Congenital Bilateral Absence of the Vas Deferens (CBAVD): A Comprehensive Disease Characterization

Disease: Congenital Bilateral Absence of the Vas Deferens (CBAVD) Identifiers: OMIM 277180 · ORPHA:48 · MONDO:0009299 · ICD-10 Q55.4 · ICD-11 LB77.0 · MeSH (vas deferens/abnormalities) Category: Reproductive System Disorder (congenital male genital malformation)

Summary

Congenital bilateral absence of the vas deferens (CBAVD) is a congenital malformation of the male reproductive ductal system in which both vasa deferentia fail to develop or degenerate before birth, producing obstructive azoospermia with preserved spermatogenesis. It accounts for 1–2% of male infertility and is estimated to affect ~0.1% of all men (likely an underestimate, since asymptomatic men are rarely evaluated). CBAVD is best understood as the genital-limited end of the cystic fibrosis (CF) / CFTR-related disorder spectrum: over 95% of men with classic CF are infertile because of vasal atresia, and isolated CBAVD is most commonly caused by biallelic CFTR variants — typically a severe CF-causing allele combined with a mild/variable allele such as the intron-8/9 poly-T (IVS8-5T / TG-T tract) or p.Arg117His.

Genetically, CBAVD is heterogeneous but CFTR-dominant. In stringently phenotyped cohorts, CFTR variants explain roughly 70–80% of cases; the X-linked adhesion G-protein-coupled receptor gene ADGRG2 accounts for ~2% of cases (a familial X-linked form), and 10–20% remain genetically unexplained. Importantly, a distinct subset of unexplained CBAVD coexists with unilateral renal agenesis / solitary kidney, pointing to an early mesonephric (Wolffian) duct developmental defect — mechanistically separate from the progressive fetal duct degeneration seen with CFTR/ADGRG2 variants. Candidate developmental genes (FREM1, WNT2B, TBX6) have recently emerged in this renal-anomaly subgroup.

Clinically, CBAVD is non-life-threatening with a normal life expectancy; its principal consequence is infertility. Because spermatogenesis is intact, biological paternity is achievable in ~85% of men via surgical sperm retrieval (MESA/PESA/TESE) combined with intracytoplasmic sperm injection (ICSI). Management is anchored by CFTR/ADGRG2 genetic testing, partner carrier screening, and genetic counseling, since ~10% of couples share pathogenic CFTR variants and risk a child with CF. A landmark 2025/2026 observation shows that prenatal CFTR-modulator therapy (elexacaftor/tezacaftor/ivacaftor, ETI) can prevent CBAVD, defining a fetal critical window; postnatal modulator therapy does not reverse established vasal agenesis. CFTR-knockout rats are the animal model that best recapitulates the human bilateral vasal absence phenotype.

1. Disease Information

CBAVD is a congenital malformation of the male reproductive tract characterized by the bilateral absence (aplasia) or atresia of the vasa deferentia — the paired muscular ducts that transport spermatozoa from the epididymis to the ejaculatory ducts. The result is a mechanical (obstructive) block to sperm transport, giving obstructive azoospermia despite normal testicular sperm production. It is frequently accompanied by anomalies of adjacent Wolffian-duct derivatives (seminal vesicles, distal epididymis, ejaculatory ducts).

Key identifiers

Synonyms / alternative names: CBAVD; congenital bilateral aplasia of the vas deferens; congenital absence of the vas deferens (CAVD, when unspecified laterality); bilateral vasal agenesis; part of the "congenital absence of the vas deferens" (CAVD) family that also includes congenital unilateral absence of the vas deferens (CUAVD).

Data source type: The evidence base is derived from aggregated disease-level resources (OMIM, Orphanet, cohort studies, meta-analyses) supplemented by individual patient reports (case reports of ADGRG2 pedigrees, MRI series, prenatal ETI case). It is a well-curated Mendelian/complex reproductive disorder rather than an EHR-derived entity.

2. Etiology

Causal factors — predominantly genetic

CBAVD is overwhelmingly a genetic disorder, principally a genital manifestation of CFTR dysfunction.

