LONG TERM CONSEQUENCES OF HEMATOPOIETIC STEM-CELL TRANSPLANTATION: CURRENT STATE OF THE SCIENCE

Hematopoietic cell transplantation (HCT) is an established curative option for a variety of hematological malignancies. Liberalization in the indications for transplantation coupled with an increase in options in the source of the hematopoietic stem cells are responsible for the increasing number of HCTs performed annually.¹ Advances in transplantation techniques and supportive care strategies have resulted in a significant improvement in survival, such that long-term survival has now become an expected outcome for patients undergoing HCT. However, exposure to chemotherapy prior to transplantation as well as at the time of transplantation, coupled with prolonged periods of immune suppression and the risk of disease recurrence, place this population at an increased risk of chronic health conditions, such as cardiac compromise, endocrine sequelae and subsequent malignancies. Furthermore, premature death remains of significant concern among healthcare providers as well as patients who have successfully survived the immediate post-HCT period. The following sections will focus on long-term survival and causes of premature death among patients undergoing HCT; on the overall burden of morbidity suffered by long-term survivors, with special emphasis on the occurrence of metabolic syndrome in this population and its impact on cardiac dysfunction and the need to understand individual variability in the risk for post-HCT complications, as we move closer to personalized medicine.

Long-term Survival and Premature Death

Long-term survival and premature deaths after allogeneic HCT were first examined for 6691 patients registered with the International Bone Marrow Transplant Registry.² This study reported the overall survival for those who were disease-free for 2 or more years after allogeneic HCT to be 89% at 5 years. The risk of death after 5 years approached that of the general population for patients with severe aplastic anemia (SAA). Recurrent disease was the chief cause of premature death. However, registry studies such as this are limited by the dependence on passive reporting of events by the participating institutions. This limitation was overcome by the Bone Marrow Transplant Survivor Study (BMTSS) where a comprehensive evaluation of cause-specific late mortality was performed in patients treated with HCT by using central resources such as the National Death Index, Social Security Death Index, supplemented by information abstracted from medical records. Late mortality was evaluated in 1479 individuals who had survived 2 or more years after allogeneic HCT.³ Median age at HCT was 25.9 years and the median length of follow-up was 9.5 years. The conditional survival probability at 15 years from HCT was 80.2%±1.9% for those who were disease-free at entry into the cohort, and the cohort was at a 9.9-fold increased risk of premature death compared with the general population. Relative mortality decreased with time from HCT, but remained significantly elevated at 15 years after HCT (standardized mortality ratio [SMR] = 2.2). Relapse of primary disease and chronic graft versus host disease (GvHD) were the leading causes of premature death. Late mortality in 854 individuals who had survived 2 or more years after autologous HCT for hematologic malignancies was also assessed in the BMTSS cohort.⁴ Median age at HCT was 36.5 years and median length of follow-up was 7.6 years. Overall survival was 68.8%±1.8% at 10 years, and the cohort was at a 13-fold increased risk for premature death when compared with the general population. Mortality rates approached those of the general population after 10 years among patients at standard risk for relapse at HCT and in patients undergoing HCT for AML. Relapse of primary disease and subsequent malignancies were leading causes of premature death. Relapse-related mortality was increased among patients with Hodgkin lymphoma (HL), non-Hodgkin lymphoma (NHL), and acute lymphoblastic leukemia (ALL). Total body irradiation (TBI) provided a protective effect.

Burden of Morbidity

Control of the underlying disease and prolongation of life among HCT survivors is not necessarily accompanied by full restoration of health. HCT survivors are at risk for long-term treatment-related complications, such as endocrinopathies, musculoskeletal disorders, cardiopulmonary compromise, and subsequent malignancies.^5–15 Understanding the burden of morbidity due to these treatment-related complications is important to the healthcare providers and policy makers in identifying and procuring resources for the long-term care of individuals with a high burden of morbidity; to the researchers in identifying common etiologic pathways that lead to the overall morbidity; and to the HCT survivors in making an informed decision regarding the quality of life concerns long-term after HCT.

