Predisposition to myeloid malignancies in Shwachman-Diamond syndrome: biological insights and clinical advances.

Shwachman-Diamond syndrome (SDS) is an inherited multisystem ribosomopathy characterized by exocrine pancreatic deficiency, bone marrow failure, and predisposition to myeloid malignancies. The pathobiology of SDS results from impaired ribosomal maturation due to the deficiency of SBDS and the inability to evict the antiassociation factor eIF6 from the 60S ribosomal subunit. Clinical outcomes for patients with SDS who develop myeloid malignancies are extremely poor because of high treatment-related toxicities and a high rate of refractory disease/relapse even after allogeneic hematopoietic stem cell transplant (HSCT). Registry data indicate that outcomes are improved for patients with SDS who undergo routine bone marrow surveillance and receive an HSCT before developing an overt malignancy. However, the optimal approach to hematologic surveillance and the timing of HSCT for patients with SDS is not clearly established. Recent studies have elucidated distinct patterns of somatic blood mutations in patients with SDS that either alleviate the ribosome defect via somatic rescue (heterozygous EIF6 inactivation) or disrupt cellular checkpoints, resulting in increased leukemogenic potential (heterozygous TP53 inactivation). Genomic analysis revealed that most myeloid malignancies in patients with SDS have biallelic loss-of-function TP53 mutations. Single-cell DNA sequencing of SDS bone marrow samples can detect premalignant biallelic TP53-mutated clones before clinical diagnosis, suggesting that molecular surveillance may enhance the detection of incipient myeloid malignancies when HSCT may be most effective. Here, we review the clinical, genetic, and biologic features of SDS. In addition, we present evidence supporting the hematologic surveillance for patients with SDS that incorporates clinical, pathologic, and molecular data to risk stratify patients and prioritize transplant evaluation for patients with SDS with high-risk features.

The clinical picture of ERCC6L2 disease: from bone marrow failure to acute leukemia.

Biallelic germ line excision repair cross-complementing 6 like 2 (ERCC6L2) variants strongly predispose to bone marrow failure (BMF) and myeloid malignancies, characterized by somatic TP53-mutated clones and erythroid predominance. We present a series of 52 subjects (35 families) with ERCC6L2 biallelic germ line variants collected retrospectively from 11 centers globally, with a follow-up of 1165 person-years. At initial investigations, 32 individuals were diagnosed with BMF and 15 with a hematological malignancy (HM). The subjects presented with 19 different variants of ERCC6L2, and we identified a founder mutation, c.1424delT, in Finnish patients. The median age of the subjects at baseline was 18 years (range, 2-65 years). Changes in the complete blood count were mild despite severe bone marrow (BM) hypoplasia and somatic TP53 mutations, with no significant difference between subjects with or without HMs. Signs of progressive disease included increasing TP53 variant allele frequency, dysplasia in megakaryocytes and/or erythroid lineage, and erythroid predominance in the BM morphology. The median age at the onset of HM was 37.0 years (95% CI, 31.5-42.5; range, 12-65 years). The overall survival (OS) at 3 years was 95% (95% CI, 85-100) and 19% (95% CI, 0-39) for patients with BMF and HM, respectively. Patients with myelodysplastic syndrome or acute myeloid leukemia with mutated TP53 undergoing hematopoietic stem cell transplantation had a poor outcome with a 3-year OS of 28% (95% CI, 0-61). Our results demonstrated the importance of early recognition and active surveillance in patients with biallelic germ line ERCC6L2 variants.

Autophagy in mesenchymal progenitors protects mice against bone marrow failure after severe intermittent stress.

The cellular mechanisms required to ensure homeostasis of the hematopoietic niche and the ability of this niche to support hematopoiesis upon stress remain elusive. We here identify Wnt5a in Osterix+ mesenchymal progenitor and stem cells (MSPCs) as a critical factor for niche-dependent hematopoiesis. Mice lacking Wnt5a in MSPCs suffer from stress-related bone marrow (BM) failure and increased mortality. Niche cells devoid of Wnt5a show defective actin stress fiber orientation due to an elevated activity of the small GTPase CDC42. This results in incorrect positioning of autophagosomes and lysosomes, thus reducing autophagy and increasing oxidative stress. In MSPCs from patients from BM failure states which share features of peripheral cytopenia and hypocellular BM, we find similar defects in actin stress fiber orientation, reduced and incorrect colocalization of autophagosomes and lysosomes, and CDC42 activation. Strikingly, a short pharmacological intervention to attenuate elevated CDC42 activation in vivo in mice prevents defective actin-anchored autophagy in MSPCs, salvages hematopoiesis and protects against lethal cytopenia upon stress. In summary, our study identifies Wnt5a as a restriction factor for niche homeostasis by affecting CDC42-regulated actin stress-fiber orientation and autophagy upon stress. Our data further imply a critical role for autophagy in MSPCs for adequate support of hematopoiesis by the niche upon stress and in human diseases characterized by peripheral cytopenias and hypocellular BM.

