Phenotypic consequences in black South African Fanconi anemia patients homozygous for a founder mutation

Fanconi anemia and dyskeratosis congenita/telomere biology disorders: Two inherited bone marrow failure syndromes with genomic instability

Inherited bone marrow failure syndromes (IBMFS) are a complex and heterogeneous group of genetic diseases. To date, at least 13 IBMFS have been characterized. Their pathophysiology is associated with germline pathogenic variants in genes that affect hematopoiesis. A couple of these diseases also have genomic instability, Fanconi anemia due to DNA damage repair deficiency and dyskeratosis congenita/telomere biology disorders as a result of an alteration in telomere maintenance. Patients can have extramedullary manifestations, including cancer and functional or structural physical abnormalities. Furthermore, the phenotypic spectrum varies from cryptic features to patients with significantly evident manifestations. These diseases require a high index of suspicion and should be considered in any patient with abnormal hematopoiesis, even if extramedullary manifestations are not evident. This review describes the disrupted cellular processes that lead to the affected maintenance of the genome structure, contrasting the dysmorphological and oncological phenotypes of Fanconi anemia and dyskeratosis congenita/telomere biology disorders. Through a dysmorphological analysis, we describe the phenotypic features that allow to make the differential diagnosis and the early identification of patients, even before the onset of hematological or oncological manifestations. From the oncological perspective, we analyzed the spectrum and risks of cancers in patients and carriers.

Fanconi anemia: current insights regarding epidemiology, cancer, and DNA repair

Fanconi anemia is a genetic disorder that is characterized by bone marrow failure, as well as a predisposition to malignancies including leukemia and squamous cell carcinoma (SCC). At least 22 genes are associated with Fanconi anemia, constituting the Fanconi anemia DNA repair pathway. This pathway coordinates multiple processes and proteins to facilitate the repair of DNA adducts including interstrand crosslinks (ICLs) that are generated by environmental carcinogens, chemotherapeutic crosslinkers, and metabolic products of alcohol. ICLs can interfere with DNA transactions, including replication and transcription. If not properly removed and repaired, ICLs cause DNA breaks and lead to genomic instability, a hallmark of cancer. In this review, we will discuss the genetic and phenotypic characteristics of Fanconi anemia, the epidemiology of the disease, and associated cancer risk. The sources of ICLs and the role of ICL-inducing chemotherapeutic agents will also be discussed. Finally, we will review the detailed mechanisms of ICL repair via the Fanconi anemia DNA repair pathway, highlighting critical regulatory processes. Together, the information in this review will underscore important contributions to Fanconi anemia research in the past two decades.

Fanconi Anemia Patients from an Indigenous Community in Mexico Carry a New Founder Pathogenic Variant in FANCG

Fanconi anemia (FA) is a rare genetic disorder caused by pathogenic variants (PV) in at least 22 genes, which cooperate in the Fanconi anemia/Breast Cancer (FA/BRCA) pathway to maintain genome stability. PV in FANCA, FANCC, and FANCG account for most cases (~90%). This study evaluated the chromosomal, molecular, and physical phenotypic findings of a novel founder FANCG PV, identified in three patients with FA from the Mixe community of Oaxaca, Mexico. All patients presented chromosomal instability and a homozygous PV, FANCG: c.511-3_511-2delCA, identified by next-generation sequencing analysis. Bioinformatic predictions suggest that this deletion disrupts a splice acceptor site promoting the exon 5 skipping. Analysis of Cytoscan 750 K arrays for haplotyping and global ancestry supported the Mexican origin and founder effect of the variant, reaffirming the high frequency of founder PV in FANCG. The degree of bone marrow failure and physical findings (described through the acronyms VACTERL-H and PHENOS) were used to depict the phenotype of the patients. Despite having a similar frequency of chromosomal aberrations and genetic constitution, the phenotype showed a wide spectrum of severity. The identification of a founder PV could help for a systematic and accurate genetic screening of patients with FA suspicion in this population.

Endocrine profiling in patients with Fanconi anemia, homozygous for a FANCG founder mutation

BACKGROUND

Fanconi anemia (FA) is phenotypically diverse, hereditary condition associated with bone marrow failure, multiple physical abnormalities, and an increased susceptibility to the development of malignancies. Less recognized manifestations of FA include endocrine abnormalities. International discourse has highlighted that these abnormalities are widespread among children and adults with FA. To date there has been no systematic study that has evaluated the endocrine abnormalities in a cohort of patients with FA, homozygous for a founder mutation (c.637_643del (p.Tyr213Lysfs*6)) in FANCG. The objectives of the study were to evaluate endocrine gland function in patients with FA of a single FA genotype, and to determine the frequency and nature of endocrine abnormalities in this group.

METHODS

Cross-sectional, descriptive study of 24 South African patients of African ancestry with FA (homozygous for a FANCG founder mutation). Outcomes measured included growth, pubertal status, growth hormone axis screening, thyroid gland function, glucose and insulin metabolism and bone age (BA).

RESULTS

Endocrine dysfunction was present in 70.8% (17 of 24), including abnormal insulin-like growth factor 1 (IGF-1)/insulin-like growth factor-binding protein 3 (IGFBP-3) in 25.0% (6 of 24), insulin resistance in 41.7% (10 of 24), abnormal thyroid function in 16.7% (4 of 24) and short stature in 45.8% (11 of 24). No abnormalities of glucose metabolism were identified. Abnormal pubertal status was seen in three males (12.5%). Abnormal BAs were present in 34.8% (8 of 23).

