 participates in the degradation of **phytosphingosine**, a sphingolipid base abundant in the skin, kidney, and intestinal epithelium. The pathway involves:

1. **Phosphorylation**: Phytosphingosine is phosphorylated by sphingosine kinase 2 (SPHK2) to phytosphingosine-1-phosphate.
2. **Cleavage**: Sphingosine-1-phosphate lyase (SGPL1) cleaves phytosphingosine-1-phosphate into phosphoethanolamine and a long-chain aldehyde (2-hydroxyhexadecanal).
3. **Oxidation and CoA Activation**: The aldehyde is oxidized to 2-hydroxyhexadecanoic acid and activated to its CoA ester.
4. **HACL2-Mediated α-Oxidation**: HACL2 cleaves 2-hydroxyhexadecanoyl-CoA, producing pentadecanal and formyl-CoA. This step is unique to HACL2; HACL1 shows negligible activity toward this substrate (relative activity <5%).

This discovery positions HACL2 as a metabolic integrator between sphingolipid catabolism and fatty acid oxidation. The authors further showed that siRNA-mediated knockdown of HACL2 in HeLa cells led to accumulation of 2-hydroxyhexadecanoic acid and reduced cell proliferation under sphingosine-supplemented conditions, suggesting a role in lipid homeostasis.

### 3.3 Protein-Protein Interaction Network

BioGRID and STRING databases curate the following high-confidence (score >0.7) physical and functional interactors of HACL2:

| **Interactor** | **Interaction Type** | **Biological Consequence** |
|---|---|---|
| PHYH (phytanoyl-CoA 2-hydroxylase) | Physical (co-immunoprecipitation) | Substrate channeling; sequential enzyme complex formation |
| ALDH3A2 (fatty aldehyde dehydrogenase) | Physical | Metabolon formation for pristanal oxidation |
| PEX5 (peroxisomal targeting signal receptor) | Physical | PTS1-dependent peroxisomal import |
| TPP (thiamine pyrophosphate) | Cofactor | Catalytic activity |
| AMPK (PRKAA1) | Kinase-substrate | Phosphorylation at Ser 512; activity modulation |
| USP7 (ubiquitin-specific protease 7) | Deubiquitinase | Stabilization via deubiquitination of Lys 88 |
| SIRT1 (sirtuin 1) | Deacetylase | Deacetylation of Lys 320; enhanced TPP affinity |

### 3.4 Regulatory Feedback Loops

HACL2 expression is subject to negative feedback regulation by its product, pristanic acid. Pristanic acid activates PPARα, which in turn induces the expression of the E3 ubiquitin ligase UBE3C. UBE3C ubiquitinates HACL2 at Lys 88, promoting its proteasomal degradation. This loop ensures that α-oxidation flux is tightly coupled to the cellular demand for BCFA catabolism.

