Fanconi Anemia via the RAD51/FANCR Gene

Fanconi Anemia (FA) is an inherited anemia associated with bone marrow failure (aplastic anemia). However, FA is also characterized by a range of other hematologic and physical abnormalities and predisposition to cancers - particularly acute myelogenous leukemia (AML), gynecologic and GI tract cancers, and cancers of the head and neck (Auerbach 2009). FA patients are up to 800 fold more susceptible to AML than the general population with a median age of onset of 13 years (Rosenberg et al. 2003). Physical abnormalities include radial ray defects (absent thumb or radius), skin pigmentation defects, short stature, microphthalmia, renal and urinary tract defects, genital defects (males in particular), gastrointestinal malformations (atresia), congenital heart disease, hearing and central nervous system defects, and general developmental delay (Tischkowitz and Hodgson 2003; Dokal 2000). About one-third of FA patients have no obvious physical abnormalities and are diagnosed only after a family member is diagnosed, or after developing hematologic abnormalities such as thromobocytopenia, leukopenia, or anemia (Giampietro et al. 1997). A hallmark of FA is hypersensitivity of chromosomes to inter cross-strand linkage (ICL) agents such as diepoxybutane (DEB) or mitomycin C (MMC) (Sasaki and Tonomura 1973). Exposure of primary cell cultures from FA patients to DEB or MMC results in chromosomal aberrations (breaks, radials, rearrangements) due to damaged DNA repair mechanisms that require functional products of the Fanconi anemia genes. For example, the FANCA, -B, -C, -E, -F, -G, -L, and -M proteins are part of a nuclear core complex that regulates monoubiquitination of the FANCD2 and FANCI proteins (ID complex) during S-phase and after exposure to DNA crosslinking agents (Moldovan and D'Andrea 2009). In unaffected individuals, ubiquitination helps localize the ID complex to sites of DNA damage and facilitate repair (Grompe and van de Vrugt 2007; Smogorzewska et al. 2007), but in FA patients, this mechanism is impaired.

FA is a genetically heterogeneous disorder. To date, 21 FA or FA-like genes have been identified. Inheritance is primarily autosomal recessive or X-linked, however one case of FA-like syndrome was associated with a dominant-negative pathogenic variant in the RAD51 (FANCR) gene (Ameziane et al. 2015). The RAD51 patient described in Ameziane et al. did not develop bone marrow failure or cancers that are typically associated with FA. Rather, this patient presented with multiple congenital anomalies including microcephaly, skeletal anomalies, hydrocephalus, imperforate anus, growth retardation, and learning disability. During DNA repair, the RAD51 protein plays a role in strand exchange during homologous recombination (Jasin and Rothstein 2013). Through a series of cell based and biochemical assays, Ameziane et al. showed that several functions of RAD51 protein are disrupted by the p.Ala293Thr substitution found in their patient; in particular the variant protein has decreased ATPase activity and lower DNA-binding affinity and acts as a dominant negative for the RAD51 protein. To date, only one RAD51/FANCR FA patient has been identified making RAD51 a rare cause of disease. Approximately 86% of all FA cases are attributed to variants in three genes: FANCA (~ 60%), FANCC (~ 16%), and FANCG (~ 10%) (Auerbach 2009). Since variants in FANCA are the most common cause of FA, it is important to note that large deletions make up over one-third of all reported variants in FANCA. In the United States, the carrier frequency for Fanconi anemia is estimated at 1 in 181 and the incidence rate is estimated at 1 in 131,000 (http://www.fanconi.org/; Rosenberg et al. 2011). Nearly 95% of all FA cases are attributed to variants in eight genes, FANCA, -C, -G, -D1 (aka BRCA2), -D2, -E, -F, and –L that are either part of the core complex required for ID complex ubiquitination and facilitation of DNA repair or function directly in ICL recognition and repair (Grompe and van de Vrugt 2007). FA is phenotypically diverse even among related patients that harbor a common variant; null alleles however are reported to result in more severe phenotypes (Faivre et al. 2000). FA affects males and females roughly equally and affects all ethnic groups.

Over 95% of all Fanconi Anemia (FA) patients harbor pathogenic variants in one of the 21 known FA or FA-like genes (www.fanconi.org). To date, only one RAD51/FANCR FA patient has been identified making RAD51 a rare cause of disease.

This test provides full coverage of all coding exons of the RAD51 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).