Biliary, Renal, Neurologic, and Skeletal Syndrome (BRENS): A Comprehensive Disease Characteristics Report

Disease: Biliary, Renal, Neurologic, and Skeletal Syndrome (BRENS) MONDO ID: MONDO:0859191 · OMIM: #619534 · Category: Mendelian (autosomal recessive) Causal gene: IFT56 (formerly TTC26; synonym DYF13), HGNC:21882, chr7q34

Summary

Biliary, Renal, Neurologic, and Skeletal Syndrome (BRENS) is an ultra-rare, autosomal-recessive, multisystem ciliopathy caused by biallelic loss-of-function variants in IFT56 (formerly TTC26), a tetratricopeptide-repeat (TPR) protein that is a core component of the intraflagellar transport (IFT)-B complex. IFT56/TTC26 is required for the anterograde transport of a selective set of cargoes along the ciliary axoneme; its loss compromises the function of both primary (signaling) cilia and motile cilia. The disorder was first delineated in 2020 by Shaheen et al., who described seven individuals from seven families with a syndromic ciliopathy featuring severe neonatal cholestasis, and identified three different homozygous TTC26 variants that fully segregated with the phenotype [PMID: 31595528].

Mechanistically, IFT56 loss dysregulates ciliary Sonic Hedgehog (Shh)–GLI signaling during embryonic development. Work in the Ttc26 hop-sterile mouse pinpointed the defect to failure of GLI dissociation from its negative regulator SUFU at the ciliary tip, downstream of normal GLI accumulation [PMID: 25340710]. A more recent study adds that TTC26 is required for ciliary localization of the methyltransferase PRMT7, which methylates GLI2 to maintain Shh–GLI2 signaling [PMID: 42178579]. Because Hedgehog signaling patterns the limb, neural tube, skeleton, and multiple viscera, and because cilia are essential in cholangiocytes, renal tubular cells, ependyma, and photoreceptors, the consequence is a congenital, variably expressed multi-organ malformation syndrome.

Clinically, BRENS produces neonatal cholestasis / fibrocystic biliary disease (potentially lethal or requiring liver transplant), renal cysts/dysplasia, brain malformations (hydrocephalus, aqueductal stenosis), pituitary stalk interruption syndrome (PSIS) with hypopituitarism and diabetes insipidus, polydactyly/syndactyly and other skeletal anomalies, plus cardiac, ocular (optic atrophy), and craniofacial (cleft lip/palate) features. Expressivity is variable — one Chinese patient had renal, neurologic, and skeletal features without biliary involvement [PMID: 39514123]. Diagnosis is molecular (WES/WGS identifying biallelic IFT56/TTC26 variants); there is no curative therapy, and management is supportive and multidisciplinary. Prognosis is guarded, dominated by the severity of the neonatal hepatic phenotype. Fewer than ~25 patients have been published worldwide as of 2025.

Key Findings

1. BRENS is an autosomal-recessive ciliopathy caused by biallelic IFT56/TTC26 variants

The disease-gene relationship is firmly established. In the foundational cohort, whole-exome sequencing of seven individuals from seven families with syndromic ciliopathy features — including severe neonatal cholestasis — revealed "three different homozygous variants in Tetratricopeptide Repeat Domain 26 (TTC26) that fully segregated with the phenotype" after positional mapping to a single locus on chromosome 7q [PMID: 31595528]. Multiple independent families have since confirmed the gene: a homozygous splice-site variant c.4-1G>C [PMID: 34177428]; the recurrent homozygous missense c.695A>G (p.Asn232Ser) [PMID: 32617964]; a homozygous intronic c.1006-5T>C [PMID: 38135897]; and compound-heterozygous alleles in a Chinese boy [PMID: 39514123]. Inheritance is autosomal recessive, and several families are consanguineous. The disease carries OMIM #619534; the gene carries OMIM 617453.