• CFTR (autosomal recessive, predominant). The majority of CBAVD men carry at least one CF-causing CFTR variant. "The majority of subjects with CAVD carry at least one cystic fibrosis-causing mutation that warrants CFTR testing" PMID: 32025909. In a large Chinese isolated-CAVD (iCAVD) cohort (n=199), "CFTR and ADGRG2 variants were identified in 74.87% of iCAVD patients, with CFTR being the predominant pathogenic gene" PMID: 42199298.
• ADGRG2 (X-linked recessive, ~2%). "Approximately 2% of the cases of CAVD are hemizygous for a loss-of-function mutation in the ADGRG2 gene that may cause a familial form of X-linked infertility" PMID: 32025909.
• Developmental / Wolffian-duct defect (subset with renal agenesis). CFTR-negative cases frequently coexist with a solitary kidney, indicating an early organogenesis disorder (see Section 5 and Section 6).
• Unexplained (10–20%). A residual fraction lacks an identified molecular cause.

Risk factors

• Genetic risk factors: biallelic CFTR variants (a severe allele such as p.Phe508del combined with a mild/variable allele — e.g., IVS8-5T poly-T with long TG tract, or p.Arg117His-T7); hemizygous truncating ADGRG2 variants; candidate developmental variants (FREM1, WNT2B, TBX6) in the renal-anomaly subtype. The IVS9-5T (poly-T 5T) allele is a major low-penetrance risk allele, especially in East Asian populations.
• Environmental risk factors: No established toxic, occupational, infectious, or lifestyle exposure causes CBAVD. It is a developmental/genetic disorder; sex is male-only (defining), and family history of CF or CBAVD is relevant.

Protective factors

• Genetic: By definition, the wild-type / functional CFTR and ADGRG2 alleles are "protective"; the poly-T 7T/9T tracts confer normal splicing/function versus the risk-associated 5T.
• Environmental / therapeutic: Prenatal CFTR-modulator therapy (ETI) prevents CBAVD in genetically susceptible fetuses (Section 6, Finding F006) — an intervention that restores CFTR channel function during the fetal critical window.

Gene–environment interactions

The clearest gene–environment interaction is pharmacologic restoration of CFTR function in utero. The poly-T/TG polymorphic tract is itself a cis-genetic modifier of splicing that determines residual CFTR activity and hence penetrance of the genital phenotype (the TG12-T5 combination reduces CFTR function; PMID: 42572672). No classical toxin-by-gene interaction has been documented.

3. Phenotypes

CBAVD presents in otherwise healthy, normally virilized men, typically discovered during infertility evaluation (adult-onset presentation of a congenital anatomic defect).

Supporting evidence. "The incidence of congenital bilateral absence of the vas deferens (CBAVD) in infertile men is 1-2%" PMID: 35109852. Seminal-vesicle involvement is variable: among 47 CBAVD patients, "29 had bilateral agenesis of the seminal vesicles, 9 had unilateral agenesis, and 9 had bilateral presence" PMID: 40533736. Epididymal involvement tracks with CFTR genotype: "patients carrying at least one non-5 T variant were associated with an 8.17-fold increased risk of epididymal partial absence compared to those having the homozygous 5 T mutation" PMID: 39592508.

Quality-of-life impact. The dominant impact is infertility and its psychosocial burden; there is no pain, disability, or systemic morbidity in isolated CBAVD. Because sperm retrieval + ICSI achieves paternity in most couples, the long-term QoL impact is limited relative to systemic diseases. Men should also be counseled about the possibility of an underlying CFTR-related disorder (e.g., pancreatitis, sinopulmonary disease) that may manifest later.

4. Genetic / Molecular Information

Causal genes

• CFTR (HGNC:1884; OMIM 602421; chr7q31.2) — cystic fibrosis transmembrane conductance regulator; a cAMP-activated Cl⁻/HCO₃⁻ channel. Predominant cause of CBAVD.
• ADGRG2 (HGNC:18023; OMIM 300572; chrXp22.13) — adhesion GPCR G2 (GPR64), epididymal- and efferent-duct-specific. X-linked cause (~2%).
• Candidate developmental genes: FREM1, WNT2B, TBX6 in CFTR-negative CBAVD with renal anomalies PMID: 40921938.

