Molecular analysis and genotype-phenotype correlation of Diamond-Blackfan anemia

Diamond-Blackfan anemia in adults: In pursuit of a common approach for a rare disease

Diamond-Blackfan anemia (DBA) is a rare bone marrow failure syndrome, usually caused by loss-of function variants in genes encoding ribosomal proteins. The hallmarks of DBA are anemia, congenital anomalies and cancer predisposition. Although DBA usually presents in childhood, the prevalence in later life is increasing due to an expanding repertoire of implicated genes, improvements in genetic diagnosis and increasing life expectancy. Adult patients uniquely suffer the manifestations of end-organ damage caused by the disease and its treatment, and transition to adulthood poses specific issues in disease management. To standardize and optimize care for this rare disease, in this review we provide updated guidance on the diagnosis and management of DBA, with a specific focus on older adolescents and adults. Recommendations are based upon published literature and our pooled clinical experience from three centres in the United Kingdom (U·K.). Uniquely we have also solicited and incorporated the views of affected families, represented by the independent patient organization, DBA U.K.

Ribosomal Biogenesis and Heterogeneity in Development, Disease, and Aging

Although reported in the literature, ribosome heterogeneity is a phenomenon whose extent and implications in cell and organismal biology is not fully appreciated. This has been the case due to the lack of the appropriate techniques and approaches. Heterogeneity can arise from alternative use and differential content of protein and RNA constituents, as well as from post-transcriptional and post-translational modifications. In the few examples we have, it is apparent that ribosomal heterogeneity offers an additional level and potential for gene expression regulation and might be a way towards tuning metabolism, stress, and growth programs to external and internal stimuli and needs. Here, we introduce ribosome biogenesis and discuss ribosomal heterogeneity in various reported occasions. We conclude that a systematic approach in multiple organisms will be needed to delineate this biological phenomenon and its contributions to growth, aging, and disease. Finally, we discuss ribosome mutations and their roles in disease.

Utility of Whole Exome Sequencing in the Early Diagnosis of Atypical Diamond-Blackfan Anemia

Diamond-Blackfan anemia (DBA) is a rare congenital bone marrow failure syndrome, with a hallmark of erythroblastopenia and congenital anomalies. DBA demonstrates genetic heterogeneity and variable phenotypic expression. We present two cases of atypical DBA harboring de novo mutations in the RPS-19 gene with c.49 G>C and c.357-1G>T allelic variants. The two cases presented confounding critical illness demonstrated by multiorgan failure, aplastic crisis, with case 2 meeting the criteria for hemophagocytic lymphohistiocytosis. We highlight the utility of genetic testing in the early diagnosis of DBA and the associated complexities and burden of disease in caring for DBA patients.

Unusual Association of Diamond-Blackfan Anemia and Severe Sinus Bradycardia in a Six-Month-Old White Infant: A Case Report and Literature Review

Diamond-Blackfan anemia is a rare (6-7 million live births), inherited condition manifesting as severe anemia due to the impaired bone marrow production of red blood cells. We present the unusual case of a six month old infant with a de novo mutation of the RPS19 gene causing Diamond-Blackfan anemia who additionally suffers from severe sinus bradycardia. The infant was diagnosed with this condition at the age of four months; at the age of 6 months, she presents with severe anemia causing hypoxia which, in turn, caused severe dyspnea and polypnea, which had mixed causes (hypoxic and infectious) as the child was febrile. After correction of the overlapping diarrhea, metabolic acidosis, and severe anemia (hemoglobin < 3 g/dL), she developed severe persistent sinus bradycardia immediately after mild sedation (before central venous catheter insertion), not attributable to any of the more frequent causes, with a heart rate as low as 49 beats/min on 24 h Holter monitoring, less than the first percentile for age, but with a regular QT interval and no arrhythmia. The echocardiogram was unremarkable, showing a small interatrial communication (patent foramen ovale with left-to-right shunting), mild left ventricular hypertrophy, normal systolic and diastolic function, and mild tricuspid regurgitation. After red cell transfusion and appropriate antibiotic and supportive treatment, the child's general condition improved dramatically but the sinus bradycardia persisted. We consider this a case of well-tolerated sinus bradycardia and foresee a good cardiologic prognosis, while the hematologic prognosis remains determined by future corticoid response, treatment-related complications and risk of leukemia.