"Whole-exome sequencing revealed three different homozygous variants in Tetratricopeptide Repeat Domain 26 (TTC26) that fully segregated with the phenotype." — [PMID: 31595528]

"We describe seven individuals from seven families with syndromic ciliopathy clinical features, including severe neonatal cholestasis (lethal in one and necessitating liver transplant in two)." — [PMID: 31595528]

2. TTC26/IFT56 is a conserved IFT-B core component required for cargo-selective transport

TTC26 (also called DYF13 in Chlamydomonas and C. elegans) is an integral component of the intraflagellar transport complex B. Ishikawa et al. showed that "TTC26/DYF13 is an IFT complex B protein in mammalian cells and Chlamydomonas reinhardtii" [PMID: 24596149]. Loss of TTC26 produces short cilia with abnormal motility while leaving IFT-particle assembly and speed largely normal; critically, "a particular set of proteins involved in motility was specifically depleted in the dyf13 mutant" — evidence that TTC26 mediates cargo-selective transport rather than bulk IFT [PMID: 24596149]. Structural/biochemical studies place TTC26/IFT56 in the IFT-B core subcomplex [PMID: 26980730] and show it dimerizes with and directly binds IFT46 [PMID: 27927754, 25340710]. Xin et al. demonstrated that "IFT56 regulates vertebrate developmental patterning by maintaining IFTB complex integrity and ciliary microtubule architecture" [PMID: 28264835].

3. Mechanism: TTC26 loss dysregulates Sonic Hedgehog signaling via impaired GLI–SUFU dissociation and GLI2 methylation

The core pathogenic mechanism is disruption of ciliary Hedgehog signal transduction. In the spontaneous Ttc26 hop-sterile mouse, the group found that "the hop mutation is located in the Ttc26 gene and impairs Hedgehog (Hh) signaling" [PMID: 25340710]. Importantly, the defect is not a failure of cilium formation — cilia number and length are preserved — but a downstream signaling failure: "hop did not interfere with Hh-induced accumulation of Gli at the tip of the primary cilium, but rather with the subsequent dissociation of Gli from its negative regulator, Sufu" [PMID: 25340710]. Patient-derived cells show cilia of variable length with dysregulated Sonic Hedgehog signaling and abnormal IFT-B staining [PMID: 31595528]. A recent study adds a molecular refinement: "TTC26 is required for the localization of protein arginine methyltransferase 7 (PRMT7) to the primary cilium, enabling methylation of GLI2" — with FLNB, to maintain Shh–GLI2 signaling [PMID: 42178579].

4. Phenotypic spectrum spans biliary, renal, neurologic, skeletal, endocrine, cardiac, ocular, and craniofacial systems

The syndrome is defined by multi-organ involvement with variable expressivity. Core features from the founding cohort include severe neonatal cholestasis (lethal in one, requiring liver transplant in two) and fibrocystic liver/biliary disease [PMID: 31595528]. Pituitary involvement is a recurrent — and now recognized as characteristic — feature: four patients with homozygous p.Asn232Ser had pituitary stalk interruption syndrome (PSIS), delineated "as a novel clinical feature of this disorder" [PMID: 32617964]. Across cases, reported manifestations include "cholestasis, cystic dilatation of intrahepatic biliary ducts, diabetes insipidus, dysmorphic facial features, optic atrophy, pituitary hypoplasia, hydrocephalus, aqueductal stenosis, hyperextensible knee joints, bilateral knee dislocation, polydactyly, and syndactyly" [PMID: 34177428], plus cleft lip/palate with probable hearing loss [PMID: 38135897]. Demonstrating the breadth of expressivity, a Chinese boy showed renal, neurologic, and skeletal features but no biliary disease — "the first description of BRENS syndrome without biliary involvement" [PMID: 39514123].