Pathogenic variants — CFTR

Isolated CBAVD is typically caused by a trans-heterozygous combination of one severe CF-causing allele plus one mild/variable allele:

• p.Phe508del (F508del) — the classic severe deletion; predominant in Europeans.
• Poly-T / TG tract (IVS8-5T; c.1210-34TG(n)T(m)) — the T5 allele with a long TG repeat reduces exon-9 inclusion and lowers functional CFTR. "the combination of T5 with longer TG repeats is associated with reduced CFTR function" — as in a compound heterozygote p.Phe508del + TG12T5 PMID: 42572672.
• p.Arg117His — associated with CBAVD and other CFTR-related disorders; a French cohort (n=179) found "83 isolated CBAVD, 67 other CFTR-related phenotypes" with an overall mild phenotype PMID: 23378603.
• IVS9-5T — the most common allele in Chinese CBAVD (~54.5%), with regional variants such as p.Gln1352His (c.4056G>C).

Variant classification follows ACMG/AMP tiers (pathogenic / likely pathogenic / VUS). Variant types include missense (p.Arg117His), in-frame deletion (p.Phe508del), splice-modulating poly-T/TG tracts, nonsense, frameshift, and deep-intronic/large rearrangements — hence the recommendation for whole-exon + flanking + rearrangement CFTR screening in CAVD PMID: 40065563. Origin is germline. Functional consequence is loss of function (reduced Cl⁻/HCO₃⁻ conductance).

Pathogenic variants — ADGRG2

Hemizygous protein-truncating variants: "c.1545dupT (p.Glu516Ter), c.2845delT (p.Cys949AlafsTer81), and c.2002_2006delinsAGA (p.Leu668ArgfsTer21)" PMID: 27476656; additional c.G118T (p.Glu40) and the nonsense c.908C>G (p.Ser303) PMID: 37273165. These are loss-of-function, X-linked, maternally inherited, and typically absent from population databases. Western blot confirms a truncated ADGRG2 protein [PMID: 37273165].

Modifier genes / epigenetics / chromosomal abnormalities

• Modifiers: the poly-T/TG tract functions as the principal cis-modifier of CFTR splicing and penetrance. SLC9A3 interacts functionally with CFTR (Sections 6/7).
• Epigenetics: No disease-specific DNA-methylation or histone signature has been established for CBAVD.
• Chromosomal abnormalities: Not a primary cause. A single case report describes a 47,XYY mosaic karyotype coexisting with CBAVD PMID: 35109852; this appears coincidental rather than causal.

5. Environmental Information

• Environmental factors / toxins / radiation / occupational exposure: None established as causal. CBAVD is a developmental-genetic disorder.
• Lifestyle factors (smoking, diet, alcohol, exercise): No demonstrated role in causation.
• Infectious agents: Not applicable — CBAVD is congenital and non-infectious. (Acquired vasal obstruction from infection or vasectomy is a separate differential diagnosis, not CBAVD.)

The only "environmental" (i.e., non-germline) modifier with proven effect is pharmacologic — prenatal CFTR modulator exposure (protective; Section 6).

6. Mechanism / Pathophysiology

Two etiologic subtypes (bimodal pathogenesis)

CBAVD arises through two mechanistically distinct routes, a key organizing insight of this investigation (Finding F005):

                          ┌─────────────────────────────────────────────┐
                          │   CBAVD  (obstructive azoospermia)           │
                          └─────────────────────────────────────────────┘
                                     │
        ┌────────────────────────────┴───────────────────────────────┐
        │                                                              │
 (A) DEGENERATIVE subtype                              (B) DEVELOPMENTAL subtype
  CFTR / ADGRG2 loss-of-function                        Mesonephric (Wolffian) duct
  → abnormal luminal Cl⁻/HCO₃⁻ &                        maldevelopment (early organogenesis)
    fluid transport                                     → ureteric bud + duct derivatives
  → progressive fetal atresia/                            affected
    degeneration of vas deferens                        → vasal agenesis + UNILATERAL
    beginning later in fetal life                         RENAL AGENESIS / solitary kidney
  → kidneys SPARED                                      candidate genes: FREM1, WNT2B, TBX6

