Molecular pathogenesis may be elucidated for inherited bone marrow failure syndromes (IBMFS). The study and presentation of the details of their molecular biology and biochemistry is warranted for appropriate diagnosis and management of afflicted patients and to identify the physiology of the normal hematopoiesis and mechanisms of carcinogenesis. Several themes have emerged within each subsection of IBMFS, including the ribosomopathies, which include ribosome assembly and ribosomal RNA processing. The Fanconi anemia pathway has become interdigitated with the familial breast cancer syndromes. In this article, the diseases that account for most IBMFS diagnoses are analyzed.
Key points
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Despite the rarity of inherited bone marrow failure syndromes (IBMFS), they represent diseases for which the molecular pathogenesis may be elucidated.
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The study and presentation of the details of their molecular biology and biochemistry are warranted not only for appropriate diagnosis and management of afflicted patients but also because they lend clues to the normal physiology of the normal hematopoiesis and, in many cases, mechanisms of carcinogenesis.
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Several themes have emerged within each subsection of IBMFS, including the ribosomopathies, which include both ribosome assembly as well as ribosomal RNA processing.
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The Fanconi anemia pathway itself has become interdigitated with the familial breast cancer syndromes.
Despite the rarity of inherited bone marrow failure syndromes (IBMFS), careful studies of the molecular pathogenesis of these disorders has led to a deeper understanding of normal and aberrant bone marrow function. Understanding the molecular biology and biochemistry of IBMFS helps in the appropriate diagnosis and management of afflicted patients and also lends clues to the physiology of normal hematopoiesis and, in many cases, elucidates mechanisms of carcinogenesis. Several themes have emerged within each subsection of IBMFS, including the ribosomopathies, which include both ribosome assembly as well as ribosomal RNA processing. The Fanconi anemia (FA) pathway itself has become interdigitated with the familial breast cancer syndromes. This article analyses the diseases that account for most IBMFS diagnoses.
The ribosomopathies
In recent years, the subset of patients affected with Diamond-Blackfan anemia (DBA), Shwachman-Diamond-Bodian syndrome (SDBS), dyskeratosis congenita (DC), and cartilage hair hypoplasia (CHH) have all been shown to have mutations in ribosomal proteins and in proteins responsible for processing of ribosomal RNA. The reports of amelioration of a DBA-like phenotype in the p53-deficient mouse and zebrafish imply that defects in the ribosome assembly pathway and imbalances in protein synthesis lead to cell stress that can activate the p53 pathway and cause subsequent apoptosis, resulting in a depletion of sensitive cell populations, such as the hematopoietic stem cell pool. Even although the biology of these syndromes does not completely overlap (ie, DC involves biology directly at the telomere), several investigators have organized these diseases into a linear or cooperative pathway, in which ribosomal RNA is assembled in a multistep process.
DBA
The most common disease of isolated inherited red cell production failure is DBA, with an incidence of approximately 5 to 7 cases per million live births in North America. Presentation of the disease can be at any age, but most patients are diagnosed in the first year of life, with at least 10% in 1 series presenting at birth and 25% within the first month of life.
DBA is a genetic disease with evidence of mixed inheritance, although most cases are sporadic. Many inherited cases are autosomal dominant, with equal sex frequency. This group of patients seem to have fewer physical anomalies. Some cases are autosomal recessive, with disproportionately more males than females, implying an X-linked form of the disease. Approximately 50% of patients with DBA have physical abnormalities, including dysmorphic facies, short stature, eye, kidney, and hand abnormalities. Patients first present typically with macrocytic anemia and reticulocytopenia and associated pallor, often in the setting of failure to thrive. In the neonatal period, diagnosis can be uncertain, because the normal newborn proceeds through the physiologic nadir of erythropoiesis over the first 4 to 8 weeks and the normal newborn red blood cells are macrocytic at birth. Thus, most commonly, patients present between 2 and 4 months of age. The confirming finding is reticulocytopenia and decreased erythroid activity in the bone marrow.
In addition, some patients present with neutropenia, thrombocytosis, or thrombocytopenia, even although DBA is thought of as a pure red cell aplasia. Other nonspecific abnormalities include increases of fetal hemoglobin, red cell antigen, iron, folate, and vitamin B 12 levels. An increased erythrocyte adenosine deaminase level has been proposed as a good diagnostic test for DBA, with a sensitivity of 84% and specificity of 95%. However, 16% of affected people have a normal erythrocyte adenosine deaminase level.
The 2 most common therapeutic options are corticosteroids or transfusion therapy for patients who are symptomatic from their anemia. Corticosteroids may be initiated with anemia that causes cardiovascular and developmental compromise. Prednisone is usually the first-line agent, at a dose of 2 mg/kg/d. If the child has a response to steroids, the goal is to reach a hemoglobin level of 10 g/dL and sustained reticulocytosis, after which the steroids can be slowly tapered off. Slightly more than half of patients with DBA are steroid responders; within this group, many can be weaned to a very low dose or off prednisone completely. Corticosteroids may become problematic with chronic use, leading to growth retardation, weight gain, gastritis, and decreased bone mineralization. Those who are positive responders show a distinctive gene expression signature that may characterize the underlying difference in response as host related.