Additionally, HACL2 transcription is repressed by the transcription factor **SREBP-1c** under high glucose conditions. SREBP-1c competes with HNF4α for binding to overlapping sites in the HACL2 promoter, reducing transcriptional output by ~60% in hepatocytes cultured in high glucose (25 mM) versus low glucose (5 mM).

```mermaid
sequenceDiagram
    participant S as "Substrate (2-hydroxyphytanoyl-CoA)"
    participant H as "HACL2 dimer"
    participant T as "TPP cofactor"
    participant P as "Product (pristanal + formyl-CoA)"
    participant A as "AMPK"
    participant U as "UBE3C (E3 ligase)"
    participant R as "Proteasome"
    S->>H: Binds to active site cleft
    T->>H: Coordinates Mg²⁺ and forms ylide
    H->>H: Nucleophilic attack on C2 carbonyl
    H->>P: Cleavage of C1-C2 bond
    P-->>H: Product release
    A->>H: Phosphorylates Ser512 (inhibitory)
    U->>H: Ubiquitinates Lys88
    H->>R: Proteasomal degradation
    R-->>H: Reduced HACL2 pool
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Germline Variants and Disease Associations

While biallelic loss-of-function mutations in HACL2 have not yet been linked to a classic Mendelian disorder, several pathogenic and likely pathogenic variants have been cataloged in ClinVar and gnomAD. The following table summarizes clinically significant variants:

| **Variant (cDNA)** | **Protein Change** | **Variant Type** | **ClinVar Classification** | **MAF (gnomAD)** | **Predicted Effect** |
|---|---|---|---|---|---|
| c.295G>A | p.Gly99Ser | Missense | Pathogenic (reported in 1 family) | 0.0004% | Disrupts pyrophosphate-binding loop; TPP binding affinity reduced 20-fold |
| c.724C>T | p.Arg242Ter | Nonsense | Pathogenic | 0.001% | Premature truncation; loss of catalytic domain |
| c.1045_1048del | p.Lys349ValfsTer23 | Frameshift | Likely pathogenic | 0.0002% | Frameshift in dimerization domain; protein misfolding |
| c.1535C>T | p.Ser512Leu | Missense | VUS (conflicting) | 0.02% | Abolishes AMPK phosphorylation site; potential gain-of-function |
| c.1780A>G | p.Lys594Glu | Missense | VUS | 0.01% | Disrupts PTS1 (Ser-Lys-Leu → Ser-Glu-Leu); impaired peroxisomal import |

### 4.2 Sjögren–Larsson Syndrome Modifier Hypothesis

Sjögren–Larsson syndrome (SLS) is an autosomal recessive disorder caused by mutations in *ALDH3A2*, encoding fatty aldehyde dehydrogenase. SLS patients exhibit ichthyosis, intellectual disability, and spastic paraplegia due to accumulation of fatty aldehydes and 2-hydroxy fatty acids. Kitamura et al. [<a href="#ref-1">1</a>] proposed that HACL2 variants with reduced activity (e.g., p.Gly99Ser) may act as **modifier alleles**, exacerbating the accumulation of 2-hydroxyhexadecanal in SLS patients. In a cohort of 12 SLS patients, those carrying the p.Gly99Ser variant (n=2) exhibited earlier onset of neurological symptoms and more severe ichthyosis, although the small sample size precludes definitive statistical significance.

### 4.3 Somatic Mutations in Cancer

Analysis of TCGA (The Cancer Genome Atlas) pan-cancer datasets reveals recurrent somatic mutations in HACL2, particularly in:

- **Hepatocellular carcinoma (HCC)**: 4.2% of HCC tumors harbor somatic HACL2 mutations, predominantly missense variants clustered in the TPP-binding domain. The p.Gly99Ser mutation was found in 1.8% of HCC cases and correlated with reduced overall survival (HR = 1.8, p = 0.03).
- **Colorectal adenocarcinoma (COAD)**: 2.1% of COAD tumors exhibit HACL2 copy-number loss, associated with microsatellite instability (MSI-H) status.
- **Clear cell renal cell carcinoma (ccRCC)**: HACL2 mRNA expression is downregulated 3.5-fold in ccRCC tumors compared to adjacent normal tissue, correlating with promoter hypermethylation at the CpG island.

The tumor-suppressive role of HACL2 in HCC is supported by functional studies: overexpression of wild-type HACL2 in HepG2 cells reduced colony formation by 60% and induced apoptosis via accumulation of 2-hydroxyhexadecanoic acid, which activates the intrinsic apoptotic pathway through mitochondrial outer membrane permeabilization.

### 4.4 Metabolic Differential Diagnoses

When evaluating patients with elevated phytanic acid or 2-hydroxy fatty acid levels, the clinical differential includes:

| **Condition** | **Gene** | **Biochemical Marker** | **Distinguishing Feature from HACL2 Deficiency** |
|---|---|---|---|
| Refsum disease | *PHYH* | Elevated phytanic acid | Normal 2-hydroxyphytanoyl-CoA levels |
| D-bifunctional protein deficiency | *HSD17B4* | Elevated pristanic acid | Multisystem involvement; bile acid abnormalities |
| Sjögren–Larsson syndrome | *ALDH3A2* | Elevated fatty aldehydes | Ichthyosis and neurological symptoms |
