Background
Childhood acute lymphoblastic leukemia (ALL) is the most common pediatric cancer, representing about 30% of all childhood malignancies. Over the past four decades, the progress in treatments for childhood ALL has made it possible to achieve a five-year survival rate over 80% [
1].
However, the aggressive treatments regimen often lead to several long-term adverse effects, including cardiac toxicity and cardiovascular issues in survivors [
2]. Physical activity has an important role in the prevention of cardiovascular diseases and in the management of metabolic disorders. A good cardiorespiratory fitness level in childhood ALL survivors has shown a significant positive impact on mental health, quality of life, bone strength and mortality [
3‐
6]. It has been shown in healthy and untrained populations that there is a great inter-individual variability in subjects’ capacity to improve their cardiac profile in response to regular exercise [
7]. This can partly be explained by genetic variations between subjects, which can ultimately have an influence on their trainability [
8]. Studies on healthy subjects and athletes have reported associations between genetic variants and changes in muscle strength [
9,
10] and cardiorespiratory fitness in response to exercise [
11]. It is essential to improve our knowledge of genetic determinants of trainability in childhood ALL survivors given that anticancer drugs, such as anthracyclines, can modify gene expression [
12] and induce an impairment of cardiac function [
13].
In this study, we investigated candidate genes linked to health-related fitness and performance and their association to high and low cardiorespiratory fitness levels in a cohort of childhood ALL survivors. Such genetic biomarkers will be important in the post-treatment follow-up of childhood ALL survivors by allowing personalized cardiorespiratory fitness risk management interventions.
Results
A total of 239 childhood ALL survivors were included in our analyses (Table
1). Participants were divided in two groups according to their median predicted
\( \dot{\mathrm{V}} \) O
2 peak. The first group opposed above and below the median of cardiorespiratory fitness level of the childhood ALL survivors (< 83.8% vs ≥83.8%) (Table
2), whereas the second group opposed the two extremes at least two standard deviations away from the median (≤48.3% vs ≥125.6%) (Additional file
1: Table S3).
Table 1
Clinical characteristics of acute lymphoblastic leukemia survivors
Total | 239 | 120 | 119 |
Age at visit (y) | 21.5 ± 6.1 | 21.5 ± 6.3 | 21.6 ± 6.0 |
Age at cancer diagnosis (y) | 6.3 ± 4.6 | 6.2 ± 4.4 | 6.4 ± 4.7 |
Time from the end of the treatment (y) | 13.0 ± 5.0 | 13.0 ± 5.3 | 12.9 ± 4.8 |
Weight (kg) | 66.8 ± 16.4 | 63.7 ± 16.6 | 70.0 ± 15.7 |
Height (cm) | 166.9 ± 10.0 | 160.3 ± 6.7 | 171.5 ± 9.6 |
\( \dot{\mathrm{V}} \)O2 peak (mL.kg− 1.min− 1) | 32.6 ± 8.4 | 27.6 ± 6.5 | 36.9 ± 7.4* |
Power at \( \dot{\mathrm{V}} \) O2 peak (W) | 174.7 ± 55.3 | 133.5 ± 28.9 | 213.8 ± 45.0* |
\( \dot{\mathrm{V}} \)O2 peak predicted (%) | 85.7 ± 17.1 | 86.8 ± 18.3* | 83.2 ± 15.4 |
Physical activity (min) | 27.8 ± 29.4 | 24.9 ± 30.3 | 30.5 ± 28.4* |
DOX (mg/m2) | 182.8 ± 119.7 | 183.4 ± 121.9 | 182.2 ± 117.9 |
DEX (mg/m2) | 2781.0 ± 440.0 | 2828.8 ± 374.4 | 2731.8 ± 499.4 |
Table 2
Clinical characteristics of acute lymphoblastic leukemia survivors according to their cardiorespiratory fitness level
Total | 119 | 120 |
Gender (Females / Males) | 54/65 | 66/54 |
Age at visit (y) | 21.6 ± 5.9 | 21.5 ± 6.4 |
Age at cancer diagnosis (y) | 6.2 ± 4.8 | 6.3 ± 4.4 |
Time from the end of the treatment (y) | 13.1 ± 5.1 | 12.8 ± 5.0 |
Weight (kg) | 67.4 ± 18.1 | 66.2 ± 14.6 |
Height (cm) | 166.1 ± 11.5 | 165.6 ± 8.3 |