For patients who do not respond to steroids or cannot tolerate them, transfusion therapy is an effective option. As is the case with chronic transfusion therapy for any disease, iron overload becomes the chief difficulty, resulting in cardiac, endocrine, and pulmonary disease. Chronic iron chelation using subcutaneous desferoxamine has been poorly tolerated and adherence limited. Since 2006, deferasirox, a daily oral chelator, has been available for treatment of iron overload, and more recently, deferiprone, an oral agent given 3 times a day, was approved by the US Food and Drug Administration for use in this setting.
Any discussion of transfusion and iron overload must be balanced against the role of bone marrow transplantation (BMT) as a curative modality, with its associated side effects. The side effects of BMT include graft-versus-host disease (GVHD), infections, and chemotherapeutic toxicity involved with conditioning. With matched sibling donors, the risk of GVHD has become markedly lower, with long-term survival of more than 80%. The optimal timing of BMT in DBA is not known. The younger patient is more likely to show less GVHD and better long-term survival but have more late effects, leading some to argue for waiting for additional time to allow for late remissions.
Growth factors have also been used in DBA. Erythropoietin has been the logical choice, without any evidence of efficacy. Interleukin 3 has shown some modest benefit amongst transfusion-dependent patients, along with side effects of deep venous thromboses and constitutional symptoms. In recent years, additional agents have been shown to be at least anecdotally beneficial, including leucine and metaclopropamide.
Patients with DBA have a reduced life expectancy from causes largely related to treatment side effects and long-term risks of malignancy. In addition, hematologic malignancy is a major cause of death because patients with DBA have a risk of acute myelogenous leukemia (AML) with a lifelong risk of up to 25%. Anecdotally, acute lymphoblastic leukemia (ALL), Hodgkin disease, and myelodysplastic syndrome (MDS) have all been reported in DBA. Reports of osteogenic sarcoma and hepatocellular carcinoma, which are likely associated with concomitant viral hepatitis infection, have also been published. An overall risk of 5.4 odds ratio for developing cancer in DBA has been calculated, with predominance in MDS, AML, colon cancer, osteogenic sarcoma, and genital cancer.
Often mentioned in the differential diagnosis of DBA are transient erythrocytopenia of childhood (TEC) and infection with parvovirus. TEC and parvovirus-induced aplasia should resolve within a month, and both are unusual in the newborn period.
The science of DBA has exploded in the last decade. The first gene identified as mutated in DBA, ribosomal protein RPS19, was cloned positionally by analyzing a patient with DBA who had a translocation interrupting the RPS19 locus. Twenty-five percent of patients with DBA have RPS19 mutations in the heterozygous state, which implies that the allele acts in a dominant negative fashion or that a dosage effect exists and that the presence of 2 mutated alleles may be lethal. As many as 8 additional mutations in ribosomal genes have been identified, both in the large as well as the small subunit of the ribosome (RPL5, RPL11, RPL15, RPL35, RPS7, RPS10, RPS17, RPS24, RPS26), leaving almost 50% of DBA cases having no identified mutation. A recent report detailed the first nonribosomal mutation found in a patient with DBA, showing a GATA splice site mutation.
The cloning of these genes alone does not answer the basic question of why DBA occurs. In general, reduced colony-forming activity in patients with DBA has been observed, but this has not proved to be a reliable predictor of DBA. Progenitors have been marked by accelerated apoptosis. Defects in the microenvironment of the marrow have also been proposed as contributors to pathogenesis because of inhibition of erythropoiesis by bone fragments from a patient with DBA. Other affected functions include growth factors, their receptors, and transcription factors.
A mouse with an RPS19 knockout showing the DBA phenotype has been created and is being studied. As noted earlier, increased understanding of DBA came through amelioration of its phenotype in a p53 knockout background in both the mouse and the zebrafish, leading to a lower rate of apoptosis and suggesting that the imbalance of ribosome synthesis or inefficiency of protein translation leads to cell stress. In addition, RPS26 has been shown through an RPL11-mediated pathway to stabilize p53 via interaction in an inhibitory fashion with MDM2, which is the E3 ubiquitin ligase for p53, thus leading to the stabilization of p53.
An interesting overlap with adult MDS has been characterized in the identification of the cause of the 5q- syndrome, which arises as a result of deletion of the RPS14 gene. This acquired defect leads to a lower-risk MDS, which has been treated with lenalidomide.