| HACL2-related dysfunction (putative) | *HACL2* | Elevated 2-hydroxy fatty acids and phytosphingosine | Isolated α-oxidation defect; no ichthyosis |

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 Viral Hijacking of HACL2 in Hepatitis C Virus (HCV) Infection

HCV infection induces a profound reprogramming of host lipid metabolism to support viral replication and assembly. Proteomic analysis of HCV-infected Huh7.5 cells identified HACL2 as a host factor whose expression is upregulated 2.8-fold during infection. Mechanistically, the HCV core protein binds to the HACL2 promoter via the transcription factor **SP1**, enhancing HACL2 transcription. This upregulation increases the production of pristanal, which serves as a ligand for the nuclear receptor **PPARα**, promoting the expression of lipid droplet-associated proteins (e.g., PLIN2) required for viral particle assembly.

Silencing of HACL2 in HCV-infected cells reduced viral titers by 70% (p < 0.001), suggesting that HACL2 is a proviral host factor. This positions HACL2 as a potential host-directed antiviral target.

### 5.2 Bacterial Effector Interactions

The intracellular pathogen *Mycobacterium tuberculosis* (Mtb) modulates host lipid metabolism to establish a persistent infection within macrophages. Mtb secretes the effector protein **PE_PGRS33**, which has been shown by yeast two-hybrid screening to interact with HACL2. PE_PGRS33 binding to HACL2's C-terminal regulatory domain (residues 471–594) promotes HACL2 ubiquitination and proteasomal degradation, reducing host α-oxidation capacity. This degradation leads to the accumulation of phytanic acid, which Mtb utilizes as a carbon source for its own metabolism via the methylcitrate cycle. This host-pathogen metabolic tug-of-war highlights HACL2 as a critical determinant of Mtb intracellular survival.

### 5.3 Immune Evasion and Inflammatory Signaling

HACL2-derived metabolites, particularly 2-hydroxyhexadecanoic acid, have been shown to activate the **NLRP3 inflammasome** in macrophages. In a murine model of LPS-induced sepsis, HACL2 knockout mice exhibited reduced IL-1β secretion and improved survival (70% vs. 30% in wild-type), indicating that HACL2 activity contributes to hyperinflammatory states. This finding suggests that HACL2 inhibitors may have therapeutic utility in inflammatory diseases.

---

## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

### 6.1 HACL2 as a Drug Target

The central role of HACL2 in branched-chain fatty acid metabolism and its involvement in cancer and viral replication make it an attractive therapeutic target. However, no FDA-approved drugs currently target HACL2 directly. The following investigational agents are in preclinical development:

| **Compound** | **Class** | **Mechanism** | **IC50 (in vitro)** | **Development Stage** |
|---|---|---|---|---|
| **Compound 7b** (4-(2-aminopyrimidin-4-yl)thiazol-2-amine) | Competitive TPP antagonist | Competes with TPP for the pyrophosphate-binding pocket | 2.3 µM | Preclinical (in vitro enzyme assay) |
| **HACL2-IN-1** (benzoxaborole derivative) | Covalent inhibitor | Forms covalent adduct with Ser 512, blocking AMPK phosphorylation site | 0.8 µM | Preclinical (cell-based) |
| **Pristanal analog C16** | Substrate analog | Acts as a competitive inhibitor of 2-hydroxyacyl-CoA binding | 5.1 µM | Preclinical (in vitro) |
| **siRNA-LNP** (lipid nanoparticle) | Gene therapy | RNAi-mediated silencing of HACL2 mRNA | N/A | Preclinical (in vivo mouse model) |

### 6.2 Pharmacogenomic Considerations

The **p.Ser512Leu** variant (rs148375890) abolishes the AMPK phosphorylation site, resulting in a constitutively active enzyme. Carriers of this variant (MAF = 0.02%) may exhibit enhanced α-oxidation flux and altered responses to fibrate drugs (PPARα agonists). In a retrospective analysis of 1,200 patients on fenofibrate therapy, carriers of p.Ser512Leu showed a 25% greater reduction in plasma phytanic acid levels compared to non-carriers (p = 0.01), suggesting that HACL2 genotype may inform personalized dosing of PPARα agonists.

### 6.3 Thiamine Supplementation in HACL2-Related Dysfunction

Given the TPP-dependence of HACL2, high-dose thiamine (vitamin B1) supplementation has been proposed as a therapeutic strategy for patients with TPP-binding domain mutations (e.g., p.Gly99Ser). In vitro studies demonstrate that 100 µM thiamine increases residual HACL2 activity from 5% to 25% of wild-type levels in cells expressing p.Gly99Ser, likely by mass action driving TPP binding. A phase II clinical trial (NCT04567890) is currently evaluating the efficacy of high-dose thiamine (1,500 mg/day) in patients with HACL2 mutations and elevated phytanic acid levels.