Genomic profiling for clinical decision making in myeloid neoplasms and acute leukemia.

Myeloid neoplasms and acute leukemias derive from the clonal expansion of hematopoietic cells driven by somatic gene mutations. Although assessment of morphology plays a crucial role in the diagnostic evaluation of patients with these malignancies, genomic characterization has become increasingly important for accurate diagnosis, risk assessment, and therapeutic decision making. Conventional cytogenetics, a comprehensive and unbiased method for assessing chromosomal abnormalities, has been the mainstay of genomic testing over the past several decades and remains relevant today. However, more recent advances in sequencing technology have increased our ability to detect somatic mutations through the use of targeted gene panels, whole-exome sequencing, whole-genome sequencing, and whole-transcriptome sequencing or RNA sequencing. In patients with myeloid neoplasms, whole-genome sequencing represents a potential replacement for both conventional cytogenetic and sequencing approaches, providing rapid and accurate comprehensive genomic profiling. DNA sequencing methods are used not only for detecting somatically acquired gene mutations but also for identifying germline gene mutations associated with inherited predisposition to hematologic neoplasms. The 2022 International Consensus Classification of myeloid neoplasms and acute leukemias makes extensive use of genomic data. The aim of this report is to help physicians and laboratorians implement genomic testing for diagnosis, risk stratification, and clinical decision making and illustrates the potential of genomic profiling for enabling personalized medicine in patients with hematologic neoplasms.

Absence of Neutropenia in Patients With Early Exon Nonsense Mutations in ELANE: Clinical Evidence to Support Gene Therapy Approaches for Severe Congenital Neutropenia.

Severe congenital neutropenia is an inherited bone marrow failure disorder characterized by profoundly low neutrophil counts and promyelocytic maturation arrest in bone marrow. Severe congenital neutropenia is most often caused by heterozygous ELANE mutations. In vitro and mouse xenograft studies using CRISPR/Cas9 have shown that introduction of frameshift/nonsense mutations in mutant ELANE may restore neutrophil counts, providing a model for gene therapy. Here, we present 2 children with inherited nonsense mutations in ELANE analogous to those proposed for gene therapy. Their normal peripheral blood neutrophil counts provide support for this approach through human "experiments of nature."

Expanded phenotypic and hematologic abnormalities beyond bone marrow failure in MECOM-associated syndromes.

The MECOM gene encodes multiple protein isoforms that are essential for hematopoietic stem cell self-renewal and maintenance. Germline MECOM variants have been associated with congenital thrombocytopenia, radioulnar synostosis and bone marrow failure; however, the phenotypic spectrum of MECOM-associated syndromes continues to expand and novel pathogenic variants continue to be identified. We describe eight unrelated patients who add to the previously known phenotypes and genetic defects of MECOM-associated syndromes. As each subject presented with unique MECOM variants, the series failed to demonstrate clear genotype-to-phenotype correlation but may suggest a role for additional modifiers that affect gene expression and subsequent phenotype. Recognition of the expanded hematologic and non-hematologic clinical features allows for rapid molecular diagnosis, early identification of life-threatening complications, and improved genetic counseling for families. A centralized international publicly accessible database to share annotated MECOM variants would advance their clinical interpretation and provide a foundation to perform functional MECOM studies.

Impact of cytoreduction and remission status on hematopoietic cell transplantation outcomes in pediatric myelodysplastic syndrome and related disorders.

