Fanconi Anemia

Introduction and Epidemiology

• Fanconi anemia (FA) is the most frequently inherited aplastic anemia and a classic inherited bone marrow failure syndrome (IBMFS).
• The estimated frequency of FA is 1 in 200,000 in most populations.
• A higher heterozygote carrier frequency is observed in specific populations due to the "founder effect," including Ashkenazi Jews (1:30,000), South African Afrikaners (1:22,000), Northern Europeans, sub-Saharan Blacks, and Spanish Gypsies.
• The classic FA phenotype is characterized by a diagnostic triad: progressive bone marrow failure, congenital physical anomalies, and an elevated predisposition to cancer (both hematologic and solid tumors).
• Despite the classic presentation, up to 40% of patients lack obvious physical abnormalities, making diagnosis challenging.
• Sibling discordance in clinical and hematologic manifestations can occur, even among affected monozygotic twins.

Pathophysiology and Genetics

Inheritance Patterns

• FA is a genetically heterogeneous disorder with mutations reported in 22 distinct genes, designated as FANC genes.
• More than 99% of cases are inherited in an autosomal recessive manner.
• Rare inheritance patterns include X-linked recessive (associated with the FANCB gene, accounting for <1% of cases) and autosomal dominant (associated with the FANCR/RAD51 gene).
• FANCA is the most common complementation group (accounting for 60–70% of cases), followed by FANCC (10–14%) and FANCG (9–10%).

Cellular Mechanisms

• The proteins encoded by wild-type FANC genes are critical components of the DNA damage recognition and repair biochemical pathways, specifically homologous recombination and DNA interstrand cross-link repair.
• The FA core complex—consisting of FANCA, B, C, E, F, G, L, and M—assembles after DNA damage is detected and is required to monoubiquitinate and activate the FANCD2 and FANCI proteins.
• The ubiquitinated FANCD2 and FANCI form a dimer that stabilizes stalled DNA replication forks and interacts with downstream proteins (such as BRCA2/FANCD1, FANCN, and FANCJ) to accomplish DNA repair.
• Pathogenic variants disrupt this cascade, leading to faulty DNA damage response, genomic instability, and hypersensitivity to chromosomal breakage.
• FA cells exhibit characteristic hypersensitivity to G2/M cell cycle arrest and chromosomal breaks when exposed to DNA cross-linking agents (clastogens) like diepoxybutane (DEB) and mitomycin C (MMC).
• Mutant cells additionally display sensitivity to oxygen-free radicals and ionizing radiation.
• The resulting genomic instability leads to accelerated apoptosis of hematopoietic stem cells (causing bone marrow failure), birth defects, and a high rate of malignant transformation.

Clinical Manifestations

Hematologic Abnormalities

• Pancytopenia is the usual hematologic finding, typically manifesting in the first decade of life.
• The median age at hematologic presentation for aplastic anemia is approximately 8 to 10 years.
• The progression of bone marrow failure follows a typical sequence: it usually presents initially with erythrocyte macrocytosis and elevated fetal hemoglobin (HbF), followed by thrombocytopenia, neutropenia, and ultimately severe aplastic anemia (SAA).