Variable Clinical Features in a Large Family With Diamond Blackfan Anemia Caused by a Pathogenic Missense Mutation in RPS19

Introduction: Diamond Blackfan anemia (DBA) is an autosomal dominant ribosomopathy caused predominantly by pathogenic germline variants in ribosomal protein genes. It is characterized by failure of red blood cell production, and common features include congenital malformations and cancer predisposition. Mainstays of treatment are corticosteroids, red blood cell transfusions, and hematologic stem cell transplantation (HSCT). Despite a better understanding of the genotype of DBA, the biological mechanism resulting in the clinical phenotype remains poorly understood, and wide heterogeneity can be seen even within a single family as depicted here.

Case Description: Thirty family members enrolled in the National Cancer Institute inherited bone marrow failure syndromes study were evaluated with detailed medical questionnaires and physical examinations, including 22 in the family bloodline and eight unrelated partners. Eight participants had been previously told they had DBA by clinical criteria. Targeted germline RPS19 testing was done on all family members. A pathogenic heterozygous missense mutation in RPS19 (p.R62Q, c.185G > A) was detected in ten family members, including one person previously presumed unaffected. Eight family members presented with macrocytic anemia in infancy; all of whom were responsive to prednisone. Four family members became treatment independent; however, one individual became transfusion-dependent 36 years later following an episode of pneumonia. One prednisone responsive individual electively discontinued steroid treatment, and lives with severe anemia. One prednisone responsive individual died at age 28 from a stroke. Two family members developed colorectal cancer in their fifties; one had never required treatment for anemia. None had major congenital anomalies.

Discussion: This large family with DBA demonstrates the heterogeneity of phenotypes that can be seen within the same genotype. Most family members presented with steroid-responsive anemia in infancy and subtle congenital malformations, findings consistent with recent genotype-phenotype studies of RPS DBA. However, two family members were relatively unaffected, underscoring the importance of further studies to assess modifier genes, and epigenetic and/or environmental factors which may result in normal erythropoiesis despite underlying ribosome dysfunction. This large, multigenerational family highlights the need for individualized treatment, the importance of early cancer surveillance even in individuals with clinically mild phenotypes, and the benefit of long-term follow-up to identify late complications.

Studies in a mosaic DBA patient and chimeric mice reveal erythroid cell-extrinsic contributions to erythropoiesis

We follow a Diamond Blackfan anemia (DBA) patient mosaic for a pathogenic RPS19 haploinsufficiency mutation with persistent transfusion-dependent anemia. Her anemia remitted on eltrombopag (EPAG), but surprisingly mosaicism was unchanged, suggesting both mutant and normal cells responded. When EPAG was held, her anemia returned. In addition to expanding hematopoietic stem/progenitor cells, EPAG aggressively chelates iron. Since DBA anemia, at least in part, results from excessive intracellular heme leading to ferroptotic cell death, we hypothesized that the excess heme accumulating in ribosomal protein-deficient erythroid precursors inhibited the growth of adjacent genetically-normal precursors, and that the efficacy of EPAG reflected its ability to chelate iron, limit heme synthesis, and thus limit toxicity in both mutant and normal cells. To test this, we studied Rpl11 haploinsufficient (DBA) mice and mice chimeric for the cytoplasmic heme export protein, FLVCR. Flvcr1-deleted mice have severe anemia, resembling DBA. Mice transplanted with ratios of DBA to wildtype marrow cells of 50:50 are anemic, like our DBA patient. In contrast, mice transplanted with Flvcr1-deleted (unable to export heme) and wildtype marrow cells at ratios of 50:50 or 80:20 have normal numbers of red cells. Additional studies suggest that heme exported from DBA erythroid cells might impede the nurse cell function of central macrophages of erythroblastic islands to impair the maturation of genetically-normal co-adherent erythroid cells. These findings have implications for the gene therapy of DBA and may provide insights into why del(5q) myelodysplastic syndrome patients are anemic despite being mosaic for chromosome 5q deletion and loss of RPS14.

Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is an inherited bone marrow failure syndrome, characterized as a rare congenital bone marrow erythroid hypoplasia (OMIM#105650). Erythroid defect in DBA results in erythroblastopenia in bone marrow as a consequence of maturation blockade between the burst forming unit-erythroid and colony forming unit-erythroid developmental stages, leading to moderate to severe usually macrocytic aregenerative (<20 × 109/L of reticulocytes) anemia. Congenital malformations localized mostly in the cephalic area and in the extremities (thumbs), as well as short stature and cardiac and urogenital tract abnormalities, are a feature of 50% of the DBA-affected patients. A significant increased risk for malignancy has been reported. DBA is due to a defect in the ribosomal RNA (rRNA) maturation as a consequence of a heterozygous mutation in 1 of the 20 ribosomal protein genes. Besides classical DBA, some DBA-like diseases have been identified. The relation between the defect in rRNA maturation and the erythroid defect in DBA has yet to be fully defined. However, recent studies have identified a role for GATA1 either due to a specific defect in its translation or due to its defective regulation by its chaperone HSP70. In addition, excess free heme-induced reactive oxygen species and apoptosis have been implicated in the DBA erythroid phenotype. Current treatment options are either regular transfusions with appropriate iron chelation or treatment with corticosteroids starting at 1 year of age. The only curative treatment for the anemia of DBA to date is bone marrow transplantation. Use of gene therapy as a therapeutic strategy is currently being explored.

A new murine Rpl5 (uL18) mutation provides a unique model of variably penetrant Diamond-Blackfan anemia

Ribosome dysfunction is implicated in multiple abnormal developmental and disease states in humans. Heterozygous germline mutations in genes encoding ribosomal proteins are found in most individuals with Diamond-Blackfan anemia (DBA), whereas somatic mutations have been implicated in a variety of cancers and other disorders. Ribosomal protein-deficient animal models show variable phenotypes and penetrance, similar to human patients with DBA. In this study, we characterized a novel ENU mouse mutant (Skax23m1Jus) with growth and skeletal defects, cardiac malformations, and increased mortality. After genetic mapping and whole-exome sequencing, we identified an intronic Rpl5 mutation, which segregated with all affected mice. This mutation was associated with decreased ribosome generation, consistent with Rpl5 haploinsufficiency. Rpl5Skax23-Jus/+ animals had a profound delay in erythroid maturation and increased mortality at embryonic day (E) 12.5, which improved by E14.5. Surviving mutant animals had macrocytic anemia at birth, as well as evidence of ventricular septal defect (VSD). Surviving adult and aged mice exhibited no hematopoietic defect or VSD. We propose that this novel Rpl5Skax23-Jus/+ mutant mouse will be useful in studying the factors influencing the variable penetrance that is observed in DBA.

Daptomycin suppresses tumor migration and angiogenesis via binding to ribosomal protein S19 in humans

We have previously reported that daptomycin (DAP), a last resort antibiotic, binds to ribosomal protein S19 (RPS19) in humans and exhibits selective anti-cancer activity against MCF7 breast cancer cells. Here, we investigated the role of RPS19 in the anti-cancer effects of DAP and have found that DAP does not induce autophagy, apoptosis or cell viability but does reduce cell proliferation. Our results suggest that an extraribosomal function of RPS19 involves the regulation of vascular endothelial growth factor (VEGF) but not EGF, PDGF or FGF. Engagement of RPS19 by DAP was shown by CETSA and ITDRF_CETSA assays, and knocking down of RPS19 with siRNA increased the potency of DAP in MCF7 cells. In addition, DAP suppressed the secretion of VEGF in cancer cells and thereby inhibited cell migration. Collectively, these data provide an outline of the underlying mechanism of how DAP exhibits anti-cancer activity and suggests that RPS19 could be a promising target for the development of new anticancer drugs.

Genotype-phenotype association and variant characterization in Diamond-Blackfan anemia caused by pathogenic variants in RPL35A

Diamond Blackfan anemia (DBA) is predominantly an autosomal dominant inherited red cell aplasia primarily caused by pathogenic germline variants in ribosomal protein genes. DBA due to pathogenic RPL35A variants has been associated with large 3q29 deletions and phenotypes not common in DBA. We conducted a multi-institutional genotypephenotype study of 45 patients with DBA associated with pathogenic RPL35A germline variants and curated the variant data on 21 additional cases from the literature. Genotype-phenotype analyses were conducted comparing patients with large deletions versus all other pathogenic variants in RPL35A. Twenty-two of the 45 cases had large deletions in RPL35A. After adjusting for multiple tests, a statistically significant association was observed between patients with a large deletion and steroid-resistant anemia, neutropenia, craniofacial abnormalities, chronic gastrointestinal problems, and intellectual disabilities (P<0.01) compared with all other pathogenic variants. Non-large deletion pathogenic variants were spread across RPL35Awith no apparent hot spot and 56% of the individual family variants were observed more than once. In this, the largest known study of DBA patients with pathogenic RPL35A variants, we determined that patients with large deletions have a more severe phenotype that is clinically different from those with non-large deletion variants. Genes of interest also deleted in the 3q29 region that could be associated with some of these phenotypic features include LMLN and IQCG. Management of DBA due to large RPL35A deletions may be challenging due to complex problems and require comprehensive assessments by multiple specialists including immunological, gastrointestinal, and developmental evaluations to provide optimal multidisciplinary care.