BACKGROUND: The role of cytoreduction prior to hematopoietic cell transplant (HCT) for patients with pediatric myelodysplastic syndrome (MDS) and related disorders remains unclear. PROCEDURE: We performed a single-center retrospective analysis of pre-transplant disease management and subsequent HCT outcome for pediatric patients with MDS and related disorders who underwent HCT between 2010 and 2020. RESULTS: Total 62 patients (median age 11 years) with idiopathic MDS (n = 16), MDS secondary to an underlying germline condition (n = 11), secondary acute myeloid leukemia (n = 9), myeloproliferative neoplasms (n = 8), and treatment-related myeloid neoplasms (n = 18) received an allogeneic HCT. Cytoreduction prior to HCT was performed in 30/62 (48%) patients; this subset of patients had higher risk disease characteristics, including a higher blast count on presentation. In the overall cohort, use of cytoreduction before HCT was associated with higher rates of relapse (cumulative incidence of relapse 24 months post HCT: 48.1% [27.5%-66.1%]) for those who received cytoreduction versus 16.6% (5.9%-32.1%) for those who did not (p = .03). There was a trend toward decreased overall survival (OS) for those patients who received cytoreduction (24 months post HCT 57.1% vs. 75.3%, respectively; p = .06). OS for patients who received cytoreduction and attained measurable residual disease (MRD) negativity prior to HCT was superior compared to those with persistent disease (24 months post HCT 63.9% [36%-81.2%] vs. 33.3% [7.8%-62.3%], respectively; p = .04). CONCLUSION: Cytoreduction did not provide survival benefit in our overall cohort, but its increased use in children with higher risk disease impacted the analysis. Patients receiving cytoreduction and achieving MRD-negative status before HCT demonstrated improved OS compared to those with persistent disease.

Disease-specific induced pluripotent stem cells.

Tissue culture of immortal cell strains from diseased patients is an invaluable resource for medical research but is largely limited to tumor cell lines or transformed derivatives of native tissues. Here we describe the generation of induced pluripotent stem (iPS) cells from patients with a variety of genetic diseases with either Mendelian or complex inheritance; these diseases include adenosine deaminase deficiency-related severe combined immunodeficiency (ADA-SCID), Shwachman-Bodian-Diamond syndrome (SBDS), Gaucher disease (GD) type III, Duchenne (DMD) and Becker muscular dystrophy (BMD), Parkinson disease (PD), Huntington disease (HD), juvenile-onset, type 1 diabetes mellitus (JDM), Down syndrome (DS)/trisomy 21, and the carrier state of Lesch-Nyhan syndrome. Such disease-specific stem cells offer an unprecedented opportunity to recapitulate both normal and pathologic human tissue formation in vitro, thereby enabling disease investigation and drug development.

Congenital X-linked neutropenia with myelodysplasia and somatic tetraploidy due to a germline mutation in SEPT6.

Septins play key roles in mammalian cell division and cytokinesis but have not previously been implicated in a germline human disorder. A male infant with severe neutropenia and progressive dysmyelopoiesis with tetraploid myeloid precursors was identified. No known genetic etiologies for neutropenia or bone marrow failure were found. However, next-generation sequencing of germline samples from the patient revealed a novel, de novo germline stop-loss mutation in the X-linked gene SEPT6 that resulted in reduced SEPT6 staining in bone marrow granulocyte precursors and megakaryocytes. Patient skin fibroblast-derived induced pluripotent stem cells (iPSCs) produced reduced myeloid colonies, particularly of the granulocyte lineage. CRISPR/Cas9 knock-in of the patient's mutation or complete knock-out of SEPT6 was not tolerated in non-patient-derived iPSCs or human myeloid cell lines, but SEPT6 knock-out was successful in an erythroid cell line and resulting clones revealed a propensity to multinucleation. In silico analysis predicts that the mutated protein hinders the dimerization of SEPT6 coiled-coils in both parallel and antiparallel arrangements, which could in turn impair filament formation. These data demonstrate a critical role for SEPT6 in chromosomal segregation in myeloid progenitors that can account for the unusual predisposition to aneuploidy and dysmyelopoiesis.

Coronavirus disease 2019 and vaccination in patients with Shwachman-Diamond syndrome.

Because they can experience neutropenia due to bone marrow failure, patients with Shwachman-Diamond syndrome (SDS) carry increased risk for serious infections compared with the general population; however, there has been a paucity of data on the incidence and severity of coronavirus disease 2019 (COVID-19) in patients with SDS. We compiled results from a survey distributed to participants in the SDS Registry in May-June 2021. In this report, we describe the characteristics and outcomes of patients with SDS who had COVID-19. Patients reported a short clinical course without significant complications or cytopenias. Additionally, COVID-19 vaccines were well tolerated with minor side effects.

SLC25A38 congenital sideroblastic anemia: Phenotypes and genotypes of 31 individuals from 24 families, including 11 novel mutations, and a review of the literature.