\( \dot{\mathrm{V}} \)O2 peak (mL.kg−1.min−1) | 28.5 ± 6.2 | 35.8 ± 8.6* |
Power at \( \dot{\mathrm{V}} \) O2 peak (W) | 162.6 ± 55.2 | 185.6 ± 53.3* |
\( \dot{\mathrm{V}} \)O2 peak predicted (%) | 71.9 ± 10.0 | 98.0 ± 11.7* |
Physical activity (min) | 22.4 ± 29.6 | 32.9 ± 28.5* |
The common variants analyses are presented in Table
3 (also see Additional file
1: Table S4). The carriers of the
TTN minor allele were overrepresented among survivors with a cardiorespiratory fitness level < 83.8%. The major allele in other genes (
LEPR, IGFBP1 and
ENO3) had, in contrast, a protective role and were represented less frequently among female survivors who had cardiorespiratory fitness level < 83.8%. Significant genetic variants were further analyzed in logistic regression (Table
3 and Additional file
1: Table S4) in which the effect of genetic variant is adjusted for age at visit, sex, cumulative doxorubicin dose and time from end of the treatment and no significant confounding factors were observed in these associations. The analysis of functionally predicted rare variants detected an association (with FDR < 10%) between the low cardiorespiratory fitness level and rare variants enrichment in the
SLC22A16 gene for the HR group (
p = 0.001) (Additional file
1: Table S5). Our analyses of rare variants showed a significant association with a low cardiorespiratory fitness in the HR group without the cardio-protective agent dexrazoxane. No other association with the appropriate FDR threshold was shown in our analysis of rare variants. In order to identify the individual contributions to the observed association signal, we explored different variant combinations with the collapsing approach. The significant signal came from the
SLC22A16 gene where three variants (rs41288594, rs61729086, rs146329765) were contributing to the signal (Additional file
1: Table S5). Our analyses reported the individual contribution of variant rs61729086 but showed no individual contributions of variants rs41288594 and rs146329765. The significant genetic variant was further analyzed in a logistic regression (Additional file
1: Table S5). Analyses revealed that there was no significant effect of co-variables in the multivariate model due to the confounding effects of other covariates and the small sample size of the low frequency genetic variant.
Table 3
Significant genetic associations between cardiorespiratory fitness and common variants
Survivors < 83.8% predicted \( \dot{\mathrm{V}} \) O2 peak | TTN | rs6723526 | 33 (14.47) | 195 (85.53) | 9 (4.55) | 189 (95.45) | 0.0005 | 0.04 | 0.1 | 3.55 (1.66–7.63) | 0.0006 | 4.47 (1.89–10.53) |
Female survivors < 83.8% predicted \( \dot{\mathrm{V}} \) O2 peak | LEPR | rs1137100 | 22 (17.19) | 106 (82.81) | 34 (37.78) | 56 (62.22) | 0.0008 | 0.03 | 0.25 | 0.34 (0.18–0.64) | 0.003 | 0.33 (0.15–0.68) |
IGFBP1 | rs4619 | 43 (33.59) | 85 (66.41) | 51 (56.67) | 39 (43.33) | 0.0008 | 0.03 | 0.42 | 0.39 (0.22–0.67) | 0.01 | 0.46(0.24–0.86) |
ENO3 | rs238239 | 41 (32.54) | 85 (67.46) | 46 (53.49) | 40 (46.51) | 0.002 | 0.05 | 0.4 | 0.42 (0.24–0.74) | 0.005 | 0.34 (0.16–0.73) |
Discussion
This study was the first to assess genetic determinants of the cardiorespiratory fitness level in childhood ALL survivors. We observed that a low cardiorespiratory fitness was associated with genes primarily related to survivors’ trainability, particularly with common variants in the TTN, LEPR, IGFBPI and ENO3 genes and with a rare variant in the SLC22A16 gene. This was particularly obvious among female survivors.
The involvement of
TTN and
LEPR, two cardiac-related genes were associated with a decrease of cardiorespiratory fitness in survivors. Our findings showed that the common variant rs6723526 in
TTN was associated to survivors with cardiorespiratory fitness level < 83.8%.