The Interplay between Drivers of Erythropoiesis and Iron Homeostasis in Rare Hereditary Anemias: Tipping the Balance

Rare hereditary anemias (RHA) represent a group of disorders characterized by either impaired production of erythrocytes or decreased survival (i.e., hemolysis). In RHA, the regulation of iron metabolism and erythropoiesis is often disturbed, leading to iron overload or worsening of chronic anemia due to unavailability of iron for erythropoiesis. Whereas iron overload generally is a well-recognized complication in patients requiring regular blood transfusions, it is also a significant problem in a large proportion of patients with RHA that are not transfusion dependent. This indicates that RHA share disease-specific defects in erythroid development that are linked to intrinsic defects in iron metabolism. In this review, we discuss the key regulators involved in the interplay between iron and erythropoiesis and their importance in the spectrum of RHA.

Human mutational constraint as a tool to understand biology of rare and emerging bone marrow failure syndromes

Inherited bone marrow failure (IBMF) syndromes are rare blood disorders characterized by hematopoietic cell dysfunction and predisposition to hematologic malignancies. Despite advances in the understanding of molecular pathogenesis of these heterogeneous diseases, genetic variant interpretation, genotype-phenotype correlation, and outcome prognostication remain difficult. As new IBMF and other myelodysplastic syndrome (MDS) predisposition genes continue to be discovered (frequently in small kindred studies), there is an increasing need for a systematic framework to evaluate penetrance and prevalence of mutations in genes associated with IBMF phenotypes. To address this need, we analyzed population-based genomic data from >125 000 individuals in the Genome Aggregation Database for loss-of-function (LoF) variants in 100 genes associated with IBMF. LoF variants in genes associated with IBMF/MDS were present in 0.426% of individuals. Heterozygous LoF variants in genes in which haploinsufficiency is associated with IBMF/MDS were identified in 0.422% of the population; homozygous LoF variants associated with autosomal recessive IBMF/MDS diseases were identified in only .004% of the cohort. Using age distribution of LoF variants and 2 measures of mutational constraint, LOEUF ("loss-of-function observed/expected upper bound fraction") and pLI ("probability of being loss-of-function intolerance"), we evaluated the pathogenicity, tolerance, and age-related penetrance of LoF mutations in specific genes associated with IBMF syndromes. This analysis led to insights into rare IBMF diseases, including syndromes associated with DHX34, MDM4, RAD51, SRP54, and WIPF1. Our results provide an important population-based framework for the interpretation of LoF variant pathogenicity in rare and emerging IBMF syndromes.

Diamond-Blackfan anemia with mutation in RPS19: A case report and an overview of published pieces of literature

Introduction

Diamond-Blackfan anemia (DBA), one of a rare group of inherited bone marrow failure syndromes, is characterized by red cell failure, the presence of congenital anomalies, and cancer predisposition. It can be caused by mutations in the RPS19 gene (25% of the cases).

Methods

This case report describes a 10-month-old boy who presented with 2 months' history of gradually increasing weakness and pallor.

Results

The patient was diagnosed as a case of DBA based on peripheral blood finding, bone marrow aspiration with trephine biopsy reports, and genetic mutation analysis of the RPS19 gene. His father refused hematopoietic stem cell transplantation for financial constraints. Patient received prednisolone therapy with oral folic acid and iron supplements.

Conclusion

Hemoglobin raised from 6.7 to 9.8g/dL after 1 month of therapeutic intervention.