5. Cross-species model organisms robustly recapitulate the ciliopathy

Multiple model systems reproduce BRENS-relevant phenotypes and have illuminated mechanism. Mouse: the spontaneous Ttc26 hop-sterile mutant is "characterized by a hopping gait, polydactyly, hydrocephalus, and male sterility" with inner dynein-arm deficiency, absent sperm flagella, and impaired Hedgehog signaling [PMID: 25340710]; a 2025 study found that Ift56 loss-of-function "has dramatic phenotypic differences depending on the genetic background in mice", establishing genetic-background modifiers [PMID: 41352382]. Zebrafish: morpholino knockdown of ttc26 "caused ciliary defects in the pronephric kidney at 27 h postfertilization and distension/dilation of pronephros" [PMID: 22718903], plus photoreceptor outer-segment defects [PMID: 36533556]. Chlamydomonas: dyf13/TTC26 mutation gives short flagella with abnormal motility and selective loss of motility proteins [PMID: 24596149]. A double Flnb;Ttc26 heterozygous mouse models adolescent idiopathic scoliosis via Shh–GLI2 [PMID: 42178579].

6. IFT56/TTC26 is LoF-tolerant in heterozygotes; BRENS is ultra-rare

gnomAD v4 constraint metrics for IFT56/TTC26 (ENSG00000105948) indicate that heterozygous loss-of-function is tolerated — pLI ≈ 3×10⁻¹⁰, observed/expected LoF (o/e) = 0.61 (90% CI 0.49–0.77; LOEUF 0.77), with 52 observed vs 85 expected pLoF variants. This tolerance is exactly what is expected for an autosomal-recessive disease gene in which carriers are unaffected. Summing 133 high-confidence pLoF allele frequencies yields a cumulative pLoF allele frequency of ≈2.36×10⁻⁴, implying a carrier frequency of roughly 1 in ~2,100 for truncating alleles alone, and a predicted homozygous/compound-het birth frequency from pLoF alleles of ≈5.6×10⁻⁸ (~1 in 18 million). Because most reported disease alleles are splice-region/missense (e.g., the recurrent c.695A>G, p.Asn232Ser) rather than canonical LoF, the true prevalence is higher than the pLoF-only estimate but remains ultra-rare (<25 published patients worldwide as of 2025).

7. Diagnosis is genetic; management is supportive with guarded prognosis

BRENS diagnosis is established by molecular genetic testing — whole-exome or whole-genome sequencing identifying biallelic IFT56/TTC26 variants (the original cohort combined positional mapping with WES [PMID: 31595528]; a later family used WGS with Sanger confirmation [PMID: 34177428]). Supportive workup findings include neonatal conjugated hyperbilirubinemia with elevated liver enzymes; liver biopsy showing ductal plate malformation/biliary fibrosis; brain MRI showing pituitary stalk interruption (thin/absent stalk, ectopic posterior pituitary, anterior pituitary hypoplasia) and hydrocephalus/aqueductal stenosis; renal imaging showing cysts/dysplasia; and an endocrine panel revealing hypopituitarism and diabetes insipidus. No disease-specific or curative therapy exists. Management is supportive and organ-directed: ursodeoxycholic acid and nutritional support for cholestasis; liver transplantation for end-stage liver disease (2 of 7 original patients required transplant, 1 died — "severe neonatal cholestasis (lethal in one and necessitating liver transplant in two)" [PMID: 31595528]); multi-hormone replacement for pituitary insufficiency, where "hormonal replacement therapy with hydrocortisone, levothyroxine, and growth hormone led to clinical stabilization" [PMID: 42460215]; ventriculoperitoneal shunting for hydrocephalus; and surgical correction of polydactyly and clefts. Prognosis is guarded; severe neonatal cholestasis can be lethal.

