Fanconi Anemia

Genetics and Pathophysiology

• Inherited bone marrow failure syndrome characterized by genomic instability.
• Autosomal recessive inheritance primarily represents over 99% of cases.
• Rare X-linked recessive (FANCB) and autosomal dominant (FANCR/RAD51) inheritance identified.
• Twenty-two FANC genes identified; FANCA, FANCC, and FANCG represent over 90% of cases.
• Gene products assemble to form core complex necessary for DNA damage repair.
• Core complex monoubiquitinates FANCD2 and FANCI, stabilizing stalled replication forks.
• Defective pathway results in faulty DNA damage response.
• Cells exhibit extreme hypersensitivity to DNA crosslinking agents, oxygen-free radicals, and ionizing radiation.

Epidemiology

• Estimated frequency represents 1 in 200,000 in most populations.
• Higher carrier frequencies noted in specific populations due to founder effects.
• Ashkenazi Jews feature high FANCC and FANCD1 mutations.
• Afrikaners and Spanish Gypsies feature high FANCA mutations.

Clinical Manifestations

Somatic Anomalies

• Up to 40% of patients lack obvious physical abnormalities.
• Skeletal anomalies: short stature, abnormal radii, thumb deformities.
• Radial ray defects represent terminal defects; abnormal radii strictly associate with thumb anomalies (hypoplastic, supernumerary, bifid, absent).
• Skin features: generalized hyperpigmentation, café-au-lait spots, hypopigmented areas.
• Craniofacial anomalies: microcephaly, microphthalmia, epicanthal folds, micrognathia, abnormal ears.
• Renal malformations: ectopic, pelvic, horseshoe, hypoplastic, or absent kidneys.
• Genitourinary defects: male hypogenitalism, hypospadias, female reproductive tract malformations, reduced fertility.
• VACTERL or VACTERL-H associations present in affected neonates.
• Endocrine dysfunction: growth hormone deficiency, hypothyroidism.

Hematologic Findings

• Median age at hematologic presentation occurs at 8-10 years.
• Initial manifestations include macrocytosis, elevated fetal hemoglobin, and thrombocytopenia.
• Progresses gradually to severe pancytopenia and aplastic anemia.

Malignancy Predisposition

• Exceedingly high risk of malignant transformation.
• Cumulative incidence of clonal/malignant myeloid transformation (myelodysplastic syndrome, acute myeloid leukemia) reaches 75% by age 18 years.
• High relative risk of solid tumors developing at remarkably young ages.
• Squamous cell carcinomas frequently affect head, neck, esophagus, vulva, anus, and cervix.
• Human papillomavirus suspected in squamous cell carcinoma pathogenesis.
• Liver tumors (adenomas, hepatomas) associate strongly with prolonged androgen therapy.

Diagnostic Evaluation

Primary Screening Modalities

• Chromosome breakage test serves as gold standard screening.
• Peripheral blood lymphocytes cultured with DNA crosslinking agents (diepoxybutane or mitomycin C).
• Exhibits increased spontaneous chromatid breaks, gaps, radials, endoreduplications, and exchanges.
• Fibroblast testing indicated if somatic mosaicism suspected; mosaicism produces false-negative lymphocyte results.
• Flow cytometry demonstrates increased G2/M cell cycle arrest upon exposure to alkylating agents.

Definitive Genetic Testing

• Next-generation sequencing gene panels or whole-exome sequencing provide definitive diagnosis.
• Complementation group analysis and targeted mutation analysis utilized historically.

Hematologic Assessment

• Complete blood counts reveal progressive cytopenias and macrocytosis.
• Bone marrow aspirate and biopsy demonstrate hypocellularity, fatty replacement, and dysplastic precursors.

Differential Diagnosis

Feature	Fanconi Anemia	Thrombocytopenia-Absent Radii Syndrome
Age of onset	Median 8-10 years	Birth to infancy
Radial ray defect	Absent radii with absent or hypoplastic thumbs (terminal defect)	Absent radii with fingers and thumbs present (intercalary defect)
Hematology	Pancytopenia, macrocytosis	Thrombocytopenia, leukemoid reactions
Chromosome breaks	Present with clastogens	Absent
Malignancy risk	High (leukemia, solid tumors)	Rare
Hemoglobin F	Increased	Normal

• Dyskeratosis congenita: distinguished by extreme telomere shortening and classic mucocutaneous triad (nail dystrophy, oral leukoplakia, reticular skin pigmentation).
• Diamond-Blackfan anemia: presents typically in first year of life as isolated pure red cell aplasia with normal chromosome breakage studies.

Management Strategies

Medical and Supportive Care

• Serial monitoring of blood counts every 3-4 months initially.
• Minimize red cell and platelet transfusions to prevent allosensitization and iron overload.
• Utilize leukocyte-depleted, irradiated, single-donor blood products.
• Androgen therapy (oxymetholone, danazol): improves bone marrow function in approximately 50-70% of patients.
• Androgens require careful tapering to lowest effective dose; side effects include virilization, peliosis hepatis, and liver tumors.
• Cytokine therapy (granulocyte colony-stimulating factor): used for severe neutropenia, though carries theoretical risk of promoting clonal myeloid expansion.

Curative Therapy

• Hematopoietic stem cell transplantation remains the exclusive curative therapy for hematologic manifestations.
• Human leukocyte antigen typing indicated at diagnosis for patient and siblings.
• Standard myeloablative conditioning highly toxic due to DNA repair defect.
• Reduced-intensity conditioning regimens (fludarabine, low-dose cyclophosphamide, antithymocyte globulin) improve survival and reduce toxicity.
• Stem cell transplantation does not cure solid tumor predisposition; actually increases post-transplant risk of squamous cell carcinomas.

Novel Therapies

• Gene therapy utilizing lentiviral vectors for FANCA correction currently under experimental clinical trials.

Long-Term Surveillance Protocol

System	Surveillance Strategy	Frequency
Hematologic	Bone marrow aspirate, biopsy, and cytogenetics (evaluate for clonal abnormalities like monosomy 7 or 13q/11q deletions)	Annually
Hematologic	Complete blood counts	Every 3-4 months
Oncologic	Comprehensive physical examination for solid tumors	Annually
Oncologic	Head, neck, and oral cancer screening	Annually (starting age 7-10 years)
Oncologic	Gynecologic examination and cervical cancer screening	Annually (starting at menarche or age 16)
Endocrine	Evaluation for growth hormone and thyroid deficiency	Annually
Hepatic	Liver ultrasound and enzyme evaluation (especially if on androgens)	Periodic

• Administration of human papillomavirus quadrivalent vaccine strongly advised to mitigate squamous cell carcinoma risk.
• Avoid exposure to environmental mutagens, insecticides, and unnecessary radiologic imaging.

Prognosis

• Hematologic survival significantly improved with modern reduced-intensity hematopoietic stem cell transplantation.
• Actuarial long-term disease-free survival for matched sibling transplantation approaches 90%.
• Morbidity and mortality heavily driven by nonhematologic solid tumors emerging in the third and fourth decades of life.
• Transplant recipients face accelerated risk of head and neck cancers, occurring approximately 15 years earlier than non-transplanted patients.