Genetic Predisposition to Myelodysplastic Syndrome in Clinical Practice

Myelodysplastic syndromes (MDSs) are a heterogeneous group of marrow failure disorders that primarily affect older persons but also occur at a lower frequency in children and young adults. There is increasing recognition of an inherited predisposition to MDS as well as other myeloid malignancies for patients of all ages. Germline predisposition to MDS can occur as part of a syndrome or sporadic disease. The timely diagnosis of an underlying genetic predisposition in the setting of MDS is important. This article delineates germline genetic causes of MDS and provides a scaffold for the diagnosis and management of patients in this context.

Diamond-Blackfan anemia caused by chromosome 1p22 deletion encompassing RPL5

Diamond-Blackfan anemia (DBA) is an inherited anemia with multiple congenital malformations, and mutations in ribosomal protein genes have been identified as the underlying cause. We describe a female patient with mild DBA due to 1p22 deletion, encompassing the gene encoding 60S ribosomal protein L5 (RPL5). Considering previously reported cases together with our patient, we suggest that RPL5 haploinsufficiency might cause a less severe form of DBA than loss-of-function mutations.

Identification of a novel RPS26 nonsense mutation in a Chinese Diamond-Blackfan Anemia patient

Background

Diamond-Blackfan anemia (DBA), a congenital pure red cell aplasia (PRCA), is characterized by normochromic macrocytic anemia, reticulocytopenia, and nearly absent erythroid progenitors in the bone marrow. DBA10, a subset of DBA, is an autosomal dominant disease caused by a mutation in RPS26. So far, there are 30 disease-causing variants in RPS26 being reported, however, only three of them are small insert mutations.

Case presentation

Here we report a three-month Chinese boy who presents with anemia from postnatal day 2. He was suspected to have Diamond-Blackfan anemia, according to the clinical result. Thus, whole-exome sequencing was performed for precise diagnosis.

Conclusion

Here, a novel insert mutation c.96dupG in RPS26 was identified by whole-exome sequencing, which caused neonatal DBA in a Chinese boy. This is the first case report of a Chinese DBA10 patient who carries a small insertion in the RPS26 gene. These findings expand the mutation diversity of RPS26 and demonstrate the clinical presentations of the Chinese DBA10 patient.

Electronic supplementary material

The online version of this article (10.1186/s12881-019-0848-1) contains supplementary material, which is available to authorized users.

The role of p53 in developmental syndromes

While it is well appreciated that loss of the p53 tumor suppressor protein promotes cancer, growing evidence indicates that increased p53 activity underlies the developmental defects in a wide range of genetic syndromes. The inherited or de novo mutations that cause these syndromes affect diverse cellular processes, such as ribosome biogenesis, DNA repair, and centriole duplication, and analysis of human patient samples and mouse models demonstrates that disrupting these cellular processes can activate the p53 pathway. Importantly, many of the developmental defects in mouse models of these syndromes can be rescued by loss of p53, indicating that inappropriate p53 activation directly contributes to their pathogenesis. A role for p53 in driving developmental defects is further supported by the observation that mouse strains with broad p53 hyperactivation, due to mutations affecting p53 pathway components, display a host of tissue-specific developmental defects, including hematopoietic, neuronal, craniofacial, cardiovascular, and pigmentation defects. Furthermore, germline activating mutations in TP53 were recently identified in two human patients exhibiting bone marrow failure and other developmental defects. Studies in mice suggest that p53 drives developmental defects by inducing apoptosis, restraining proliferation, or modulating other developmental programs in a cell type-dependent manner. Here, we review the growing body of evidence from mouse models that implicates p53 as a driver of tissue-specific developmental defects in diverse genetic syndromes.

How I manage children with Diamond-Blackfan anaemia

Diamond-Blackfan anaemia (DBA) is a rare inherited marrow failure disorder, characterized by hypoplastic anaemia, congenital anomalies and a predisposition to cancer as a result of ribosomal dysfunction. Historically, treatment is based on glucocorticoids and/or blood transfusions, which is accompanied by significant toxicity and long-term sequelae. Currently, stem cell transplantation is the only curative option for the haematological DBA phenotype. Whereas this procedure has been quite successful in the last decade in selected patients, novel therapies and biological insights are still warranted to improve clinical care for all DBA patients. In addition to paediatric haematologists, other physicians (e.g. endocrinologist, gynaecologist) should ideally be involved in the care of this chronic condition from an early age, to improve lifelong management of haematological and non-haematological symptoms, and screen for DBA-associated malignancies. Here we provide an overview of current knowledge and recommendations for the day-to-day care of DBA patients.