DC
DC is a genetically heterogeneous disorder related to defects in telomere maintenance. The classic X-linked recessive form in particular seems to be associated with the classic DC findings of hyperpigmented reticular lacy pigmented skin, dystrophic nails, and leukoplakia. Up to 90% of patients develop bone marrow failure, although the time to recognition of classic findings is variable. The inheritance pattern can also be autosomal dominant or autosomal recessive. Two particularly severe variants are Hoyeraal-Hreidarsson (HH), presenting as systemic DC with cerebellar hypoplasia and developmental dysfunction, and Revesz syndrome, similar to HH coupled with exudative retinopathy. Reported cases have included probands with intrauterine growth retardation and microcephaly. In addition, these patients have a predisposition to myelodysplasia, myeloid leukemia, and an assortment of solid tumors, including squamous cell cancers of the head, neck, and anogenital region. As in numerous genetic cancer predisposition syndromes, sporadic mutations have been found in DC genes in cancers, suggesting the importance of the telomere broadly in cancer outside DC.
On a cellular level, DC cells are characterized by progressive telomere shortening that accumulates in an age-dependent fashion, thus leading to variability in the phenotype. The age of presentation has been reported to be as high as 75 years. When a patient with bone marrow failure does present, given the clinical heterogeneity of many of these disorders in question, telomere analysis should be a part of the workup for all bone marrow failure. Although some variability exists in telomere length analysis, overall, it is a sensitive test for DC, and combination analysis with flow cytometry to measure telomeres in multiple cell subsets can enhance its usefulness.
The first identified gene, DKC, is X linked (and the most common form of DC), encoding the dyskerin protein that resides in ribonucleoprotein (RNP) complexes at the telomere as well as in ribosomal RNA. This finding implies a duality in mechanism of the proteins involved in DC. Dyskerin binds to the RNA that serves as the template for the TTAGGG repeats found at telomeres. Other members of the telomerase RNP found to be causative when mutant in DC include NOP10, NHP2, TCAB1, TERC, and TERT (telomerase reverse transcriptase), all of which are autosomal-recessive inheritance. Dyskerin also seems to be involved in the pseudouridylation of ribosomal RNA (rRNA), but the specific function in the ribosome of this modification is unclear.
The other genes responsible for DC reside in the shelterin complex, which protects telomeres from degradation. Shelterin is a 6-member protein complex, which includes the DC causative gene TIN2. The function of the shelterin complex is to protect telomeres from exonuclease activity, thus enabling the integrity of telomere length.
The classic presentation of DC includes 2 of the 3 following findings: dystrophic nails, a reticular lacy rash, leukoplakia. Other features recognized to be part of the DC spectrum include premature graying hair, pulmonary fibrosis, liver disease, leukemia or MDS, esophageal stenosis, urethral stenosis, short stature, and developmental delay. Recently described is the high prevalence of neuropsychiatric disorders in this population, which have previously been reported as 25% in children but in a recent small prospective study performed by the National Cancer Institute, the lifetime risk for children with DC was 83% for disorders including attention-deficit/hyperactivity disorder, pervasive developmental disorder, anxiety, and mood disorders.
The diagnosis of DC can be difficult, with still close to 50% of patients lacking a known mutation in the DC pathway. The measurement of telomere lengths in lymphocyte subsets has become an important element that supports a diagnosis of DC. Patients with DC have very short telomeres, well below the normal range, in contrast to patients with other IBMFS, who often have short telomeres compared with the general population but that are not significantly shorter than the first percentile. Clinicians must use a combination of physical examination findings, family history, and features of leukemia or bone marrow failure or very short telomere lengths. Often, the family history can be particularly useful in anticipating new patients, who may have worse disease than their forebears, perhaps because the telomere inheritance becomes shorter with each generation.
Management of DC generally centers around treating the symptoms of marrow failure. Because the treatment options have significant morbidity, a watch-and-wait approach is often used. When cytopenias cause symptoms, androgens can be used to good effect, despite masculinizing side effects and the need for careful monitoring for liver tumors later in life. The use of growth factors has achieved some temporary improvement in counts and related symptoms, but worry of malignant transformation in the resultant stressed marrow remains a concern.
If those therapies do not help, BMT can be considered. Outcomes for BMT have been poor based both on patients’ sensitivities to conditioning regimens as well as the underlying disease. Patients with DC are unique in having very late pulmonary and liver complications, likely caused by their underlying disease, which may be worsened or accelerated by the transplant process. Recent data from 2013 looking at 34 patients treated from 1981 to 2009 reported a 10-year survival probability of 30%. Unrelated donors or mismatched related donors and regimen intensity were independent risk factors for early mortality.
Shwachman-Diamond Syndrome
Shwachman-Diamond syndrome (SDS) is a syndrome of neutropenia, pancreatic exocrine function insufficiency, and metaphyseal dysostosis. Anemia and thrombocytopenia can accompany the neutropenia. Patients with SDS can also have a wide array of physical findings, including short stature, failure to thrive, skin rashes and macules, teeth and palatal defects, and syndactyly.