8. Identifiers and IFT56 protein architecture

Disease identifiers: MONDO:0859191 ("biliary, renal, neurologic, and skeletal syndrome"); OMIM #619534; MedGen C1794200; UMLS C5561990; MalaCards entry present. No dedicated Orphanet ORPHA code or specific ICD-10/ICD-11/MeSH term is currently mapped (classified broadly under ciliopathy/congenital malformation syndromes). Gene: IFT56 (formerly TTC26; synonym DYF13), HGNC:21882, OMIM 617453, Ensembl ENSG00000105948, chr7q34. Protein: UniProt A0AVF1, "Intraflagellar transport protein 56," 554 aa, containing four tetratricopeptide-repeat (TPR) motifs (aa 57–90, 92–125, 151–184, 468–501). The recurrent founder missense p.Asn232Ser lies in the inter-repeat region; splice-site variants predominate among disease alleles.

9. Anatomical, cellular, and subcellular map

Documented across cases [PMIDs 31595528, 34177428, 32617964, 38135897, 39514123], with ontology suggestions:

Subcellular localization is confirmed: "We localized Ttc26 to the transition zone of photoreceptor and to the transition zone of cilia in cultured murine inner medullary collecting duct 3 (mIMCD3) renal cells" [PMID: 22718903]. Tissue expression supports hepatobiliary primacy: "strong expression of Ttc26 in the embryonic mouse liver in a pattern consistent with its proposed role in the normal development of the intrahepatic biliary system" [PMID: 31595528].

10. Natural history and variant landscape

BRENS is congenital/neonatal in onset (cholestasis and malformations present at or near birth), chronic and lifelong, without spontaneous remission. The hepatic component is often progressive (cholestasis → biliary fibrosis → end-stage liver disease/transplant), whereas malformations (polydactyly, hydrocephalus, PSIS) are static-congenital and endocrine deficiency is stable but permanent [PMID: 31595528, 32617964]. Critical intervention windows: the embryonic period fixes malformations (irreversible), and the neonatal period is critical for life-saving hormone replacement and cholestasis/transplant management. The variant landscape comprises splice-region and missense loss-of-function changes (c.4-1G>C, c.695A>G p.Asn232Ser, c.1006-5T>C, c.1069+5G>A, c.511A>G, c.1099T>C, plus three homozygous founding-cohort variants); ClinVar lists 200+ submissions, predominantly population VUS/benign, consistent with a recessive, LoF-tolerant gene. Etiology is exclusively genetic (autosomal recessive); no environmental, infectious, epigenetic, or chromosomal mechanism is implicated.

Section-by-Section Report

1. Disease Information

BRENS is an ultra-rare autosomal-recessive multisystem ciliopathy affecting biliary, renal, neurologic, and skeletal systems (plus endocrine, cardiac, ocular, and craniofacial). Key identifiers: MONDO:0859191, OMIM #619534, MedGen C1794200, UMLS C5561990. No dedicated Orphanet/ICD/MeSH code is currently mapped. Synonyms: "TTC26 ciliopathy," "biliary ciliopathy (TTC26-related)." Information is derived from aggregated disease-level resources and published individual case reports (not EHR cohorts) — fewer than ~25 patients worldwide.

2. Etiology

The primary and sole established cause is genetic: biallelic loss-of-function variants in IFT56/TTC26. No environmental, infectious, lifestyle, or toxic contributing factors are implicated. Genetic risk requires two pathogenic alleles (autosomal recessive); consanguinity is a strong risk factor (several reported families are consanguineous, and homozygous founder alleles such as p.Asn232Ser recur). No protective variants or gene–environment interactions are described. Modifier effects are documented at the model-organism level: genetic background dramatically alters phenotype severity in Ift56/Ttc26 mutant mice [PMID: 41352382], and FLNB acts as a genetic modifier of the Shh–GLI2 axis [PMID: 42178579].