The Genetic Landscape of Diamond-Blackfan Anemia

Diamond-Blackfan anemia (DBA) is a rare bone marrow failure disorder that affects 7 out of 1,000,000 live births and has been associated with mutations in components of the ribosome. In order to characterize the genetic landscape of this heterogeneous disorder, we recruited a cohort of 472 individuals with a clinical diagnosis of DBA and performed whole-exome sequencing (WES). We identified relevant rare and predicted damaging mutations for 78% of individuals. The majority of mutations were singletons, absent from population databases, predicted to cause loss of function, and located in 1 of 19 previously reported ribosomal protein (RP)-encoding genes. Using exon coverage estimates, we identified and validated 31 deletions in RP genes. We also observed an enrichment for extended splice site mutations and validated their diverse effects using RNA sequencing in cell lines obtained from individuals with DBA. Leveraging the size of our cohort, we observed robust genotype-phenotype associations with congenital abnormalities and treatment outcomes. We further identified rare mutations in seven previously unreported RP genes that may cause DBA, as well as several distinct disorders that appear to phenocopy DBA, including nine individuals with biallelic CECR1 mutations that result in deficiency of ADA2. However, no new genes were identified at exome-wide significance, suggesting that there are no unidentified genes containing mutations readily identified by WES that explain >5% of DBA-affected case subjects. Overall, this report should inform not only clinical practice for DBA-affected individuals, but also the design and analysis of rare variant studies for heterogeneous Mendelian disorders.

Biallelic mutations in EXOC3L2 cause a novel syndrome that affects the brain, kidney and blood

BACKGROUND

Dandy-Walker malformation features agenesis/hypoplasia of the cerebellar vermis, cystic dilatation of the fourth ventricle and enlargement of posterior fossa. Although Dandy-Walker malformation is relatively common and several genes were linked to the syndrome, the genetic cause in the majority of cases is unknown.

OBJECTIVE

To identify the mutated gene responsible for Dandy-Walker malformation, kidney disease and bone marrow failure in four patients from two unrelated families.

METHODS

Medical assessment, sonographic, MRI and pathological studies were used to define phenotype. Chromosomal microarray analysis and whole-exome sequence were performed to unravel the genotype.

RESULTS

We report four subjects from two unrelated families with homozygous mutations in the Exocyst Complex Component 3-Like-2 gene (EXOC3L2).EXOC3L2 functions in trafficking of post-Golgi vesicles to the plasma membrane. In the first family a missense mutation in a highly conserved amino acid, p.Leu41Gln, was found in three fetuses; all had severe forms of Dandy-Walker malformation that was detectable by prenatal ultrasonography and confirmed by autopsy. In the second family, the affected child carried a nonsense mutation, p.Arg72*, and no detected protein. He had peritrigonal and cerebellar white matter abnormalities with enlargement of the ventricular trigones, developmental delay, pituitary hypoplasia, severe renal dysplasia and bone marrow failure.

CONCLUSION

We propose that biallelic EXOC3L2 mutations lead to a novel syndrome that affects hindbrain development, kidney and possibly the bone marrow.

A functional assay for the clinical annotation of genetic variants of uncertain significance in Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a rare genetic hypoplasia of erythroid progenitors characterized by mild to severe anemia and associated with congenital malformations. Clinical manifestations in DBA patients are quite variable and genetic testing has become a critical factor in establishing a diagnosis of DBA. The majority of DBA cases are due to heterozygous loss-of-function mutations in ribosomal protein (RP) genes. Causative mutations are fairly straightforward to identify in the case of large deletions and frameshift and nonsense mutations found early in a protein coding sequence, but diagnosis becomes more challenging in the case of missense mutations and small in-frame indels. Our group recently characterized the phenotype of lymphoblastoid cell lines established from DBA patients with pathogenic lesions in RPS19 and observed that defective pre-rRNA processing, a hallmark of the disease, was rescued by lentiviral vectors expressing wild-type RPS19. Here, we use this complementation assay to determine whether RPS19 variants of unknown significance are capable of rescuing pre-rRNA processing defects in these lymphoblastoid cells as a means of assessing the effects of these sequence changes on the function of the RPS19 protein. This approach will be useful in differentiating pathogenic mutations from benign polymorphisms in identifying causative genes in DBA patients.