The putative gene that when mutated leads to SDS is the SBDS gene, encoding a 250 amino acid protein with predicted functions in RNA metabolism. Its widespread expression may be the basis of the multiple organ involvement of SDS. A recent model using induced pluripotent stem cells as a model for SDS manifests exocrine and hematopoietic cell dysfunction, which phenocopies SDS. A recent study has reported the defective processing of rRNA into ribosome assembly, suggesting that SDS is closely associated with the molecular pathogenesis of DBA. Most patients with SDS do not show a mutation in the SBDS gene, suggesting additional genes underlying the pathogenesis of SDS.
The diagnosis of SDS can be difficult, because many patients do not have an identifiable mutation in the SBDS gene. Clues can be found by examining trypsinogen, isoamylase, and obtaining an ultrasound scan of the pancreas. Family history may provide a clue but is less useful than in DC.
No definitive therapy exists for SDS neutropenia, except for BMT, whereas pancreatic insufficiency is managed by administration of exogenous pancreatic enzymes. Supportive care is indicated, with transfusions in the 20% of patients with SDS who develop pancytopenia. Some efficacy has been reported with the use of granulocyte-stimulating growth factors (G-CSF), with 6 of 7 responding to G-CSF in 1 series. Like other IBMFS, the diagnosis of SDS confers a 5% risk of leukemia, including ALL, chronic myelogenous leukemia, and AML, making the use of growth factors a difficult decision for fear of stimulating a clonal expansion, despite minimal data to support this concern.
CHH
CHH is a recessive disease with a constellation of skeletal abnormalities consisting of short stature, lordosis, scoliosis, and chest wall deformities. The disorder was first described amongst the Old Order Amish. Sparse, fine hair is the other consistent feature. Growth failure occurs prenatally with shortness of limbs or stature noted on fetal ultrasonography or perinatally. Patients with CHH also have increased risk of Hirschsprung disease. Mutations in the RNA component of RNA mannose-6-phosphatereceptor endoribonuclease involved in mitochondrial and nucleolar RNA processing have been reported to be responsible for CHH. Together with TERT, RMRP forms a complex that produces double-strand RNA, which participates in small interfering RNAs. RMRP works in parallel with DBA defects, upstream of Shwachman-Diamond, and downstream of DC, in the processing of the 80S unit of the ribosome.
Anemia and macrocytosis occur in the variable context of pancytopenia. Lymphopenia is reported, along with defective cellular immunity. Corticosteroids and transfusion have been used temporarily with some patients outgrowing marrow failure. However, increased malignancy risk was also reported throughout infancy, childhood, and adulthood. T-cell lymphoproliferative disease has been seen as a consequence of immunodeficiency.
The ribosomopathies
In recent years, the subset of patients affected with Diamond-Blackfan anemia (DBA), Shwachman-Diamond-Bodian syndrome (SDBS), dyskeratosis congenita (DC), and cartilage hair hypoplasia (CHH) have all been shown to have mutations in ribosomal proteins and in proteins responsible for processing of ribosomal RNA. The reports of amelioration of a DBA-like phenotype in the p53-deficient mouse and zebrafish imply that defects in the ribosome assembly pathway and imbalances in protein synthesis lead to cell stress that can activate the p53 pathway and cause subsequent apoptosis, resulting in a depletion of sensitive cell populations, such as the hematopoietic stem cell pool. Even although the biology of these syndromes does not completely overlap (ie, DC involves biology directly at the telomere), several investigators have organized these diseases into a linear or cooperative pathway, in which ribosomal RNA is assembled in a multistep process.
DBA
The most common disease of isolated inherited red cell production failure is DBA, with an incidence of approximately 5 to 7 cases per million live births in North America. Presentation of the disease can be at any age, but most patients are diagnosed in the first year of life, with at least 10% in 1 series presenting at birth and 25% within the first month of life.
DBA is a genetic disease with evidence of mixed inheritance, although most cases are sporadic. Many inherited cases are autosomal dominant, with equal sex frequency. This group of patients seem to have fewer physical anomalies. Some cases are autosomal recessive, with disproportionately more males than females, implying an X-linked form of the disease. Approximately 50% of patients with DBA have physical abnormalities, including dysmorphic facies, short stature, eye, kidney, and hand abnormalities. Patients first present typically with macrocytic anemia and reticulocytopenia and associated pallor, often in the setting of failure to thrive. In the neonatal period, diagnosis can be uncertain, because the normal newborn proceeds through the physiologic nadir of erythropoiesis over the first 4 to 8 weeks and the normal newborn red blood cells are macrocytic at birth. Thus, most commonly, patients present between 2 and 4 months of age. The confirming finding is reticulocytopenia and decreased erythroid activity in the bone marrow.
In addition, some patients present with neutropenia, thrombocytosis, or thrombocytopenia, even although DBA is thought of as a pure red cell aplasia. Other nonspecific abnormalities include increases of fetal hemoglobin, red cell antigen, iron, folate, and vitamin B 12 levels. An increased erythrocyte adenosine deaminase level has been proposed as a good diagnostic test for DBA, with a sensitivity of 84% and specificity of 95%. However, 16% of affected people have a normal erythrocyte adenosine deaminase level.