CONCLUSION

Endocrine abnormalities occur at a high frequency in patients with FA, homozygous for a FANCG founder mutation, similar to other FA cohorts. Our data are specific to FA patients with a single genotype, and therefore provide the first genotype-phenotype information on endocrine abnormalities in South African patients, homozygous for a FANCG founder mutation. Recommendations regarding endocrine screening in this patient subgroup are made, including, but not limited to, baseline testing of thyroid function, fasted insulin and glucose, and IGF-1 and IGFBP-3.

Common and Founder Mutations for Monogenic Traits in Sub-Saharan African Populations

This review highlights molecular genetic studies of monogenic traits where common pathogenic mutations occur in black families from sub-Saharan Africa. Examples of founder mutations have been identified for oculocutaneous albinism, cystic fibrosis, Fanconi anemia, and Gaucher disease. Although there are few studies from Africa, some of the mutations traverse populations across the continent, and they are almost all different from the common mutations observed in non-African populations. Myotonic dystrophy is curiously absent among Africans, and nonsyndromic deafness does not arise from mutations in GJB2 and GJB7. Locus heterogeneity is present for Huntington disease, with two common triplet expansion loci in Africa, HTT and JPH3. These findings have important clinical consequences for diagnosis, treatment, and genetic counseling in affected families. We currently have just a glimpse of the molecular etiology of monogenic diseases in sub-Saharan Africa, a proverbial “ears of the hippo” situation.

Biallelic BRCA2 mutations in two black South African children with Fanconi anaemia

Fanconi anaemia (FA) is a genotypically and phenotypically heterogeneous genetic condition, characterized cytogenetically by chromosomal instability and breakage secondary to impaired DNA repair mechanisms. Affected individuals typically manifest growth restriction and congenital physical abnormalities and most progress to hematological disease including bone marrow aplasia. A rare genetic subtype of FA (FA-D1) is caused by biallelic mutations in the BRCA2 gene. Affected individuals manifest severe congenital anomalies and significant pigmentary changes and are additionally at risk for early onset leukemia and certain solid organ malignancies, including Wilms tumors and brain tumors. Parents of affected individuals are obligate carriers for heterozygous BRCA2 mutations and are thus potentially at risk for adult onset cancers which fall within the hereditary breast and ovarian cancer spectrum. We present two cases of black South African patients with FA diagnosed with biallelic BRCA2 mutations and discuss the phenotypic consequences and implications for them and their families. Recognition of this severe end of the phenotypic spectrum of FA is critical in allowing for confirmation of the diagnosis as well as cascade screening and appropriate care of family members.

Founder haplotype analysis of Fanconi anemia in the Korean population finds common ancestral haplotypes for a FANCG variant

A common ancestral haplotype is strongly suggested in the Korean and Japanese patients with Fanconi anemia (FA), because common mutations have been frequently found: c.2546delC and c.3720_3724delAAACA of FANCA; c.307+1G>C, c.1066C>T, and c.1589_1591delATA of FANCG. Our aim in this study was to investigate the origin of these common mutations of FANCA and FANCG. We genotyped 13 FA patients consisting of five FA-A patients and eight FA-G patients from the Korean FA population. Microsatellite markers used for haplotype analysis included four CA repeat markers which are closely linked with FANCA and eight CA repeat markers which are contiguous with FANCG. As a result, Korean FA-A patients carrying c.2546delC or c.3720_3724delAAACA did not share the same haplotypes. However, three unique haplotypes carrying c.307+1G>C, c.1066C > T, or c.1589_1591delATA, that consisted of eight polymorphic loci covering a flanking region were strongly associated with Korean FA-G, consistent with founder haplotypes reported previously in the Japanese FA-G population. Our finding confirmed the common ancestral haplotypes on the origins of the East Asian FA-G patients, which will improve our understanding of the molecular population genetics of FA-G. To the best of our knowledge, this is the first report on the association between disease-linked mutations and common ancestral haplotypes in the Korean FA population.

Hematological consequences of a FANCG founder mutation in Black South African patients with Fanconi anemia

Fanconi anemia (FA) is a rare disorder of DNA repair, associated with various somatic abnormalities but characterized by hematological disease that manifests as bone marrow aplasia and malignancy. The mainstay of treatment, in developed nations, is hematopoietic stem cell transplantation (HSCT) with subsequent surveillance for solid organ and non-hematological malignancies. In South Africa, FA in the Black population is caused by a homozygous deletion mutation in the FANCG gene in more than 80% of cases. Many affected patients are not diagnosed until late in the disease course when severe cytopenia and bone marrow aplasia are already present. Most patients are not eligible for HSCT at this late stage of the disease, even when it is available in the state health care system. In this study, the hematological presentation and disease progression in 30 Black South African patients with FA, confirmed to have the FANCG founder mutation, were evaluated and compared to those described in other FA cohorts. Our results showed that patients, homozygous for the FANCG founder mutation, present with severe cytopenia but progress to bone marrow failure at similar ages to other individuals affected with FA of heterogeneous genotype. Further, the incidence of myelodysplastic syndrome is similar to that which has been previously described in other FA cohorts. Although severe cytopenia at presentation may be predicted by a higher number of somatic anomalies, the recognition of the physical FA phenotype in Black South African patients is challenging and may not be useful in expediting referral of suspected FA patients for tertiary level investigations and care. Given the late but severe hematological presentation of FA in Black South African patients, an investigative strategy is needed for earlier recognition of affected individuals to allow for possible HSCT and management of bone marrow disease.