Evidence for the split: "An important proportion of these unexplained CAVDs coexist with a solitary kidney suggesting an early organogenesis disorder (Wolffian duct), unlike CAVDs related to CFTR or ADGRG2 mutations, which might be the result of progressive degeneration that begins later in fetal life" PMID: 32025909. The shared embryology of the developmental subtype: "The embryonic insult that results in unilateral renal agenesis may involve not only the ureteral bud but also other mesonephric duct derivatives, including the seminal vesicles, vas deferens, and epididymis" PMID: 16985610. MRI data support acquired/progressive vasal agenesis in the CFTR-type: "Preliminary findings in this study are consistent with the theory of acquired vasal agenesis in CBAVD" PMID: 41255074.

Molecular pathways and protein dysfunction

• CFTR pathway (degenerative subtype). CFTR is a cAMP-activated Cl⁻ and HCO₃⁻ channel governing the luminal microenvironment of the male tract. Loss of function disturbs anion/fluid secretion and the HCO₃⁻/soluble adenylyl cyclase/cAMP/CREB and NF-κB/COX-2/PGE₂ signaling axes relevant to tract development and sperm function: "CFTR is emerging as a versatile player with roles in mediating different signaling pathways... in addition to its long-recognized role in electrolyte and fluid transport that regulates the luminal microenvironment of the male reproductive tract" PMID: 22709980. Abnormal luminal fluid handling is thought to drive the progressive atresia of the vas: "cystic fibrosis (CF) leads to infertility in over 95% of cases, due to early and progressive atresia of the vas deferens, resulting in obstructive azoospermia" PMID: 42380629.
• ADGRG2 pathway (X-linked subtype). ADGRG2 is an adhesion GPCR expressed apically in non-ciliated efferent-duct epithelium: "ADGRG2 expression was restricted to the apical membranes of non-ciliated epithelia in human efferent ducts" PMID: 32314195. Loss of function causes obstructive infertility by disrupting efferent-duct fluid reabsorption (recapitulated in Adgrg2-knockout mice: "Adgrg2-knockout male mice develop obstructive infertility" PMID: 27476656).
• SLC9A3 (NHE3) interaction. SLC9A3 loss reduces CFTR protein and causes obstruction: "depleted Slc9a3 in male mice causes infertility due to the abnormal dilated lumen of the rete testis and efferent ductules" PMID: 28384194 — implicating a shared ion-transport/CFTR-stability module.

Cellular processes, cell types, and compartments

• Cell types (CL): ductal epithelial cells of the vas deferens/epididymis; non-ciliated efferent-duct epithelial cells (ADGRG2⁺); seminal-vesicle epithelium.
• Cellular process: disrupted transepithelial anion/fluid transport → luminal microenvironment failure → epithelial/duct degeneration and atresia (degenerative subtype) or failed duct morphogenesis (developmental subtype).
• Subcellular compartments (GO CC): apical plasma membrane (GO:0016324) — site of CFTR and ADGRG2 function; anion channel activity.
• Suggested GO BP: GO:0006821 (chloride transport), GO:0055085 (transmembrane transport), GO:0048754 (branching morphogenesis of an epithelial tube), GO:0035239 (tube morphogenesis), GO:0007283 (spermatogenesis — preserved).

Critical window — a therapeutic mechanism

The degenerative subtype is preventable in utero. In a male CF infant (homozygous F508del) whose carrier mother began ETI at 27+4 weeks: "ultrasound at 8 weeks demonstrated bilateral vas deferens, a structure typically absent in nearly all male patients with CF at birth" and "These findings suggest that prenatal CFTR modulation - even when initiated late" can preserve the duct PMID: 41654435. Conversely, "At present male patients taking CFTR modulators have not shown improvement in infertility" PMID: 39288989 — establishing that restoration must occur before the duct is lost.