The 2 most common therapeutic options are corticosteroids or transfusion therapy for patients who are symptomatic from their anemia. Corticosteroids may be initiated with anemia that causes cardiovascular and developmental compromise. Prednisone is usually the first-line agent, at a dose of 2 mg/kg/d. If the child has a response to steroids, the goal is to reach a hemoglobin level of 10 g/dL and sustained reticulocytosis, after which the steroids can be slowly tapered off. Slightly more than half of patients with DBA are steroid responders; within this group, many can be weaned to a very low dose or off prednisone completely. Corticosteroids may become problematic with chronic use, leading to growth retardation, weight gain, gastritis, and decreased bone mineralization. Those who are positive responders show a distinctive gene expression signature that may characterize the underlying difference in response as host related.
For patients who do not respond to steroids or cannot tolerate them, transfusion therapy is an effective option. As is the case with chronic transfusion therapy for any disease, iron overload becomes the chief difficulty, resulting in cardiac, endocrine, and pulmonary disease. Chronic iron chelation using subcutaneous desferoxamine has been poorly tolerated and adherence limited. Since 2006, deferasirox, a daily oral chelator, has been available for treatment of iron overload, and more recently, deferiprone, an oral agent given 3 times a day, was approved by the US Food and Drug Administration for use in this setting.
Any discussion of transfusion and iron overload must be balanced against the role of bone marrow transplantation (BMT) as a curative modality, with its associated side effects. The side effects of BMT include graft-versus-host disease (GVHD), infections, and chemotherapeutic toxicity involved with conditioning. With matched sibling donors, the risk of GVHD has become markedly lower, with long-term survival of more than 80%. The optimal timing of BMT in DBA is not known. The younger patient is more likely to show less GVHD and better long-term survival but have more late effects, leading some to argue for waiting for additional time to allow for late remissions.
Growth factors have also been used in DBA. Erythropoietin has been the logical choice, without any evidence of efficacy. Interleukin 3 has shown some modest benefit amongst transfusion-dependent patients, along with side effects of deep venous thromboses and constitutional symptoms. In recent years, additional agents have been shown to be at least anecdotally beneficial, including leucine and metaclopropamide.
Patients with DBA have a reduced life expectancy from causes largely related to treatment side effects and long-term risks of malignancy. In addition, hematologic malignancy is a major cause of death because patients with DBA have a risk of acute myelogenous leukemia (AML) with a lifelong risk of up to 25%. Anecdotally, acute lymphoblastic leukemia (ALL), Hodgkin disease, and myelodysplastic syndrome (MDS) have all been reported in DBA. Reports of osteogenic sarcoma and hepatocellular carcinoma, which are likely associated with concomitant viral hepatitis infection, have also been published. An overall risk of 5.4 odds ratio for developing cancer in DBA has been calculated, with predominance in MDS, AML, colon cancer, osteogenic sarcoma, and genital cancer.
Often mentioned in the differential diagnosis of DBA are transient erythrocytopenia of childhood (TEC) and infection with parvovirus. TEC and parvovirus-induced aplasia should resolve within a month, and both are unusual in the newborn period.
The science of DBA has exploded in the last decade. The first gene identified as mutated in DBA, ribosomal protein RPS19, was cloned positionally by analyzing a patient with DBA who had a translocation interrupting the RPS19 locus. Twenty-five percent of patients with DBA have RPS19 mutations in the heterozygous state, which implies that the allele acts in a dominant negative fashion or that a dosage effect exists and that the presence of 2 mutated alleles may be lethal. As many as 8 additional mutations in ribosomal genes have been identified, both in the large as well as the small subunit of the ribosome (RPL5, RPL11, RPL15, RPL35, RPS7, RPS10, RPS17, RPS24, RPS26), leaving almost 50% of DBA cases having no identified mutation. A recent report detailed the first nonribosomal mutation found in a patient with DBA, showing a GATA splice site mutation.
The cloning of these genes alone does not answer the basic question of why DBA occurs. In general, reduced colony-forming activity in patients with DBA has been observed, but this has not proved to be a reliable predictor of DBA. Progenitors have been marked by accelerated apoptosis. Defects in the microenvironment of the marrow have also been proposed as contributors to pathogenesis because of inhibition of erythropoiesis by bone fragments from a patient with DBA. Other affected functions include growth factors, their receptors, and transcription factors.
A mouse with an RPS19 knockout showing the DBA phenotype has been created and is being studied. As noted earlier, increased understanding of DBA came through amelioration of its phenotype in a p53 knockout background in both the mouse and the zebrafish, leading to a lower rate of apoptosis and suggesting that the imbalance of ribosome synthesis or inefficiency of protein translation leads to cell stress. In addition, RPS26 has been shown through an RPL11-mediated pathway to stabilize p53 via interaction in an inhibitory fashion with MDM2, which is the E3 ubiquitin ligase for p53, thus leading to the stabilization of p53.