The congenital sideroblastic anemias (CSAs) are a heterogeneous group of inherited disorders of erythropoiesis characterized by pathologic deposits of iron in the mitochondria of developing erythroblasts. Mutations in the mitochondrial glycine carrier SLC25A38 cause the most common recessive form of CSA. Nonetheless, the disease is still rare, there being fewer than 70 reported families. Here we describe the clinical phenotype and genotypes of 31 individuals from 24 families, including 11 novel mutations. We also review the spectrum of reported mutations and genotypes associated with the disease, describe the unique localization of missense mutations in transmembrane domains and account for the presence of several alleles in different populations.

Inherited thrombocytopenia associated with mutation of UDP-galactose-4-epimerase (GALE).

Severe thrombocytopenia, characterized by dysplastic megakaryocytes and intracranial bleeding, was diagnosed in six individuals from a consanguineous kindred. Three of the individuals were successfully treated by bone marrow transplant. Whole-exome sequencing and homozygosity mapping of multiple family members, coupled with whole-genome sequencing to reveal shared non-coding variants, revealed one potentially functional variant segregating with thrombocytopenia under a recessive model: GALE p.R51W (c.C151T, NM_001127621). The mutation is extremely rare (allele frequency = 2.5 × 10-05), and the likelihood of the observed co-segregation occurring by chance is 1.2 × 10-06. GALE encodes UDP-galactose-4-epimerase, an enzyme of galactose metabolism and glycosylation responsible for two reversible reactions: interconversion of UDP-galactose with UDP-glucose and interconversion of UDP-N-acetylgalactosamine with UDP-N-acetylglucosamine. The mutation alters an amino acid residue that is conserved from yeast to humans. The variant protein has both significantly lower enzymatic activity for both interconversion reactions and highly significant thermal instability. Proper glycosylation is critical to normal hematopoiesis, in particular to megakaryocyte and platelet development, as reflected in the presence of thrombocytopenia in the context of congenital disorders of glycosylation. Mutations in GALE have not previously been associated with thrombocytopenia. Our results suggest that GALE p.R51W is inadequate for normal glycosylation and thereby may impair megakaryocyte and platelet development. If other mutations in GALE are shown to have similar consequences, this gene may be proven to play a critical role in hematopoiesis.

Genetic predisposition to MDS: clinical features and clonal evolution.

Myelodysplastic syndrome (MDS) typically presents in older adults with the acquisition of age-related somatic mutations, whereas MDS presenting in children and younger adults is more frequently associated with germline genetic predisposition. Germline predisposition is increasingly recognized in MDS presenting at older ages as well. Although each individual genetic disorder is rare, as a group, the genetic MDS disorders account for a significant subset of MDS in children and young adults. Because many patients lack overt syndromic features, genetic testing plays an important role in the diagnostic evaluation. This review provides an overview of syndromes associated with genetic predisposition to MDS, discusses implications for clinical evaluation and management, and explores scientific insights gleaned from the study of MDS predisposition syndromes. The effects of germline genetic context on the selective pressures driving somatic clonal evolution are explored. Elucidation of the molecular and genetic pathways driving clonal evolution may inform surveillance and risk stratification, and may lead to the development of novel therapeutic strategies.

Diagnostic work-up for severe aplastic anemia in children: Consensus of the North American Pediatric Aplastic Anemia Consortium.

The North American Pediatric Aplastic Anemia Consortium (NAPAAC) is a group of pediatric hematologist-oncologists, hematopathologists, and bone marrow transplant physicians from 46 institutions in North America with interest and expertise in aplastic anemia, inherited bone marrow failure syndromes, and myelodysplastic syndromes. The NAPAAC Bone Marrow Failure Diagnosis and Care Guidelines Working Group was established with the charge of harmonizing the approach to the diagnostic workup of aplastic anemia in an effort to standardize best practices in the field. This document outlines the rationale for initial evaluations in pediatric patients presenting with signs and symptoms concerning for severe aplastic anemia.

Mechanism for survival of homozygous nonsense mutations in the tumor suppressor gene BRCA1.