The burden of morbidity due to chronic health conditions was evaluated in HCT survivors and a healthy sibling comparison group participating in the BMTSS.¹⁶ HCT survivors and siblings completed a 255-item questionnaire, which covers the following general areas: diagnosis of physical health conditions with age at diagnosis (endocrinopathies; central nervous system compromise; cardiopulmonary dysfunction; gastrointestinal and hepatic sequelae; musculoskeletal abnormalities; and subsequent malignancies); diagnosis and extent of chronic GvHD; access to and use of medical care; and sociodemographic characteristics (education, marital status, employment, household income, and insurance). The reliability and validity of the BMTSS questionnaire has been tested, and the responses have demonstrated a high level of sensitivity and specificity, confirming that survivors are able to report the occurrence of adverse medical conditions with accuracy.¹⁷ The prevalence and severity of chronic health conditions reported by individuals who had undergone HCT between 1976 and 1998 and survived 2 years (n=1022), and their siblings (n=309) is described here. A severity score (grades 1 [mild] through 4 [life-threatening]) was assigned to each health condition using the Common Terminology Criteria for Adverse Events (version 3). The mean age at enrollment was 43.1 and 44.8 years for survivors and siblings, respectively. Among HCT survivors, 66% had at least one chronic condition, and 18% had a severe or life-threatening condition; comparable figures in siblings were 39% and 8%, respectively (p<0.001). HCT survivors were 3.5 times more likely to develop severe/ life threatening conditions (95% CI, 2.3 to 5.4), when compared with age- and sex-matched siblings. The prevalence of any chronic health condition was significantly higher among allogeneic HCT recipients when compared with autologous HCT recipients (any chronic health conditions: 71.1% versus 60.7%, p=0.001; grade 3 or 4 conditions: 20.6% versus 15.5%, p=0.04). The cumulative incidence of a chronic health condition among patients, who had survived the first 2 years after HCT, was 32% (95% CI, 30% to 35%) and 59% (95% CI, 56% to 62%) at 2 and 10 years after HCT, respectively. HCT survivors are more likely to report difficulty in holding jobs (allogeneic HCT survivors report a 14-fold increased risk compared with the siblings, while autologous HCT recipients report a 9-fold increase risk). Furthermore, HCT survivors report greater difficulty in obtaining health insurance (7-fold greater difficulty) or life insurance (9-fold greater difficulty) compared with the sibling comparison group.^3–4

BMTSS data reveals that long-term survivors of HCT are at a 2.9-fold increased risk of reporting severe or life-threatening cardiovascular disease when compared with age- and gender-matched siblings.¹⁶ Furthermore, female recipients of autologous HCT are at a 4.4-fold increased risk of premature death from cardiac causes when compared with age-matched general population.⁴ The next few sections focus on the magnitude of risk and pathogenetic risk factors for the development of cardiovascular complications in HCT survivors.

Metabolic Syndrome after Hematopoietic Cell Transplant

Metabolic syndrome is a constellation of central obesity, insulin resistance, glucose intolerance, dyslipidemia, and hypertension, and is associated with a substantially increased risk for type 2 diabetes mellitus and atherosclerotic cardiovascular disease (CVD).^18–20 Data from the third National Cholesterol Education Program Adult Treatment Panel III (ATP III)^21–22 (Table 1) show that the prevalence of metabolic syndrome in the US population of adults is 21.7%, with little difference by sex. A comparative analysis of NHANES III with NHANES 1999–2000 has shown that the age adjusted prevalence of metabolic syndrome in the US has increased from 24.1% in to 26.7% (P=0.088).²³

Table 1.

ATP III Criteria for Metabolic Syndrome – indicated by 3 or more positive findings

Criterion	Adults	Adolescents*
High Triglyceride Level, mg/dL	≥150	≥110
Low HDL-C level, mg/dL
Males	<40	≤40
Females	<50	≤40
Abdominal obesity, waist circumference, cm
Males	>102	≥90th Percentile
Females	>88	≥90th Percentile
High fasting glucose level, mg/dL	≥100)**	≥100**
High blood pressure, mm Hg	≥130/85	≥90th Percentile

^*ATP III criteria modification for adolescents (age 12–19 years) as described by Cook et al, 2003²⁴

^**American Diabetes Association 2003 definition lowers abnormal fasting glucose level to 100mg/dL⁷³ and this change has been incorporated into the current definition of metabolic syndrome⁷⁴

The BMTSS demonstrated that, after adjustment for age, gender and BMI, allogeneic HCT survivors were 3.7 times (95% CI: 1.8–7.3) more likely to report diabetes and 2.1 times (95% CI: 1.4–3.0) more likely to report hypertension compared to siblings. Allogeneic HCT survivors were also more 2.3 times more likely to report hypertension (95% CI: 1.5–3.7) when compared with autologous recipients; finally, TBI was associated with an increased risk of diabetes (OR=3.4, 95% CI: 1.6–7.5).