An interesting overlap with adult MDS has been characterized in the identification of the cause of the 5q- syndrome, which arises as a result of deletion of the RPS14 gene. This acquired defect leads to a lower-risk MDS, which has been treated with lenalidomide.
DC
DC is a genetically heterogeneous disorder related to defects in telomere maintenance. The classic X-linked recessive form in particular seems to be associated with the classic DC findings of hyperpigmented reticular lacy pigmented skin, dystrophic nails, and leukoplakia. Up to 90% of patients develop bone marrow failure, although the time to recognition of classic findings is variable. The inheritance pattern can also be autosomal dominant or autosomal recessive. Two particularly severe variants are Hoyeraal-Hreidarsson (HH), presenting as systemic DC with cerebellar hypoplasia and developmental dysfunction, and Revesz syndrome, similar to HH coupled with exudative retinopathy. Reported cases have included probands with intrauterine growth retardation and microcephaly. In addition, these patients have a predisposition to myelodysplasia, myeloid leukemia, and an assortment of solid tumors, including squamous cell cancers of the head, neck, and anogenital region. As in numerous genetic cancer predisposition syndromes, sporadic mutations have been found in DC genes in cancers, suggesting the importance of the telomere broadly in cancer outside DC.
On a cellular level, DC cells are characterized by progressive telomere shortening that accumulates in an age-dependent fashion, thus leading to variability in the phenotype. The age of presentation has been reported to be as high as 75 years. When a patient with bone marrow failure does present, given the clinical heterogeneity of many of these disorders in question, telomere analysis should be a part of the workup for all bone marrow failure. Although some variability exists in telomere length analysis, overall, it is a sensitive test for DC, and combination analysis with flow cytometry to measure telomeres in multiple cell subsets can enhance its usefulness.
The first identified gene, DKC, is X linked (and the most common form of DC), encoding the dyskerin protein that resides in ribonucleoprotein (RNP) complexes at the telomere as well as in ribosomal RNA. This finding implies a duality in mechanism of the proteins involved in DC. Dyskerin binds to the RNA that serves as the template for the TTAGGG repeats found at telomeres. Other members of the telomerase RNP found to be causative when mutant in DC include NOP10, NHP2, TCAB1, TERC, and TERT (telomerase reverse transcriptase), all of which are autosomal-recessive inheritance. Dyskerin also seems to be involved in the pseudouridylation of ribosomal RNA (rRNA), but the specific function in the ribosome of this modification is unclear.
The other genes responsible for DC reside in the shelterin complex, which protects telomeres from degradation. Shelterin is a 6-member protein complex, which includes the DC causative gene TIN2. The function of the shelterin complex is to protect telomeres from exonuclease activity, thus enabling the integrity of telomere length.
The classic presentation of DC includes 2 of the 3 following findings: dystrophic nails, a reticular lacy rash, leukoplakia. Other features recognized to be part of the DC spectrum include premature graying hair, pulmonary fibrosis, liver disease, leukemia or MDS, esophageal stenosis, urethral stenosis, short stature, and developmental delay. Recently described is the high prevalence of neuropsychiatric disorders in this population, which have previously been reported as 25% in children but in a recent small prospective study performed by the National Cancer Institute, the lifetime risk for children with DC was 83% for disorders including attention-deficit/hyperactivity disorder, pervasive developmental disorder, anxiety, and mood disorders.
The diagnosis of DC can be difficult, with still close to 50% of patients lacking a known mutation in the DC pathway. The measurement of telomere lengths in lymphocyte subsets has become an important element that supports a diagnosis of DC. Patients with DC have very short telomeres, well below the normal range, in contrast to patients with other IBMFS, who often have short telomeres compared with the general population but that are not significantly shorter than the first percentile. Clinicians must use a combination of physical examination findings, family history, and features of leukemia or bone marrow failure or very short telomere lengths. Often, the family history can be particularly useful in anticipating new patients, who may have worse disease than their forebears, perhaps because the telomere inheritance becomes shorter with each generation.
Management of DC generally centers around treating the symptoms of marrow failure. Because the treatment options have significant morbidity, a watch-and-wait approach is often used. When cytopenias cause symptoms, androgens can be used to good effect, despite masculinizing side effects and the need for careful monitoring for liver tumors later in life. The use of growth factors has achieved some temporary improvement in counts and related symptoms, but worry of malignant transformation in the resultant stressed marrow remains a concern.
If those therapies do not help, BMT can be considered. Outcomes for BMT have been poor based both on patients’ sensitivities to conditioning regimens as well as the underlying disease. Patients with DC are unique in having very late pulmonary and liver complications, likely caused by their underlying disease, which may be worsened or accelerated by the transplant process. Recent data from 2013 looking at 34 patients treated from 1981 to 2009 reported a 10-year survival probability of 30%. Unrelated donors or mismatched related donors and regimen intensity were independent risk factors for early mortality.