7. Anatomical Structures Affected

• Organ level (primary): vas deferens (UBERON:0001000) — bilateral. Secondary/associated: epididymis (UBERON:0001301), seminal vesicle (UBERON:0000998), ejaculatory duct, efferent ducts (UBERON:0003074). Kidney (UBERON:0002113) in the developmental subtype (unilateral renal agenesis).
• Body system: male reproductive/genital system (UBERON:0000079); urinary system involvement in the renal-anomaly subtype.
• Tissue / cell level: ductal epithelium (transporting epithelium) and surrounding smooth muscle of the vas; non-ciliated efferent-duct epithelial cells (ADGRG2⁺). Testicular seminiferous tissue is preserved (spermatogenesis intact).
• Subcellular: apical plasma membrane (GO:0016324); anion channel machinery.
• Localization / lateralization: bilateral by definition (CBAVD); the CAVD family also includes unilateral (CUAVD) and asymmetric presentations. Seminal-vesicle and epididymal involvement is frequently asymmetric PMID: 40533736.

8. Temporal Development

• Onset: Congenital — the anatomic defect is present at (or develops before) birth. In the degenerative CFTR subtype, atresia is "early and progressive" during fetal life [PMID: 42380629]; the developmental subtype originates at early organogenesis. Clinically, however, the disorder is usually detected in adulthood during infertility work-up (insidious, asymptomatic until then).
• Progression: The structural defect is fixed/non-progressive after birth (the vas is already absent). There are no post-natal "stages." Disease course is stable and lifelong; the associated infertility is chronic unless overcome by ART.
• Patterns / critical periods: The single actionable critical period is fetal — CFTR function must be preserved during gestation to prevent vasal loss (prenatal ETI, [PMID: 41654435]). No spontaneous remission occurs.

9. Inheritance and Population

Epidemiology

• Prevalence: "The prevalence of CAVDs in men is reported to be approximately 0.1%" PMID: 32025909 (likely underestimated).
• Frequency in male infertility: CBAVD accounts for 1–2% of infertile men PMID: 35109852.
• Incidence: Not precisely established (congenital, detected at reproductive age).

Inheritance (genetic etiology)

• Penetrance / expressivity: Variable and incomplete for the genital phenotype; sweat chloride does not correlate with severity in p.Arg117His carriers PMID: 23378603. For ADGRG2, an obligate-carrier male with the p.Ser303 variant had normal fertility*, illustrating variable expressivity PMID: 37273165.
• Genetic anticipation / mitochondrial inheritance: Not applicable.
• Founder effects / geographic allele spectra: Population-specific. p.Phe508del predominates in Europeans; IVS9-5T is the most common allele in Chinese CBAVD (~54.5%) with regional p.Gln1352His. "There are no obvious hotspot CFTR mutations in Chinese CBAVD patients besides the IVS9-5 T allele" PMID: 35119551.
• Consanguinity / carrier frequency: CFTR carrier frequency is high in populations with elevated CF prevalence (up to ~1/25 in Europeans), underpinning the reproductive-risk concern. ~10% of iCAVD couples share pathogenic CFTR variants: "10.14% of couples carried shared pathogenic or likely pathogenic CFTR variants" PMID: 42199298.

Population demographics

• Sex: male only (defining).
• Geographic variation of variants: European vs East Asian allele spectra differ markedly (above).
• Age distribution: presents at reproductive age (typically 20s–40s at infertility work-up).