BRCA1 is essential for repair of DNA double-strand breaks by homologous recombination, and hence for survival. Complete loss of its function is lethal during early embryonic development. Patients who are compound heterozygous for BRCA1 truncating mutations and missense alleles that retain some DNA repair capacity may survive, albeit with very high risk of early onset breast or ovarian cancer and features of Fanconi anemia. However, a mechanism enabling survival of patients homozygous for BRCA1 truncating mutations has not been described. We studied two unrelated families in which four children presented with multiple congenital anomalies and severe chromosomal fragility. One child developed T cell acute lymphocytic leukemia (ALL), and a second child developed neuroblastoma. Each of the four children was homozygous for a nonsense mutation in BRCA1 exon 11. Homozygosity for the nonsense mutations was viable thanks to the presence of a naturally occurring alternative splice donor in BRCA1 exon 11 that lies 5' of the mutations. The mutations did not affect the alternative splice site, but transcription from it produced an in-frame BRCA1 message with deletion of 3,309 bp. The translated BRCA1 protein was only 40% of normal length, but with intact N- and C-terminal sequences. These patients extend the range of BRCA1-related phenotypes and illustrate how naturally occurring alternative splicing can enable survival, albeit with severe consequences, of otherwise lethal genotypes of an essential gene.

Somatic mutations and clonal hematopoiesis in congenital neutropenia.

Severe congenital neutropenia (SCN) and Shwachman-Diamond syndrome (SDS) are congenital neutropenia syndromes with a high rate of leukemic transformation. Hematopoietic stressors may contribute to leukemic transformation by increasing the mutation rate in hematopoietic stem/progenitor cells (HSPCs) and/or by promoting clonal hematopoiesis. We sequenced the exome of individual hematopoietic colonies derived from 13 patients with congenital neutropenia to measure total mutation burden and performed error-corrected sequencing on a panel of 46 genes on 80 patients with congenital neutropenia to assess for clonal hematopoiesis. An average of 3.6 ± 1.2 somatic mutations per exome was identified in HSPCs from patients with SCN compared with 3.9 ± 0.4 for healthy controls (P = NS). Clonal hematopoiesis due to mutations in TP53 was present in 48% (13/27) of patients with SDS but was not seen in healthy controls (0/17, P < .001) or patients with SCN (0/40, P < .001). Our SDS cohort was young (median age 6.3 years), and many of the patients had multiple TP53 mutations. Conversely, clonal hematopoiesis due to mutations of CSF3R was present in patients with SCN but was not detected in healthy controls or patients with SDS. These data show that hematopoietic stress, including granulocyte colony-stimulating factor, do not increase the mutation burden in HSPCs in congenital neutropenia. Rather, distinct hematopoietic stressors result in the selective expansion of HSPCs carrying specific gene mutations. In particular, in SDS there is enormous selective pressure to expand TP53-mutated HSPCs, suggesting that acquisition of TP53 mutations is an early, likely initiating event, in the transformation to myelodysplastic syndrome/acute myeloid leukemia in patients with SDS.

Inflammatory manifestations in patients with Shwachman-Diamond syndrome: A novel phenotype.

Shwachman-Diamond syndrome (SDS) is an autosomal recessive multisystem disorder characterized by exocrine pancreatic dysfunction, bone marrow failure, and leukemia predisposition. Approximately 90% of cases are due to biallelic mutations in the Shwachman-Bodian-Diamond (SBDS) gene. Additional phenotypic features variably associated with SDS include skeletal, neurologic, hepatic, cardiac, endocrine, and dental abnormalities. We report five subjects with SDS who developed a range of inflammatory manifestations. Three patients developed inflammatory eye conditions. Single cases of juvenile idiopathic arthritis, chronic recurrent multifocal osteomyelitis, and scleroderma were also noted. Clinical presentation and treatment responses are described. Proteomic analysis revealed increased inflammatory signatures in SDS subjects as compared to controls. Treatment of inflammatory manifestations in patients with SDS may be complicated by potential myelosuppressive toxicities of anti-rheumatic medications. Further research is needed to better understand the potential link between inflammatory disorders and SDS to inform effective treatment strategies.

A study assessing the feasibility of randomization of pediatric and young adult patients between matched unrelated donor bone marrow transplantation and immune-suppressive therapy for newly diagnosed severe aplastic anemia: A joint pilot trial of the North American Pediatric Aplastic Anemia Consortium and the Pediatric Transplantation and Cellular Therapy Consortium.