In adolescent population in the United States, the prevalence of metabolic syndrome (Table 1) was 4.2% (6.1% in males and 2.1% in females),²⁴ with an increase to 6.8% per the NHANES 1999–2000 (P=0.02).²⁵ More importantly, nearly 30% of overweight adolescents meet the criteria for metabolic syndrome. There is preliminary evidence to suggest that childhood cancer survivors are at an increased risk for premature development of metabolic syndrome.^26–28 In adult survivors of childhood ALL, various factors, including female sex,²⁹ genetic predisposition³⁰, exposure to steroids,³¹ and cranial radiation therapy,^29,32 have been implicated in the development of obesity and other conditions characteristic of metabolic syndrome. The Childhood Cancer Survivor Study (CCSS) reports that childhood cancer survivors who have received >20 Gy of cranial radiation are 2.6-fold (females) and 1.9-fold (males) more likely to be obese, when compared with age- and race-matched siblings. The risk for obesity is greatest among females exposed to radiation exceeding 20 Gy at 0–4 years of age.²⁹ While the total dose of TBI exposure in HCT protocols is typically in the range of 10 Gy to 14 Gy, the dose rate is higher, which may have a greater impact at the cellular level than a higher dose given over a longer time period.³³

In addition to significantly higher weight and body fat, childhood cancer survivors have been shown to have higher fasting plasma glucose and insulin levels, and lower serum high density lipoprotein (HDL) cholesterol.²⁶ A combination of obesity, hyperinsulinemia, and low HDL cholesterol was seen in 16% of the survivors, but in none of the controls (P=0.01). Of the survivors with indicators of metabolic syndrome, 50% had received cranial radiation, and also had markedly reduced spontaneous growth hormone (GH) secretion. Similar results were found in survivors of allogeneic HCT performed in childhood. Hyperinsulinemia, impaired glucose tolerance (by oral glucose tolerance test), hypertriglyceridemia, low HDL-cholesterol, and abdominal obesity were more common among the HCT patients than among either the non-HCT group of leukemia patients or healthy controls.³⁴ Core signs of metabolic syndrome were found in 39% of HCT survivors vs. 8% of leukemia controls and 0% of healthy controls. Fifty-two percent of HCT patients were found to have hyperinsulinemia and 43% had abnormal glucose metabolism, compared to none of the healthy controls (p=0.0002, and 0.001 respectively). Variables associated with hyperinsulinemia in the HCT patients were time from transplantation (p=0.01), presence of chronic GVHD (p=0.01), and hypogonadism (p=0.04). Another study found that the patients who received TBI had a significantly higher first phase insulin response and insulinemia/glycemia ratio on glucose tolerance testing as compared to patients who received only lymphoid radiation, no radiation, or controls,³⁵ suggesting that TBI may play a role in the development of insulin resistance.

Presence of metabolic syndrome places survivors at an elevated risk for a number of adverse health outcomes, such as overt diabetes and cardiovascular disease, which when combined with prior exposure to cardiotoxic agents such as chest radiation and anthracyclines and cyclophosphamide,^36–37 could have a potentially devastating consequence on the survivors.²⁸ Preclinical and clinical evidence of the impact of the components of metabolic syndrome on radiation-related atherosclerotic heart disease and anthracycline-related congestive hear failure is described in detail below.