3. Phenotypes

See Finding 4 table. Onset is neonatal/congenital. Severity is variable (from lethal neonatal cholestasis to milder biliary-sparing presentations). Progression: hepatic disease progressive; malformations static; endocrine deficits permanent. Quality-of-life impact is severe where hypopituitarism, hydrocephalus, and liver disease coexist, requiring lifelong hormone replacement and organ-directed care. Suggested HPO terms: Neonatal cholestasis (HP:0011967), Hepatic fibrosis (HP:0001395), Polydactyly (HP:0100259), Hydrocephalus (HP:0000238), Aqueductal stenosis (HP:0002410), Anterior pituitary hypoplasia (HP:0010626), Diabetes insipidus (HP:0000873), Optic atrophy (HP:0000648), Renal cyst (HP:0000107), Cleft lip/palate (HP:0000175), Congenital heart defect (HP:0001627), Joint hyperlaxity (HP:0001388).

4. Genetic/Molecular Information

Causal gene: IFT56/TTC26 (HGNC:21882, OMIM 617453). Variant types: predominantly splice-site and missense (loss-of-function). Classification per ACMG/AMP: reported disease alleles are pathogenic/likely pathogenic; the gene shows abundant benign/VUS population variation. Allele frequencies in gnomAD are consistent with an ultra-rare recessive disorder (see Finding 6). Origin is germline. Functional consequence is loss of function (disrupted IFT-B integrity and cargo transport). Modifier genes: FLNB and genetic background (from models). No epigenetic or chromosomal mechanism is implicated.

5. Environmental Information

Not applicable — BRENS is a purely Mendelian disorder. No environmental, lifestyle, or infectious agents contribute.

6. Mechanism / Pathophysiology

Ordered causal chain: 1. Biallelic loss-of-function variants in IFT56/TTC26 → loss of functional IFT56 protein. 2. Loss of IFT56 → destabilization of the IFT-B core subcomplex and impaired ciliary microtubule architecture [PMID: 28264835] → cargo-selective failure of anterograde intraflagellar transport (motility-related and signaling cargoes) [PMID: 24596149]. 3. Impaired IFT → dysfunctional primary (signaling) and motile cilia across tissues (short/variable-length cilia; abnormal motility) [PMID: 31595528, 24596149]. 4a. In primary cilia → failure of GLI dissociation from SUFU at the ciliary tip (inferred to also involve loss of ciliary PRMT7 → reduced GLI2 methylation) → dysregulated Sonic Hedgehog–GLI signaling [PMID: 25340710, 42178579]. 4b. In motile cilia → inner dynein-arm deficiency and impaired ciliary/flagellar motility → ependymal/CSF-flow and reproductive defects (demonstrated in mouse) [PMID: 25340710]. 5. Dysregulated Shh–GLI patterning during embryogenesis → abnormal development of limb (polydactyly), neural tube/brain (hydrocephalus, aqueductal stenosis), pituitary (PSIS), intrahepatic biliary tree (ductal plate malformation/cholestasis), kidney (cysts), heart, eye, and craniofacial structures. 6. Postnatally → progressive biliary fibrosis and end-stage liver disease; permanent hypopituitarism and diabetes insipidus; static malformations → the BRENS clinical phenotype.

Molecular pathways: Hedgehog/GLI (central). Cellular processes: ciliogenesis, intraflagellar transport, developmental patterning. Protein dysfunction: loss of function of an IFT-B TPR scaffold destabilizing the complex. Suggested GO terms: intraciliary transport (GO:0042073), smoothened signaling pathway (GO:0007224), cilium assembly (GO:0060271), determination of left/right symmetry (GO:0007368). Suggested CL terms: cholangiocyte (CL:0002326), ependymal cell (CL:0000065), photoreceptor cell (CL:0000210), kidney epithelial cell (CL:1000454).

7. Anatomical Structures Affected

See Finding 9. Primary organs: liver/intrahepatic bile ducts, kidney, brain (ventricular system), pituitary. Secondary/associated: heart, eyes, skeleton/limbs, craniofacial structures. Involvement is generally bilateral. Subcellular: primary and motile cilia (axoneme, transition zone, ciliary tip), IFT-B particle.

8. Temporal Development

Onset congenital/neonatal; course chronic-lifelong. Hepatic disease progressive; malformations static; endocrine deficits permanent. No remission. Critical windows: embryonic (malformation fixation, irreversible) and neonatal (life-saving hormone replacement, cholestasis/transplant management).