Shwachman-Diamond Syndrome
Shwachman-Diamond syndrome (SDS) is a syndrome of neutropenia, pancreatic exocrine function insufficiency, and metaphyseal dysostosis. Anemia and thrombocytopenia can accompany the neutropenia. Patients with SDS can also have a wide array of physical findings, including short stature, failure to thrive, skin rashes and macules, teeth and palatal defects, and syndactyly.
The putative gene that when mutated leads to SDS is the SBDS gene, encoding a 250 amino acid protein with predicted functions in RNA metabolism. Its widespread expression may be the basis of the multiple organ involvement of SDS. A recent model using induced pluripotent stem cells as a model for SDS manifests exocrine and hematopoietic cell dysfunction, which phenocopies SDS. A recent study has reported the defective processing of rRNA into ribosome assembly, suggesting that SDS is closely associated with the molecular pathogenesis of DBA. Most patients with SDS do not show a mutation in the SBDS gene, suggesting additional genes underlying the pathogenesis of SDS.
The diagnosis of SDS can be difficult, because many patients do not have an identifiable mutation in the SBDS gene. Clues can be found by examining trypsinogen, isoamylase, and obtaining an ultrasound scan of the pancreas. Family history may provide a clue but is less useful than in DC.
No definitive therapy exists for SDS neutropenia, except for BMT, whereas pancreatic insufficiency is managed by administration of exogenous pancreatic enzymes. Supportive care is indicated, with transfusions in the 20% of patients with SDS who develop pancytopenia. Some efficacy has been reported with the use of granulocyte-stimulating growth factors (G-CSF), with 6 of 7 responding to G-CSF in 1 series. Like other IBMFS, the diagnosis of SDS confers a 5% risk of leukemia, including ALL, chronic myelogenous leukemia, and AML, making the use of growth factors a difficult decision for fear of stimulating a clonal expansion, despite minimal data to support this concern.
CHH
CHH is a recessive disease with a constellation of skeletal abnormalities consisting of short stature, lordosis, scoliosis, and chest wall deformities. The disorder was first described amongst the Old Order Amish. Sparse, fine hair is the other consistent feature. Growth failure occurs prenatally with shortness of limbs or stature noted on fetal ultrasonography or perinatally. Patients with CHH also have increased risk of Hirschsprung disease. Mutations in the RNA component of RNA mannose-6-phosphatereceptor endoribonuclease involved in mitochondrial and nucleolar RNA processing have been reported to be responsible for CHH. Together with TERT, RMRP forms a complex that produces double-strand RNA, which participates in small interfering RNAs. RMRP works in parallel with DBA defects, upstream of Shwachman-Diamond, and downstream of DC, in the processing of the 80S unit of the ribosome.
Anemia and macrocytosis occur in the variable context of pancytopenia. Lymphopenia is reported, along with defective cellular immunity. Corticosteroids and transfusion have been used temporarily with some patients outgrowing marrow failure. However, increased malignancy risk was also reported throughout infancy, childhood, and adulthood. T-cell lymphoproliferative disease has been seen as a consequence of immunodeficiency.
FA/BRCA pathway
FA is an autosomal and X-linked recessive disorder characterized by bone marrow failure, AML, solid tumors, and developmental abnormalities. At the molecular level, cells derived from patients with FA show hypersensitivity to DNA cross-linking agents, resulting in increased numbers of chromosomal abnormalities, including translocations and radial chromosomes. This hypersensitivity has made successful treatment of patients with FA a challenge in the past because traditional treatments of symptoms resulted in more harm than good. In recent years, care of the patient with FA has improved with modern blood banking, antibiotics, and hematopoietic stem cell transplantation (SCT), which has been used in patients with FA for almost 30 years. Despite greater survival of children into adulthood as a result of SCT, the specter of the potential for solid tumors, such as squamous cell carcinomas (SCC) of the head, neck, and genitourinary track, remains a serious problem.
Even although the classic features of thumb abnormalities and radius absence generally characterize these patients, children with FA typically present in the first decade of life on recognition of aplastic anemia. A sizable subset of patients with FA have no discernible abnormalities at all. As a result, the index of suspicion of the clinician must be high to recognize the diagnosis of FA in the differential diagnosis of aplastic anemia.
The gold standard tests for FA quantify chromosomal breakage in cells exposed to cross-linking agents to which FA cells are hypersensitive, typically diepoxybutane (DEB). On occasion, despite the strong suspicion of FA being present in a patient, the chromosome fragility test can be negative because of somatic reversion. In the face of this possibility, if a negative DEB or mitomycin C result has been obtained in the setting of strong suspicion of an FA diagnosis, then a skin biopsy should be obtained for culture and subsequent DEB testing.