Congenital Physical Anomalies

• Congenital anomalies are present in about 60% of patients and can involve nearly any organ system.
• Body and Growth (40%): Short stature, delicate features, low birth weight (intrauterine growth restriction), and underweight status.
• Skin (40%): Generalized hyperpigmentation on the trunk, neck, and intertriginous areas; café-au-lait spots; and vitiligo/hypopigmented areas.
• Upper Limbs (35%): Absent or hypoplastic thumbs, supernumerary or bifid thumbs, triphalangeal or tubular thumbs. Radial anomalies (absent or hypoplastic) occur in 7% of patients but are only seen in conjunction with abnormal thumbs. Clinodactyly, hypoplastic thenar eminence, and absent/weak radial pulses are also seen.
• Head and Face (20%): Microcephaly, hydrocephalus, micrognathia, triangular or birdlike face, frontal bossing, and dental abnormalities.
• Eyes (20%): Microphthalmia (small eyes), strabismus, epicanthal folds, ptosis, cataracts, and hypertelorism.
• Kidneys (20%): Ectopic, pelvic, horseshoe, hypoplastic, or absent kidneys; hydronephrosis and duplicated collecting systems.
• Gonads and Urogenital: Males (25%) may have hypogenitalia, undescended testes, hypospadias, micropenis, and azoospermia (all are infertile). Females (2%) may have hypogenitalia, bicornuate uterus, or atresia of the vagina/uterus (reduced fertility).
• Ears (10%): Deafness (usually conductive), abnormal ear shape, or low-set ears.
• Lower Limbs (5%): Toe syndactyly, clubfeet, congenital hip dislocation.
• Gastrointestinal (5%): High-arched palate, esophageal/duodenal atresia, tracheoesophageal fistula, and imperforate anus.
• Neurological: Developmental delay or cognitive delays are seen in approximately 10% of patients.
• Neonates may present with a cluster of malformations consistent with the VACTERL-H association (vertebral, anal, cardiac, tracheoesophageal fistula, renal, limb, and hydrocephalus).

Cancer Predisposition

• FA patients have a markedly elevated, premature risk of both hematologic and solid malignancies.
• Hematologic Malignancies: The cumulative incidence of clonal myeloid transformation (including myelodysplastic syndrome [MDS] and acute myeloid leukemia [AML]) is approximately 75% by age 18, and the cumulative incidence of leukemia by age 40 is 33%.
• Solid Tumors: There is a 600-fold higher risk for squamous cell carcinomas (SCC) of the head and neck, a 2000-fold higher risk for upper esophageal carcinoma, and a 3000-fold higher risk for vulvar cancer.
• The median age of onset for SCC in the FA population is 33 years, compared to 60–70 years in the general population.
• Human papillomavirus (HPV) is highly suspected in the pathogenesis of SCC in FA patients.
• Benign and malignant liver tumors (adenomas, hepatomas) frequently occur, typically associated with the use of androgen therapy for aplastic anemia.
• The risk of developing solid tumors further increases after hematopoietic stem cell transplantation (HSCT).

Diagnostic Evaluation

Screening and Definitive Tests

• Chromosomal Breakage Test: The gold-standard diagnostic screening test involves culturing peripheral blood T-lymphocytes with DNA cross-linking agents (DEB or MMC). Positive results show significantly enhanced chromosomal fragility, including chromatid breaks, gaps, rearrangements, radials, and endoreduplication.
• Flow Cytometry: Alkylating agent–treated cells can be analyzed via flow cytometry to detect an abnormal accumulation of cells arrested in the G2/M phase of the cell cycle.
• Somatic Mosaicism: Approximately 10–15% of patients possess somatic mosaicism (a mixed population of cells where some hematopoietic stem cells underwent spontaneous gene correction). These patients may yield false-negative chromosomal breakage results in peripheral blood. In such cases, testing must be performed on cultured skin fibroblasts.
• Molecular Diagnosis: Next-generation sequencing (NGS) gene panels, whole exome sequencing (WES), complementation group analysis, and targeted mutation analysis are utilized to establish the definitive genetic diagnosis and exact FANC gene involved.
• Prenatal Diagnosis: The DEB test or genetic testing can be performed on amniotic fluid cells or chorionic villus samples.

Additional Investigations

• Complete Blood Count (CBC): Reveals pancytopenia, absolute reticulocytopenia, and elevated fetal hemoglobin (HbF).
• Bone Marrow Aspirate and Biopsy: Required to assess overall cellularity (fatty replacement is seen in severe aplasia) and to evaluate for multilineage dysplasia or leukemic transformation.
• Cytogenetics/FISH: Essential to screen for clonal abnormalities predictive of leukemia (e.g., monosomy 7, 3q abnormalities, structural rearrangements).
• Systemic Screening: Upon diagnosis, all patients require baseline echocardiography, abdominal and renal ultrasound, hearing evaluation, and endocrine assessment (growth hormone, thyroid function).