Cardiovascular Effects of Metabolic Syndrome

Atherosclerosis is a complex process involving inflammation and cellular proliferation in arterial walls. The development and progression of atherosclerosis is mediated by a variety of growth factors, cytokines, thrombotic factors, and vasoactive substances. Insulin resistance is a suspected causal pathway because of its clinical association with increased rates of macrovascular disease and subsequent cardiovascular morbidity and mortality.³⁸ Insulin resistance is associated with endothelial dysfunction and impaired insulin-mediated nitric oxide-dependent vasodilation.³⁹ Endothelial dysfunction occurs early in the pathogenesis of atherosclerosis and is associated with many cardiovascular pathological disease states. Strong evidence exists showing the association between arterial stiffness and atherosclerosis, cardiovascular disease and mortality.^40–41 Radiation therapy, including TBI, may lead to arterial stiffness and therefore potentially to atherosclerosis. In an ongoing study we have measured insulin resistance, fasting glucose, insulin, lipids, anthropometry, blood pressure, and carotid artery compliance and distensibility in 106 survivors of HCT performed in childhood (current age 26.6 years) and 72 healthy sibling controls (current age 23.7 years). Preliminary analysis found two or more components of metabolic syndrome were present in 37% survivors and only 13.9% controls (OR, 2.7, 95% CI 1.2–5.9, p=0.02). HCT survivors who had TBI with or without cranial radiation had significantly higher total cholesterol, LDL cholesterol, triglycerides and insulin; lower HDL cholesterol levels and they were also more insulin resistant. However, for the subjects who did not receive any radiation prior to or during HCT, there were no differences in any of the cardiovascular risk factors compared to controls. Carotid artery distensibility was decreased in survivors who received TBI compared to controls with even greater negative impact in those who received TBI and pre-HCT cranial radiation. These findings are concerning and suggest that even at a relatively young age, and independent of obesity, HCT survivors of childhood hematologic malignancies have increased cardiovascular risk factors present as well as adverse vascular changes, which are associated with exposure to TBI, with or without cranial radiation. These abnormalities may ultimately contribute to a higher risk of early cardiovascular morbidity and mortality and thus early screening and management of modifiable cardiovascular risk factors should be considered in HCT survivors.

Anthracycline-related congestive heart failure after HCT

Congestive heart failure (CHF) is a well-described sequela during the immediate post-HCT period. Mortality attributed early CHF ranges from 1% to 9% while morbidity ranges from 5% to 43%.^42–44 Risk factors for early CHF include reduced pre-HCT ejection fraction (EF), conditioning with high dose cyclophosphamide (HD CY) and total body irradiation (TBI).^44–46 The occurrence of CHF after the first year (late CHF) is less well studied, with few reports describing this outcome hampered by relatively short lengths of follow-up and small sample size.^47–48 Potential mediators of late CHF include pre-transplantation exposure to known cardiotoxic agents such as anthracyclines, alkylating agents, and mediastinal radiation, compounded by HD CY and TBI at the time of HCT and the presence of GvHD after HCT.^49–50 Furthermore, the magnitude of risk could be moderated by the presence of comorbid conditions and host characteristics such as age at exposure and sex of recipient.⁴⁹

We recently examined the independent role of pre-HCT exposure to therapeutic agents, transplant-related conditioning and co-morbidities in the development of late CHF after HCT.⁵¹ Using a nested case control design, we identified cases with late CHF from a cohort of nearly 3,000 1+ year survivors who underwent HCT. Median age at CHF diagnosis was 51 years (range 22.5 to 71 years), and median time from HCT was 3 years (range, 1 to 19 years); all had clinical CHF per the American Heart Association (AHA)/American College of Cardiology (ACC) guidelines.⁵² We found pre-HCT exposure to anthracyclines and presence of post-HCT co-morbidities to be primarily responsible for the risk of late CHF after HCT, while conditioning-related exposures did not appear to contribute significantly to the risk. The cardiotoxic effect of anthracyclines was highest for autologous HCT recipients, with a cumulative dose of ≥250 mg/m² being associated with a 30-fold increased risk of late CHF. Overall survival was less than 50% at 2 years after CHF diagnosis.

Role of genetic susceptibility in the development of Cardiac Complications after HCT

It is becoming increasingly recognized that risks for many diseases result from an interaction between inherited gene variants and environmental factors, including chemical, physical, and behavioral factors. However, there continue to be large gaps in knowledge with regards to the pathogenesis of therapy-related adverse events. These gaps can be filled only by approaching these problems in a systematic, comprehensive manner that not only helps further the understanding of disease biology but also identifies those at highest risk of these adverse outcomes, raising the possibility of targeted disease prevention and health promotion efforts for individuals at high risk because of their genetic makeup.

