| Domain | Best-supported finding | Evidence type and strength | Knowledge-base ontology suggestions | Key caveat |
|---|---|---|---|---|
| Identifiers | HH21 is catalogued as **hypogonadotropic hypogonadism 21 with or without anosmia**, associated with **FLRT3**: OMIM disease **615271**, MONDO **MONDO:0014107**, and Ensembl gene **ENSG00000125848**. (pqac-00000000, pqac-00000001) | **Direct database association; limited-to-moderate evidence.** Open Targets aggregates four ClinVar or EVA records principally linked to PMID 23643382. | MONDO:0014107; parent MONDO:0018555; FLRT3 | Disease-specific ICD-10, ICD-11, MeSH, and Orphanet identifiers were not established in the retrieved evidence; broader hypogonadotropic-hypogonadism codes may be required. |
| Gene validity | FLRT3 has a historical human association with HH21, but a 2023 review of the NHS panel framework places it among genes with insufficient evidence for routine clinical reporting rather than among the 14 clinically reportable green genes. (pqac-00000012, pqac-00000013) | **Direct human reports with limited replication; weak-to-moderate clinical validity.** No recent independent case series or definitive segregation and functional validation was retrieved. | Gene–disease association; ACMG/AMP evidence codes after variant-level reassessment | An OMIM disease number does not establish definitive contemporary validity. Exact variants, segregation, penetrance, functional results, and population frequencies could not be recovered from the available source text. |
| Phenotype | Expected manifestations are congenital GnRH deficiency, low sex steroids with low or inappropriately normal LH and FSH, absent or arrested puberty, and infertility. Anosmia or hyposmia may occur but is not obligatory; neonatal male clues include micropenis and cryptorchidism. (pqac-00000010, pqac-00000012, pqac-00000016, pqac-00000018) | **General-CHH extrapolation; strong syndrome-level evidence but not FLRT3-specific.** | HP:0000044 Hypogonadotropic hypogonadism; HP:0000823 Delayed puberty; HP:0000786 Primary amenorrhea; HP:0000027 Azoospermia; HP:0000054 Micropenis; HP:0000028 Cryptorchidism; HP:0000458 Anosmia; HP:0004409 Hyposmia | FLRT3-specific phenotype frequencies, sex distribution, severity, nonreproductive manifestations, and genotype–phenotype correlations are unknown. |
| Mechanism | FLRT3 is a single-pass cell-surface adhesion and guidance protein containing 10 leucine-rich repeats and an FNIII domain. Models support homophilic adhesion, shed-FLRT–UNC5 repulsion, Rnd-dependent cadherin regulation, and enhancement of FGF–ERK/MAPK signaling. Altered olfactory or GnRH-lineage guidance is a plausible route to HH21. (pqac-00000019, pqac-00000023, pqac-00000025) | **Direct FLRT3 molecular and model evidence; indirect disease mechanism.** Adhesion, repulsion, and neural migration functions are demonstrated, but the FLRT3-to-GnRH-to-HH21 chain is inferred. | GO:0007155 cell adhesion; GO:0001764 neuron migration; GO:0007411 axon guidance; GO:0000165 MAPK cascade; GO:0005886 plasma membrane; CL:0000540 neuron | No retrieved experiment demonstrated defective GnRH-neuron development caused by a human HH21 variant. Immune, metabolic, tissue-necrosis, and disease-specific epigenetic mechanisms remain unsupported. |
| Anatomy | Inferred primary sites are the embryonic nasal and olfactory placode, GnRH migratory pathway, forebrain and hypothalamus, pituitary gonadotropes, and downstream gonads. Olfactory bulb or sulcus abnormalities may accompany Kallmann-spectrum disease. (pqac-00000009, pqac-00000017) | **General developmental-neuroendocrine evidence; indirect for FLRT3.** | UBERON:0000004 nose; UBERON:0005725 olfactory epithelium; UBERON:0002264 olfactory bulb; UBERON:0001898 hypothalamus; UBERON:0000007 pituitary gland; UBERON:0000473 testis; UBERON:0000992 ovary | Direct localization of pathogenic FLRT3 effects in human olfactory or GnRH tissue has not been demonstrated; characteristic lateralization is not reported. |