Differential Diagnosis

• Thrombocytopenia-Absent Radius (TAR) Syndrome: TAR strongly mimics FA but can be distinguished because patients with TAR have bilateral absent radii but always possess normal, present thumbs. In FA, radial anomalies only occur in conjunction with abnormal or absent thumbs. TAR patients also do not exhibit increased chromosomal breakage.
• Dyskeratosis Congenita (DC): Features the classic mucocutaneous triad (nail dystrophy, lacy reticular pigmentation, oral leukoplakia) and is diagnosed by extremely short telomeres (<1st percentile), lacking the DEB hypersensitivity seen in FA.
• Diamond-Blackfan Anemia (DBA): Presents primarily as pure red cell aplasia in the first year of life with elevated erythrocyte adenosine deaminase (eADA) activity, without the chromosomal breakage defects of FA.
• Other Causes: Acquired severe aplastic anemia, Shwachman-Diamond syndrome, and VACTERL association.

Management and Treatment

Surveillance and Supportive Care

• Hematologic Monitoring: Patients with mild to moderate cytopenias require CBCs every 3 to 4 months and annual bone marrow aspirations (with cytogenetics/FISH) to monitor for MDS or AML.
• Cancer Screening: Annual rigorous physical exams targeting the oral cavity, skin, and thyroid. Post-pubertal females require annual gynecologic examinations and breast exams. Regular endoscopic screening of the esophagus is advised for older patients.
• Vaccination: Administration of the HPV quadrivalent vaccine is strongly advised to reduce the risk of head, neck, and genital squamous cell carcinomas.
• Transfusion Therapy: Red blood cell and platelet transfusions should be judicious and minimized to prevent alloimmunization and iron overload. All blood products must be leukodepleted, irradiated, and preferably from single unrelated donors.
• Environmental Precautions: Patients must avoid exposure to potential mutagens, DNA-damaging carcinogens, ionizing radiation (limit unnecessary CT scans; use MRI when possible), and tobacco.

Medical Therapy

• Androgen Therapy: Oxymetholone (2–5 mg/kg/day) or Danazol can induce a hematologic response in approximately 50–70% of patients.
• Reticulocytosis and a rise in hemoglobin are typically seen within 1–2 months, followed by improvements in white blood cell and platelet counts.
• Androgens are not curative and serve primarily as a bridge to transplant. Patients eventually become refractory due to progressive stem cell depletion.
• Severe side effects require close monitoring and include virilization/masculinization, growth spurts (premature epiphyseal closure), mood swings, elevated liver enzymes, cholestasis, peliosis hepatis, and hepatic adenomas/carcinomas.
• Cytokines: Granulocyte Colony-Stimulating Factor (G-CSF) at 5 μg/kg/day can be used to treat severe neutropenia but carries a theoretical risk of promoting clonal expansion and transformation to MDS/AML (such as monosomy 7 clones).

Hematopoietic Stem Cell Transplantation (HSCT)

• Allogeneic HSCT is the only curative therapy for the bone marrow failure, MDS, and AML associated with FA.
• Early HLA-typing of the patient and family members is mandatory at the time of diagnosis.
• Because FA cells are exquisitely sensitive to DNA-damaging agents, traditional myeloablative conditioning regimens cause fatal toxicity. Specialized reduced-intensity conditioning (RIC) regimens utilizing fludarabine, low-dose cyclophosphamide, and antithymocyte globulin (ATG)—usually avoiding total body irradiation—are standard.
• Transplants performed using an HLA-matched unaffected sibling donor yield a long-term disease-free survival rate exceeding 90%.
• Due to significant improvements in high-resolution HLA typing and T-cell depletion strategies, outcomes using matched unrelated donors (MUD) and haploidentical donors have improved dramatically, achieving survival rates greater than 80%.
• Ideal timing for HSCT is before the patient receives massive amounts of transfusions (preventing alloimmunization) and prior to the development of MDS or overt leukemia, preferably before age 10.
• Post-HSCT, patients remain at a highly accelerated risk for secondary solid tumors (particularly head and neck SCC), compounded by the effects of conditioning chemotherapy and chronic graft-versus-host disease (GVHD).

Investigational Therapies

• Gene Therapy: Clinical trials utilizing autologous CD34+ hematopoietic stem cells transduced ex vivo with lentiviral vectors (e.g., to correct the FANCA gene) have shown preliminary promise, though cell collection yields remain a limiting factor.