Recent advances in the sequencing of the human genome^53–54 have created many opportunities for investigating the relationship between polymorphic genetic variants present in the human population and their impact on the pathogenesis of various medical conditions.^55–57 The traditional approach has been to use the candidate gene approach, in which a gene or pathway is targeted as potentially important based on a priori hypotheses about their etiological role in development of disease.⁵⁸ However, the sequencing of the human genome has greatly expanded the ability of researchers to broaden the focus of genomic studies and perform genome-wide association studies (GWAS), allowing the study of genetic variation across the entire genome and risk of disease.⁵⁹ Here we present the example of anthracycline-related congestive heart failure (A-CHF) to demonstrate how genetics could potentially play a role in explaining the observed inter-individual variability in risk of developing A-CHF.

A-CHF is thought to be related to direct myocardial injury due to formation of free radicals.⁶⁰ Well-recognized clinical risk factors for A-CHF include female sex, younger age at exposure, and radiation therapy involving the heart.^61–63 However, these clinical risk factors do not fully explain the wide inter-individual variability in susceptibility to A-CHF. Significant cardiotoxicity has been reported at cumulative doses of less than 250 mg/m²,⁶⁴ while doses that exceed 1000 mg/m² have been tolerated without long-term sequelae by some.⁶⁵ Among long-term HCT survivors,⁵¹ 40% of cases with clinical CHF had received a cumulative dose of less than 250 mg/m².This heterogeneity could be explained, in part, by genetic susceptibility that alter the metabolism of anthracyclines, the myocardial response to the drug, as well as others thought to play a role in susceptibility to de novo disease.^66–68

The strongest support for the role of these genes as regulators of response to anthracyclines has come from transgenic and knock-out mouse strains with altered sensitivity to the drugs. For example, transgenic overexpression of the multiple drug resistance gene (MDR1) is protective against the cardiotoxic effect of anthracyclines,⁶⁹ while deficiency and overexpression of anthracycline metabolizing enzyme, carbonyl reductase (CBR) in mice had opposing effects (protecting and enhancing) on susceptibility to A-CHF.^66,70–71 A limitation of information obtained from such genetically engineered mouse strains is the strong penetrance of introduced mutations, which likely does not reflect more subtle genetic variability in humans.⁶⁷

Two recent studies in humans have evaluated the role that these genetic alterations play in the development of clinical A-CHF. These studies suggest that polymorphisms in genes involved in anthracycline metabolism, transport, and superoxide generation may have an impact on an individual’s risk of A-CHF. In the first study,⁶⁸ 82 genes involved in the metabolism of reactive oxygen species (ROS), DNA repair, drug transport and metabolism, endothelial physiology, the rennin-angiotensin system, muscle contraction and structure, and inflammation were evaluated. The risk of chronic cardiotoxicity was significantly increased (OR: 2.5) in individuals with a polymorphism for the NADPH oxidase subunit, NCF4, which is responsible for down-regulation of the enzyme involved in a multi-enzyme complex regulating ROS generation. Inherited NADPH oxidase deficiency may result in impaired ROS defense capacity and therefore lead to increased intracardiac ROS levels after anthracycline exposure.⁶⁸

The second study⁶⁶ examined polymorphisms in genes such as: catalase (CAT), superoxide dismutase 1 (SOD1), NADPH:quinone oxidoreductase (NQO1), and carbonyl reductase 3 (CBR3). A polymorphism in CBR3 (V244M) was associated with a markedly increased risk of A-CHF among homozygous and heterozygous individuals (OR: 8.2 and 5.4, respectively). It is believed that the functional CBR3 polymorphism may modulate intracardiac formation of cardiotoxic anthracycline alcohol metabolites, leading to myocardial injury and eventual cardiac dysfunction. These two studies illustrate how certain “at-risk” alleles in key candidate genes could identify individuals at the highest risk of developing anthracycline-related cardiotoxicity. If these findings are replicated and confirmed by others in independent study samples, they could set the stage for identifying a subgroup of patients up front who would perhaps need alternative treatment for management of their cancer; while for those who have already received the anthracyclines, identification of high-risk alleles would warrant closer surveillance for cardiotoxicity and use of medications that modulate cardiac function.

The data presented here demonstrates that HCT survivors have a high burden of morbidity; especially as it relates to development of cardiovascular disease, and that attention needs to focus on instituting systematic and targeted follow-up of those at high risk.⁷²