9. Inheritance and Population

Autosomal recessive. Ultra-rare (<25 published patients; estimated carrier frequency ~1/2,000 for truncating alleles). Penetrance appears complete for biallelic pathogenic genotypes, with variable expressivity (biliary-sparing cases exist [PMID: 39514123]). Consanguinity and founder alleles (p.Asn232Ser) contribute. No confirmed sex bias in the syndrome itself (male sterility is seen in the mouse model). No genetic anticipation (not a repeat-expansion disorder).

10. Diagnostics

Molecular diagnosis by WES/WGS (biallelic IFT56/TTC26 variants). Supportive: conjugated hyperbilirubinemia, elevated liver enzymes; liver biopsy (ductal plate malformation/fibrosis); brain MRI (PSIS triad, hydrocephalus/aqueductal stenosis); renal ultrasound (cysts/dysplasia); endocrine panel (hypopituitarism, diabetes insipidus). Differential diagnosis: other syndromic ciliopathies (Meckel, Joubert, Bardet-Biedl, nephronophthisis-related; TTC12/TTC21B multisystem ciliopathies [PMID: 36273201]), Alagille syndrome, and other causes of neonatal cholestasis with malformations. Screening: cascade carrier testing in families; prenatal/preimplantation testing where the familial variant is known.

11. Outcome/Prognosis

Guarded. Neonatal cholestasis can be lethal; liver transplantation may be required (2/7 transplanted, 1 death in the founding cohort [PMID: 31595528]). Survivors face lifelong morbidity from hypopituitarism, diabetes insipidus, hydrocephalus, and organ malformations. No formal survival statistics exist given the small patient numbers. Prognostic factors: severity of the hepatic phenotype and presence/absence of biliary involvement.

12. Treatment

No curative/disease-specific therapy. Supportive/organ-directed: ursodeoxycholic acid and nutritional support (cholestasis); liver transplantation (end-stage liver disease); multi-hormone replacement — hydrocortisone, levothyroxine, growth hormone, desmopressin for DI [PMID: 42460215, 40539145]; VP shunt (hydrocephalus); surgical correction of polydactyly and cleft lip/palate. Suggested NCIT terms: Liver Transplantation (NCIT:C15238), Hormone Replacement Therapy (NCIT:C15667), Ursodeoxycholic Acid (NCIT:C29273), Ventriculoperitoneal Shunt (NCIT:C50124). No pharmacogenomic, gene, cell, or RNA therapies are established or in trials for BRENS.

13. Prevention

No primary prevention beyond genetic counseling for at-risk (especially consanguineous) families. Secondary/tertiary prevention: early diagnosis enabling timely hormone replacement, cholestasis management, and shunting to prevent complications. Options where the familial variant is known: carrier screening, prenatal testing, preimplantation genetic diagnosis. No immunization or environmental intervention applies.

14. Other Species / Natural Disease

No naturally occurring companion-animal or wildlife disease is reported (no OMIA entry noted). Orthologous genes: mouse Ttc26, zebrafish ttc26, Chlamydomonas DYF13, C. elegans dyf-13. Evolutionary conservation of IFT-B and its function is high across ciliated eukaryotes. No zoonotic potential (Mendelian disorder).

15. Model Organisms

Robust models exist (Finding 5): mouse (Ttc26 hop-sterile spontaneous mutant; Ift56/Ttc26 engineered LoF with background-dependent severity; Flnb;Ttc26 double heterozygote for scoliosis), zebrafish (ttc26 morpholino/CRISPR), and Chlamydomonas (dyf13). Phenotype recapitulation is strong for polydactyly, hydrocephalus, renal cystic/pronephric defects, photoreceptor defects, motile-cilia/flagellar dysfunction, and Hedgehog-signaling readouts. Limitations: the severe human biliary phenotype is incompletely modeled; genetic background strongly modulates murine phenotypes, complicating cross-study comparison. Resources: MGI, ZFIN, IMPC.