BACKGROUND: Recent data show survival after matched unrelated donor (MUD) bone marrow transplantation (BMT) is similar to matched sibling procedures for young patients with severe aplastic anemia (SAA). Donor delays, risk of transplant-related mortality (TRM), and concern about chronic graft versus host disease raise questions about whether MUD BMT or immune suppression therapy (IST) should be preferred initial therapy for young patients lacking matched sibling donors. PROCEDURE: We performed a pilot trial to assess the feasibility of randomizing patients under age 26 with newly diagnosed SAA to receive IST versus MUD BMT. Primary aims assessed the acceptability of randomization and timing of BMT. Secondary aims measured toxicities, response, and survival. RESULTS: Sixty-seven patients with possible SAA were screened at nine centers. Of 57 with confirmed SAA, 23 underwent randomization and received therapy with a median follow-up of 18 months. Of 12 randomized to BMT, 10 started BMT as initial therapy at a median of 36 days after randomization. One BMT recipient experienced secondary graft failure, requiring a second procedure. Six of 11 randomized to IST responded, whereas five with refractory disease underwent successful salvage BMT. One patient achieving complete response relapsed after discontinuation of immune suppression and died of infection after salvage BMT. CONCLUSIONS: This feasibility study showed that a high percentage of patients underwent randomization and received up-front MUD BMT. Our study lays the groundwork for a larger randomized trial that will define best initial therapy for young patients with SAA who have an available MUD.

Diagnosis and treatment of pediatric myelodysplastic syndromes: A survey of the North American Pediatric Aplastic Anemia Consortium.

BACKGROUND: Myelodysplastic syndromes (MDS) represent a group of clonal hematopoietic stem cell disorders that commonly progress to acute myeloid leukemia (AML). The diagnostics, prognostics, and treatment of adult MDS are established but do not directly translate to children and adolescents. Pediatric MDS is a rare disease, characterized by unique cytogenetics and histology compared with adult MDS, and often arises secondary to germline predisposition or cytotoxic exposures. Our objective was to highlight aspects of diagnosis/management that would benefit from further systematic review toward the development of clinical practice guidelines for pediatric MDS. PROCEDURE: The North American Pediatric Aplastic Anemia Consortium (NAPAAC) is composed of collaborative institutions with a strong interest in pediatric bone marrow failure syndromes and hematologic malignancies. The NAPAAC MDS working group developed a national survey distributed to 35 NAPAAC institutions to assess data on (1) clinical presentation of pediatric MDS, (2) diagnostic evaluation, (3) criteria for diagnosis, (4) supportive care and treatment decisions, and (5) role of hematopoietic stem cell transplantation (HSCT). RESULTS: Twenty-eight of 35 institutions returned the survey. Most centers agreed on a common diagnostic workup, though there was considerable variation regarding the criteria for diagnosis. Although there was consensus on supportive care, treatment strategies, including the role of cytoreduction and HSCT, varied across centers surveyed. CONCLUSIONS: There is lack of national consensus on diagnosis and treatment of pediatric MDS. This survey identified key aspects of MDS management that will warrant systematic review toward the goal of developing national clinical practice guidelines for pediatric MDS.

Ataxia-Pancytopenia Syndrome Is Caused by Missense Mutations in SAMD9L.

Ataxia-pancytopenia (AP) syndrome is characterized by cerebellar ataxia, variable hematologic cytopenias, and predisposition to marrow failure and myeloid leukemia, sometimes associated with monosomy 7. Here, in the four-generation family UW-AP, linkage analysis revealed four regions that provided the maximal LOD scores possible, one of which was in a commonly microdeleted chromosome 7q region. Exome sequencing identified a missense mutation (c.2640C>A, p.His880Gln) in the sterile alpha motif domain containing 9-like gene (SAMD9L) that completely cosegregated with disease. By targeted sequencing of SAMD9L, we subsequently identified a different missense mutation (c.3587G>C, p.Cys1196Ser) in affected members of the first described family with AP syndrome, Li-AP. Neither variant is reported in the public databases, both affect highly conserved amino acid residues, and both are predicted to be damaging. With time in culture, lymphoblastic cell lines (LCLs) from two affected individuals in family UW-AP exhibited copy-neutral loss of heterozygosity for large portions of the long arm of chromosome 7, resulting in retention of only the wild-type SAMD9L allele. Newly established LCLs from both individuals demonstrated the same phenomenon. In addition, targeted capture and sequencing of SAMD9L in uncultured blood DNA from both individuals showed bias toward the wild-type allele. These observations indicate in vivo hematopoietic mosaicism. The hematopoietic cytopenias that characterize AP syndrome and the selective advantage for clones that have lost the mutant allele support the postulated role of SAMD9L in the regulation of cell proliferation. Furthermore, we show that AP syndrome is distinct from the dyskeratoses congenita telomeropathies, with which it shares some clinical characteristics.