| Diagnostics | Evaluate pubertal development, morning sex steroids, LH and FSH, other pituitary hormones, formal olfaction, and pituitary or olfactory anatomy by MRI when indicated. Multigene NGS, exome, or genome analysis is preferable to isolated FLRT3 testing because CHH is genetically heterogeneous and can be oligogenic. (pqac-00000010, pqac-00000012, pqac-00000017, pqac-00000018) | **General-CHH clinical evidence; strong. FLRT3 result interpretation remains uncertain.** | Relevant HPO terms; LOINC concepts for LH, FSH, testosterone, estradiol, inhibin B, and AMH; NCIT:C16809 magnetic resonance imaging; NCIT:C17147 genetic testing | A heterozygous FLRT3 VUS alone should not establish HH21. Confirm candidate variants, evaluate segregation and population frequency, and assess established CHH genes and copy-number variants. |
| Treatment | Management is phenotype-based: gradual testosterone induction or replacement in males; estradiol followed by cyclic progestogen in females; pulsatile GnRH or hCG plus FSH for fertility; and orchidopexy for cryptorchidism. Infant mini-puberty replacement remains specialist-led. (pqac-00000012, pqac-00000016, pqac-00000027, pqac-00000030, pqac-00000032) | **General-CHH evidence; moderate-to-strong. No FLRT3-specific treatment evidence.** Approximately 75% fertility restoration is reported across treated CHH populations; orchidopexy success is about 90%. | NCIT:C2300 testosterone; NCIT:C483 estradiol; NCIT:C2302 progesterone; NCIT:C644 human chorionic gonadotropin; NCIT:C717 follicle-stimulating hormone; NCIT:C614 gonadotropin-releasing hormone; NCIT:C15218 orchidopexy | No approved FLRT3-targeted drug, gene therapy, RNA therapy, cell therapy, immunotherapy, or genotype-guided pharmacogenomic strategy exists. Treatment outcomes cannot be assumed to differ by FLRT3 genotype. |
| Epidemiology and course | CHH overall is estimated at approximately 1 per 15,000–50,000 people, with a male-to-female ratio near 3.6:1. Finnish Kallmann estimates are about 1 per 30,000 males and 1 per 125,000 females. Approximately 10–20% of IHH cases may recover axis function, sometimes followed by relapse. (pqac-00000012, pqac-00000015, pqac-00000018) | **General-CHH epidemiology; moderate evidence. No HH21-specific denominator.** | Rare-disease classification; HP:0003674 Onset in adolescence; HP:0003581 Adult onset for uncommon later presentations | No reliable FLRT3-HH21 prevalence, incidence, carrier frequency, penetrance, founder effect, ancestry distribution, survival, or mortality estimate is available. |
| Models | Flrt3-null mice exhibit early morphogenetic abnormalities and embryonic lethality. Neural studies support migration and repulsion functions, while combined Flrt2 and Flrt3 deletion disrupts cortical interneuron streams through UNC5-related signaling. (pqac-00000011, pqac-00000019, pqac-00000021, pqac-00000023) | **Direct model-organism and in-vitro FLRT biology; moderate mechanistic evidence.** | NCBI Taxon:10090 Mus musculus; GO:0001764 neuron migration; GO:0007411 axon guidance; CL:0000099 interneuron | No model directly reproduces GnRH deficiency, pubertal failure or infertility, and optional anosmia together. FLRT-family redundancy and early lethality complicate interpretation. |
| Omics and advanced technologies | No HH21-specific transcriptomic, single-cell, spatial-transcriptomic, proteomic, metabolomic, lipidomic, methylomic, multi-omic, organoid, iPSC, or CRISPR-screen signature was identified. | **Evidence absent.** | CL:0000540 neuron; GO:0010468 regulation of gene expression; relevant EFO terms for single-cell RNA sequencing and spatial transcriptomics | General tissue-expression and cortical-development findings should not be represented as an HH21 molecular profile without variant-specific patient or engineered-model evidence. |


*Table: Compact assessment of FLRT3-related HH21 across identifiers, validity, clinical features, mechanism, diagnostics, management, epidemiology, and models. It distinguishes direct FLRT3 evidence from broader CHH extrapolation.*