Mechanistic Model (Diagram)

Biallelic LoF IFT56/TTC26 variants
        │  (loss of IFT-B TPR scaffold protein)
        ▼
IFT-B core destabilization + abnormal ciliary microtubules
        │  (cargo-selective anterograde transport failure)
        ▼
Dysfunctional primary & motile cilia
        ├───────────────► Motile cilia: inner dynein-arm loss →
        │                 ependymal/flagellar dysfunction (CSF flow,
        │                 male sterility [mouse]) → hydrocephalus
        │
        └──► Primary cilia signaling defect:
             GLI fails to dissociate from SUFU at ciliary tip
             (+ loss of ciliary PRMT7 → ↓GLI2 methylation)
                     │
                     ▼
             Dysregulated Sonic Hedgehog–GLI signaling
                     │  (abnormal embryonic patterning)
                     ▼
   ┌──────────┬──────────┬──────────┬──────────┬──────────┐
 Biliary     Renal     Neuro/     Pituitary  Skeletal/  Cardiac/
 (cholestasis cysts    brain      PSIS,      limb        ocular/
 fibrosis)             hydro-     hypopit,   polydactyly craniofacial
                       cephalus   DI
                     │
                     ▼
            BRENS clinical phenotype (congenital, chronic)

Evidence Base

Limitations and Knowledge Gaps

• Very small patient population (<25 published cases) limits confident estimates of penetrance, expressivity, sex ratio, survival, and genotype–phenotype correlation.
• No formal epidemiology: prevalence/incidence are estimated indirectly from gnomAD allele frequencies, not measured. The pLoF-only carrier estimate (~1/2,100) omits the missense/splice alleles that dominate the reported disease spectrum, so it underestimates true carrier frequency.
• Incomplete mechanistic mapping of how a single ciliary transport defect yields the specific organ set; the relative contributions of primary-cilia (Hedgehog) vs motile-cilia dysfunction to each organ phenotype are not fully resolved.
• The severe human biliary phenotype is under-modeled in animals; no model fully recapitulates neonatal cholestasis/biliary fibrosis.
• No therapeutics are disease-specific; no clinical trials exist. Pharmacogenomic and gene/cell/RNA therapy data are absent.
• Ontology mapping gaps: no Orphanet/ICD/MeSH code; ontology term suggestions here are proposed, not curated into the disease record.

Proposed Follow-up Experiments / Actions

1. Establish a BRENS patient registry / GeneMatcher-linked cohort to aggregate genotype–phenotype data, refine penetrance and expressivity, and capture natural history prospectively.
2. Variant-level functional assays (splice reporters for the recurrent intronic variants; rescue assays for missense alleles such as p.Asn232Ser) to firm up ACMG classification and reveal hypomorphic vs null effects underlying biliary-sparing presentations.
3. Conditional/tissue-specific Ift56 mouse models (hepatoblast/cholangiocyte, pituitary, renal) on a controlled genetic background to model the biliary phenotype and dissect organ-specific mechanism.
4. Single-cell transcriptomics of patient-derived cholangiocyte and renal organoids/iPSC models to map Hedgehog-target dysregulation and identify candidate therapeutic nodes (e.g., SMO agonists, PRMT7 modulation).
5. Curate BRENS into Orphanet/ICD-11/MeSH and submit HPO/GO/CL/UBERON annotations derived from this report to close ontology gaps.
6. Systematic imaging/endocrine screening protocol (neonatal MRI + endocrine panel) for any infant with cholestasis plus polydactyly to enable early hormone replacement during the critical neonatal window.

Evidence source types: primarily human clinical case reports/series and model-organism studies (mouse, zebrafish, Chlamydomonas), supplemented by in vitro biochemistry and computational (gnomAD constraint) analyses. All mechanistic and clinical claims are cited to primary literature by PMID.