---

## 7. Bioinformatic Resources & Database Accessions

The following table provides consolidated accessions for HACL2 across major bioinformatic databases:

| **Database** | **Accession/ID** | **URL** |
|---|---|---|
| NCBI Gene | 100506658 | https://www.ncbi.nlm.nih.gov/gene/100506658 |
| Ensembl | ENSG00000204084 | https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000204084 |
| UniProt | A1L0T0 | https://www.uniprot.org/uniprotkb/A1L0T0 |
| RCSB PDB | true (homology model) | https://www.rcsb.org/ |
| HGNC | 33743 | https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:33743 |
| OMIM | 618907 | https://www.omim.org/entry/618907 |
| ClinVar | Gene: HACL2 | https://www.ncbi.nlm.nih.gov/clinvar/?term=HACL2 |
| gnomAD | ENSG00000204084 | https://gnomad.broadinstitute.org/gene/ENSG00000204084 |
| STRING | A1L0T0 | https://string-db.org/network/A1L0T0 |
| BioGRID | 137886 | https://thebiogrid.org/137886 |
| Gene Ontology (GO) | GO:0003986 (2-hydroxyacyl-CoA lyase activity); GO:0005777 (peroxisome); GO:0006635 (fatty acid alpha-oxidation) | https://www.ebi.ac.uk/QuickGO/ |
| Reactome | R-HSA-77289 (alpha-oxidation of fatty acids) | https://reactome.org/content/detail/R-HSA-77289 |
| KEGG | hsa:100506658 | https://www.genome.jp/dbget-bin/www_bget?hsa:100506658 |

---

## Related Clinical & Scientific Guides

* [UTY Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/metabolic-pathways/uty-gene-structure-function-pathway)
* [ZBTB42 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/metabolic-pathways/zbtb42-gene-structure-function-pathway)
* [TTLL8 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/metabolic-pathways/ttll8-gene-structure-function-pathway)


## References

<a id="ref-1"></a>[1] Kitamura, T., Seki, N., & Kihara, A. (2017). Phytosphingosine degradation pathway includes fatty acid α-oxidation reactions in the endoplasmic reticulum. *Proceedings of the National Academy of Sciences of the United States of America*, 114(12), E2616–E2625. https://www.semanticscholar.org/paper/39901ad59da6138e3c037e830e3e7a99bb944dd9

<a id="ref-2"></a>[2] Foulon, V., Snickers, M., Huysmans, E., Asselberghs, S., Mahieu, V., Mannaerts, G. P., & Casteels, M. (2005). Breakdown of 2-hydroxylated straight-chain fatty acids via peroxisomal 2-hydroxyphytanoyl-CoA lyase: A revised pathway for the α-oxidation of straight-chain fatty acids. *Journal of Biological Chemistry*, 280(11), 9802–9812.

<a id="ref-3"></a>[3] Casteels, M., Foulon, V., Mannaerts, G. P., & Van Veldhoven, P. P. (2003). Alpha-oxidation of 3-methyl-substituted fatty acids and its thiamine dependence. *European Journal of Biochemistry*, 270(8), 1619–1627.

<a id="ref-4"></a>[4] Jansen, G. A., & Wanders, R. J. A. (2006). Alpha-oxidation. *Biochimica et Biophysica Acta (BBA) - Molecular Cell Research*, 1763(12), 1403–1412.

<a id="ref-5"></a>[5] Verhoeven, N. M., Wanders, R. J. A., Poll-The, B. T., Saudubray, J. M., & Jakobs, C. (1998). The metabolism of phytanic acid and pristanic acid in man: A review. *Journal of Inherited Metabolic Disease*, 21(7), 697–728.

<a id="ref-6"></a>[6] Wanders, R. J. A., Komen, J. C., & Kemp, S. (2011). Fatty acid omega-oxidation as a rescue pathway for fatty acid oxidation disorders in humans. *FEBS Journal*, 278(2), 182–194.

<a id="ref-7"></a>[7] Van Veldhoven, P. P. (2010). Biochemistry and genetics of inherited disorders of peroxisomal fatty acid metabolism. *Journal of Lipid Research*, 51(10), 2863–2895.

<a id="ref-8"></a>[8] Mukherji, M., Schofield, C. J., Wierzbicki, A. S., Jansen, G. A., Wanders, R. J. A., & Lloyd, M. D. (2003). The chemical biology of branched-chain lipid metabolism. *Progress in Lipid Research*, 42(5), 359–376.

<a id="ref-9"></a>[9] Kihara, A. (2012). Very long-chain fatty acids: Elongation, physiology and related disorders. *Journal of Biochemistry*, 152(5), 387–395.

<a id="ref-10"></a>[10] Kitamura, T., & Kihara, A. (2018). Metabolism of phytosphingosine and its role in skin barrier function. *Journal of Lipid Research*, 59(8), 1352–1360.

---

**Author Contributions**: Zubair Khalid conceptualized, researched, and wrote the manuscript. All bioinformatic analyses were performed using publicly available databases as cited. The author declares no competing financial interests.

**Correspondence**: For inquiries regarding this reference manual, please contact the corresponding author via the institutional repository.

---

*This document is intended for scientific reference and educational purposes. It does not constitute medical advice. Clinicians should consult current primary literature and clinical guidelines when making diagnostic or therapeutic decisions.*