Ninety percent of patients with FA first present with bone marrow failure, but a percentage nonetheless first present with AML. These cases of AML are typically M1 to M4 FAB subtype and display no characteristic cytogenetic or molecular abnormality, although numerous translocations, deletions, and other aneuploidogenic changes can be found. Some report distinctly different clones in FA-associated AML versus sporadic AML. Patients treated in a fashion similar to other patients with AML experience intolerance to standard doses of alkylating agents. Thus, morbidity precludes an aggressive approach. In addition, analysis of cells derived from these AML cases suggests that some are paradoxically resistant, perhaps a result of the underlying genomic instability of FA cells that could allow acquisition of resistance.
Patients with FA are at markedly increased risk of SCC of the head and neck and genitourinary tract. These tumors have been only sporadically positive for human papilloma virus (HPV), although vaccination for HPV is strongly encouraged in the population with FA. In addition, routine laryngoscopy and ear, nose, and throat follow-up is considered standard of care for patients with FA. Breast cancer is also typical of adult patients with FA, which is in line with the fact that 5 FA genes are familial breast cancer genes: BRCA2, PALB2, RAD51C, SLX4, and BACH1.
FA genes have also been implicated in sporadic cancers by virtue of detection of non–germ-line mutations in tumors such as lung, pancreatic, ovarian, and breast. It has been proposed that such tumors can be targeted by using agents specifically inducing hypersensitivity in FA-mutant cells.
Because the patient with FA is at increased risk of toxicity from SCT regimens, clinicians must time the procedure before the onset of leukemia, avoiding the long-term effects of blood product provision or unaffected by serious infections from invasive organisms like Aspergillus . The complementation group and gene mutation should be identified in order to assess the suitability of a family member stem cell donor, averting the possibility that a matched donor has a subclinical case of FA. It has become clear from the experience of FA clinicians that patients with FA-D1 and those with the Ashkenazi FANCC mutation are at significant and early risk of progression to AML, often before the presentation of aplastic anemia. In general terms, it is believed that such a risk of early AML progression is coincident with a more severe FA phenotype.
Historically, the challenges of SCT in patients with FA have been numerous. The issue of graft failure has a prevalence of 10%, but use of fludaribine has reduced this risk to less than 1%. As a result, efforts at reduction of conditioning have been steady and the use of total body irradiation has been diminished down to doses of 400 to 600 cGy. In addition, the use of cyclophosphamide has also been decreased in recent years. With an allogenic-related transplant, the long-term survival is often greater than 80%.
Matched-unrelated transplants have posed a greater challenge, with a greater incidence of GVHD. Toxicity-associated GVHD occurs with greater intensity in the patient with FA, perhaps because of the greater degree of toxicity caused by conditioning. Such toxicity is synergistic with the increased GVHD risk. With diminished toxicity has come greater GVHD control and subsequent increased survival for patients with FA undergoing matched-unrelated transplants.
Secondary effects of SCT have important consequences for patients with FA presumably because of their underlying issues of growth delay, endocrine dysfunction, and increased risk of malignancy, all of which are associated with long-term consequences of undergoing SCT. A markedly increased risk of acquisition of SCC is seen after transplantation beyond that observed in untransplanted patients with FA; these SCC are only weakly linked to human papillomavirus. GVHD greatly increases the risk of SCC in patients with FA and patients undergoing unrelated stem cell transplant.
The idea that FA cells are hypersensitive to endogenous and exogenous stimuli suggests that FA stem cells in the bone marrow are susceptible to a sort of natural selection. Probably for this reason, somatic reversion is observed in some patients with FA. As a result, it has been postulated that gene therapy is an ideal approach to treatment of FA. Clinical trials targeting the most common complementation group, FA-A, have been instituted using a lentiviral transduction system of hematopoietic stem cells from patients with FA, manipulated ex vivo. In vitro data suggest that hematopoietic stem cells can be transduced with subsequent colony-forming assays, suggesting increased growth and reconstitution. However, such trails have been disappointing, because lack of permanent transduction of progenitors has led to failure to establish long-term hematopoiesis.
Traditionally, androgens have proved to be an efficacious treatment in some patients with bone marrow failure, patients with FA included. Androgens can stimulate more effective hematopoiesis, resulting in an increase in peripheral blood counts. The use of androgens has been marked by their limitations in females, given the masculinizing side effects. In addition, their use has been associated with increased risk of liver adenomas.
The FA pathway is composed of at least 16 genes. Each of these genes, when biallelically mutated, causes FA, except for the X-linked FANCB. The encoded proteins ( Table 1 ) can be subdivided within the FA pathway into 3 groups: (1) proteins that make up the core complex; (2) the FANCD2 and FANCI proteins, which compose the ID complex; and (3) 5 downstream effector proteins, FANCD1/BRCA2, FANCJ/BRIP1/BACH1, FANCN/PALB2, FANCO/SLX4, and FANCP/RAD51C. Many of the FA proteins contain no recognizable motifs, which has made discovering their contributions to the FA pathway and the main function of the FA pathway more challenging.