10. Diagnostics

Clinical tests

• Physical exam: bilateral non-palpable vasa deferentia — the cornerstone finding.
• Semen analysis (laboratory): azoospermia, low ejaculate volume, low (acidic) pH, low/absent fructose (reflecting seminal-vesicle contribution). Normal serum FSH/testosterone and normal testicular volume support an obstructive (not spermatogenic) cause.
• Imaging: scrotal ultrasound (confirms absent/atretic vas, evaluates epididymis), transrectal ultrasound (seminal vesicles, ejaculatory ducts), and MRI for the intra-abdominal vas and seminal-vesicle pathology — "detailed findings are obtained by MRI even in the evaluation of the intra-abdominal part of the VD" PMID: 41255074. Renal ultrasound is mandatory to detect solitary kidney (developmental subtype).
• Biopsy/histopathology: testicular biopsy shows preserved spermatogenesis (not routinely needed).

Genetic testing

• Recommended approach: Comprehensive CFTR analysis first — including full exon + flanking-region sequencing, the poly-T/TG tract, and large-rearrangement detection — because standard panels miss deep-intronic and rearrangement alleles: "the urgent need for extensive CFTR screening, including sequencing of whole exons and flanking regions and detection of large rearrangements and deep intronic CF-causing variants" PMID: 40065563. If CFTR is negative and renal ultrasound is normal, test ADGRG2: "Pathogenic variants in ADGRG2 are important to look for when CFTR analysis is negative and renal ultrasonography is normal" PMID: 41886210.
• Modalities: targeted CFTR single-gene/panel testing; WES/WGS for unexplained cases and developmental candidate genes (FREM1, WNT2B, TBX6); karyotype only if a broader syndrome is suspected.

Clinical criteria & differential diagnosis

• Diagnosis rests on the triad of non-palpable vasa + obstructive azoospermia + low-volume acidic fructose-negative semen, confirmed by imaging and genetics.
• Differential diagnosis: other causes of obstructive azoospermia (ejaculatory-duct obstruction, post-infectious or post-vasectomy obstruction, Young syndrome) and non-obstructive azoospermia (distinguished by normal FSH/testicular volume and preserved spermatogenesis in CBAVD).

Screening

• Partner CFTR carrier screening is essential before ART (couple co-carrier risk ~10%).
• Cascade testing of family members for ADGRG2 pedigrees.

11. Outcome / Prognosis

• Survival / mortality: Isolated CBAVD carries no excess mortality; life expectancy is normal. Prognosis for survival is excellent. (Men should nonetheless be evaluated for a broader CFTR-related disorder that could have its own morbidity.)
• Morbidity / function: The sole functional impact is infertility; no disability or organ failure in isolated disease.
• Fertility outcome (the key prognostic domain): Excellent with ART. Because spermatogenesis is preserved, surgical sperm retrieval + ICSI is highly successful: in a Chinese cohort, "Spermatozoa were successfully retrieved in 46 patients, and 39 of the patients had their own offspring through ICSI" (≈85% paternity) PMID: 35119551.
• Prognostic factors:
• Residual CFTR activity predicts retrieval success — MESA extraction failure was higher in CF than CFTR-RD: "Extraction failure rates were 18.6% for cystic fibrosis and 3.9% for CFTR-RD (P = 0.01)", and worse with no residual activity (27.9% vs 3.7% failure, P<0.001) PMID: 40850271.
• Sperm motility predicts ICSI outcome: "the clinical pregnancy rates, embryo implantation rates, and live birth rates in the high motility group were significantly increased" PMID: 34313208.
• Complications: principally reproductive/ART-related (e.g., risk of transmitting CF to offspring; rare ART complications such as monochorionic/conjoined twinning after embryo transfer, PMID: 42215739).

12. Treatment

CBAVD has no medical cure for the anatomic defect; management is fertility-focused plus genetic counseling.

Assisted reproduction (mainstay)

• Surgical sperm retrieval: MESA (microsurgical epididymal sperm aspiration), PESA (percutaneous epididymal sperm aspiration), TESE (testicular sperm extraction). (NCIT: Sperm Retrieval; Testicular Sperm Extraction.)
• ICSI (intracytoplasmic sperm injection): the definitive route to biological paternity; ~85% success in achieving offspring [PMID: 35119551]. (NCIT: Intracytoplasmic Sperm Injection.)
• Optimization: select high-motility sperm to maximize live-birth rate [PMID: 34313208].