TTN, which encodes the protein titin, one of the most abundant proteins in the human striated muscle myofilament [
43] and plays an essential role in skeletal and cardiac muscle [
44]. The HERITAGE family study demonstrated a similar influence of the
TTN gene in regards to cardiac function during endurance training [
45]. We also found that the common variant rs1137100 in the LEPR gene encoding the leptin receptor involved in body weight regulation [
46], insulin resistance [
47] and physiological adaptation response to exercise [
48], was associated with a cardiorespiratory fitness level < 83.8% and ≤ 48.3% in female survivors. The
LEPR gene had been reported by the HERITAGE study as associated with a lower
\( \dot{\mathrm{V}} \) O
2 max training response [
49] and was also associated with significantly lower physical activity levels in young children in another report [
50]. Thus, the
TTN and the
LEPR genes are respectively associated with cardiac function during endurance training and a lower
\( \dot{\mathrm{V}} \) O
2 max training response.
Another gene linked to insulin resistance is
IGFBP1. This gene is also associated with lower cardiorespiratory fitness in female survivors. Indeed, we found that the common variant rs4619 in
IGFBP1 was associated with a cardiorespiratory fitness level < 83.8% and ≤ 48.3%, especially in female survivors. This gene is a member of the insulin-like growth factor binding protein family which plays an important role in cell migration and metabolic processes, influencing cardiorespiratory fitness level due to its role in energy metabolism [
51]. It has been reported that while there is no significant change in
IGFBP1 levels in response to exercises with high resistance [
52], changes are observed in response to exercises that are short in duration [
53].
Similarly to the positive association between the expression of the IGFBP1 gene and physical inactivity [
54], the enolase 3 (ENO3) gene is positively associated with exercise intolerance [
55]. The
ENO3 gene is especially important because of its role in muscle development and regeneration [
56]. It encodes β-enolase and is mainly expressed in adult striated muscle (i.e. skeletal and cardiac muscle) [
57,
58]. It has been found that the absolute cellular level of enolase activity is higher in skeletal muscle than in other tissues (e.g., liver, kidney, spleen and neural tissues) [
59] due to the heavy metabolic requirement of skeletal muscle [
55]. Our findings showed that the common variant rs238239 in the
ENO3 gene was associated with a cardiorespiratory fitness level < 83.8% in female survivors.
LEPR,
IGFBP1 and
ENO3 genes were found to be only significantly associated with a lower cardiorespiratory fitness in female survivors. As discussed previously, these genes play important roles in energy metabolism, body weight regulation and muscle development and regeneration [
46,
51,
56]. Female survivors showed a greater impairment of total daily energy expenditure after treatments with anthracyclines [
2]. Indeed, it is reported that in ALL survivors, cardiorespiratory levels are significantly lower in comparison to a similarly aged non-cancer population, especially in female survivors [
60]. Many studies have reported a deterioration of the cardiorespiratory fitness in female survivors [
60‐
63], which is consistent with our genetic association.
The absence of results in the common variants analysis with stratification by prognostic risk group (i.e. SR, HR and HR + DEX) was unexpected and could be due to lack of power. Indeed, several studies have shown genetic contributions to cardiac toxicity induced by higher cumulative dose of anthracyclines in pediatric cancer patients [
64‐
66]. These findings confirmed that the doxorubicin dose can be an important risk factor [
65,
67] which can affect the cancer subjects’ trainability [
68] and their cardiovascular response [
69‐
71]. It has been shown that the
SLC22A16 gene could contribute to variations in the doxorubicin pharmacokinetics, resulting in a possible higher incidence of adverse effects [
72].
SLC22A16 also plays a key role in mitochondrial fat oxidation by transporting carnitine, which is important during exercise [
73]. Previous studies have shown that its supplementation of carnitine has the potential to improve β-oxidation during exercise in order to improve performance by making ATP available for mechanical work [
74‐
77]. In this study, we found that a low cardiorespiratory fitness was detected in association with the presence of rare variants in the
SLC22A16 gene. Our findings revealed that there was no significant effect of co-variables in the multivariate model due to the confounding effects of other covariates. A previous study reported that the
SLC22A16 gene was associated with the doxorubicin dose in female cancer patients [
78]. This could explain that for survivors in our HR group, the
SLC22A16 gene was negatively associated with cardiorespiratory fitness.
It should be noted that our statistical power is limited by the relatively small sample size (
n = 239). The strengths of our study is the analysis of French Canadian population because it is characterized by its homogeneity, not only at the genetic level but also in terms of lifestyle [
15]. Moreover, no significant differences between the frequency of common variant alleles between French-Canadian and other populations of European descent have been reported [
79].