CFTR modulator therapy — a disease-modifying frontier

• Prenatal ETI (elexacaftor/tezacaftor/ivacaftor) can prevent CBAVD when administered to the fetus via a carrier mother during gestation PMID: 41654435. This is investigational and raises ethical/consent questions for heterozygous fetuses PMID: 39543810.
• Postnatal modulators do NOT reverse established CBAVD/infertility PMID: 39288989.

Pharmacogenomics / personalized medicine

• Genotype-guided counseling: CFTR genotype informs residual function, retrieval prognosis, CFTR-RD surveillance, and modulator eligibility.
• Couple-level genotyping guides preimplantation genetic testing (PGT) to avoid transmitting CF.

Not applicable

Gene therapy, cell therapy, RNA therapeutics, immunotherapy, and chemotherapy have no established role in CBAVD.

13. Prevention

• Primary prevention: Prenatal CFTR-modulator therapy is the only demonstrated means of preventing the vasal defect itself (investigational) [PMID: 41654435]. Broadly, primary prevention is limited because CBAVD is congenital/genetic.
• Secondary prevention / early detection: Genetic diagnosis at infertility work-up enables timely ART and CFTR-RD surveillance.
• Tertiary prevention: Optimizing sperm-retrieval/ICSI protocols and PGT to prevent CF offspring.
• Genetic screening & counseling (central): CFTR carrier screening of both partners before ART is essential; ~10% of couples are co-carriers [PMID: 42199298]. Options include preimplantation genetic diagnosis and prenatal testing. Comprehensive CFTR screening (exons + flanking + rearrangements) is recommended before ART [PMID: 40065563].
• Immunization / public-health / environmental measures: Not applicable (non-infectious, non-environmental).

14. Other Species / Natural Disease

• Taxonomy affected (models): Rattus norvegicus (rat, NCBI:txid10116), Mus musculus (mouse, NCBI:txid10090), plus large-animal CF models — ferret, pig, sheep, rabbit.
• Orthologous genes: Cftr (rat/mouse), Adgrg2, Slc9a3 orthologs.
• Natural / comparative disease: CF animal models frequently show absent vas deferens/epididymis with normal testicular histology, mirroring human CBAVD: "CFTR-knockout rats more closely reproduce the human phenotype, showing bilateral absence of the vas deferens and epididymal hypoplasia, although they exhibit more pronounced hypospermatogenesis than observed in men" PMID: 42380629. Evolutionary conservation of CFTR-dependent duct development underlies the cross-species recapitulation.
• Transmission / zoonosis: Not applicable (genetic malformation).

15. Model Organisms

Supporting quotes. "knock-in or partial knockout models usually remain fertile, whereas complete knockouts may develop vas deferens atresia with aging" PMID: 42380629. "Adgrg2-knockout male mice develop obstructive infertility" PMID: 27476656. "depleted Slc9a3 in male mice causes infertility due to the abnormal dilated lumen of the rete testis and efferent ductules" PMID: 28384194.

Applications: these models allow study of CFTR-dependent duct morphogenesis, the fetal critical window for modulator rescue, efferent-duct fluid handling (ADGRG2/SLC9A3), and CFTR-modulator pharmacology. Databases: MGI, RGD, IMPC/KOMP, IMSR.

Mechanistic Model / Interpretation

CBAVD is best conceptualized as a convergent obstructive-azoospermia phenotype reached by two upstream routes:

UPSTREAM CAUSE                    MID-STREAM MECHANISM                    DOWNSTREAM PHENOTYPE
─────────────────────────────────────────────────────────────────────────────────────────────
CFTR biallelic LoF ──►  ↓ apical Cl⁻/HCO₃⁻ & fluid transport ──►  progressive fetal ─┐
(severe + mild allele;   (HCO₃⁻/sAC/cAMP; NF-κB/COX-2)             vasal atresia      │
 poly-T/TG modifier)                                              (kidneys spared)    │
                                                                                      ├─► Bilateral
ADGRG2 hemizygous LoF ─► efferent-duct epithelial dysfunction ──► efferent/vasal ─────┤   absent vas
(X-linked, ~2%)          (adhesion GPCR, fluid reabsorption)      obstruction         │   → obstructive
                                                                                      │   azoospermia
Wolffian-duct           failed duct + ureteric-bud morphogenesis ─► vasal agenesis ───┘   (SPERMATOGENESIS
maldevelopment          (FREM1/WNT2B/TBX6?)                        + UNILATERAL RENAL     PRESERVED)
(developmental subtype)                                            AGENESIS

Upstream vs downstream: the genetic lesion (CFTR/ADGRG2 LoF or a developmental-gene defect) is upstream; disrupted epithelial ion/fluid transport (or failed morphogenesis) is the mid-stream mechanism; duct atresia/agenesis and consequent obstructive azoospermia are downstream. The testis is not in the causal chain — spermatogenesis is preserved, which is precisely why sperm retrieval + ICSI works. The presence/absence of a solitary kidney is the single most useful clinical discriminator between the developmental and degenerative subtypes and should redirect genetic testing (renal-anomaly → developmental genes; normal kidneys + CFTR-negative → ADGRG2).

Evidence Base

Consistency: Findings are mutually reinforcing across independent European and East Asian cohorts, case reports, MRI series, and multiple animal models. No major contradictions were identified; the chief tension is the "progressive degeneration" vs "developmental agenesis" debate, which the two-subtype model reconciles (degenerative = CFTR/ADGRG2, kidneys spared; developmental = Wolffian-duct defect, renal agenesis).

Limitations and Knowledge Gaps

1. Unexplained fraction (10–20%). A substantial minority of CBAVD lacks a molecular diagnosis; developmental genes (FREM1, WNT2B, TBX6) are candidates but not yet validated at scale.
2. Single-case evidence for prenatal prevention. The ETI-prevents-CBAVD observation rests on one infant [PMID: 41654435]; timing, dosing, the true critical window, and long-term/heterozygote safety are unknown, with unresolved ethical questions.
3. Penetrance/expressivity poorly quantified. The poly-T/TG modifier and the normal-fertility ADGRG2 carrier show incomplete penetrance that is not yet predictable at the individual level.
4. Model limitations. No model perfectly reproduces isolated human CBAVD; rats over-express hypospermatogenesis, and most mouse models remain fertile.
5. Epidemiology. Prevalence (~0.1%) is likely underestimated; incidence and non-European/non-East-Asian allele spectra are under-characterized.
6. Epigenetics. No disease-specific epigenetic signature has been defined.
7. Long-term offspring outcomes after prenatal modulator exposure are unstudied.

Proposed Follow-up Experiments / Actions

1. Systematic renal imaging + developmental-gene panel (FREM1, WNT2B, TBX6, and broader WES/WGS) in all CFTR/ADGRG2-negative CBAVD to validate the developmental subtype and expand the gene set.
2. Registry/prospective study of prenatal CFTR-modulator exposure with structured male genital-tract follow-up (vas patency by ultrasound, later fertility) to define the fetal critical window, efficacy, and safety.
3. Functional dissection of the ADGRG2–CFTR–SLC9A3 module in efferent-duct organoids/animal models to map shared fluid-transport mechanisms.
4. Genotype-stratified sperm-retrieval outcome studies to formalize residual-CFTR-activity and motility as pre-procedure prognostic tools (building on [PMID: 40850271], [PMID: 34313208]).
5. Universal comprehensive CFTR screening protocol (exons + flanking + poly-T/TG + rearrangements + deep-intronic) with mandatory partner carrier screening and PGT counseling before ART.
6. Population-specific allele catalogues beyond European/Chinese cohorts to improve carrier-screening panels globally.
7. Longitudinal CFTR-RD surveillance of isolated-CBAVD men to quantify later pancreatic/sinopulmonary risk.

Report compiled from a 5-iteration autonomous investigation: 9 confirmed findings, 27 papers reviewed. Evidence types span human clinical cohorts, case reports, imaging series, in vitro studies, and model-organism data.