Survival Probability of Adults With Cystic Fibrosis Depending on Their Biological Status

Background: Cystic brosis (CF) is one of the most common autosomal recessive disease, and the type of mutation is recognized as one of the most important factors determining the survival rate. Factors contributing to disease exacerbations, and survival rate are poor nutritional status, lung failure, and infection development by Pseudomonas Aeruginosa. The study aimed to evaluate the effect of the severity of mutation, nutritional status, lung function, and Pseudomonas aeruginosa infection on survival rate in adult patients with Cystic Fibrosis. Material and methods: A study of 124 (68 (cid:0) and 56 (cid:0) ) CF patients aged from 18 to 51 years were evaluated for: a) type of mutation in the CFTR gene, b) nutritional status (BMI), c) lung function (FEV1%), and d) Pseudomonas aeruginosa (PA) infection. For statistical calculations, Kaplan-Meier analysis of survival and Chi-squared test for multiple samples were used. Results: Both the type of mutation (Chi²=12.73, df= 3, p=0.005), lung function (Chi² = 15.20, df = 2, p = 0.0005), PA infection (Chi²= 11.48, df= 3, p= 0.009), and BMI (Chi²=31.08, df=4, p<0.000) signicantly differentiated the probability of survival of CF patients. The shortest life expectancy was observed in patients with a severe type of mutation on both alleles, FEV1% between 40-70%, subjects in whom Pseudomonas culture was extensively drug-resistant or pandrug-resistant, and patients whose BMI was lower than 18.5 kg/m². The period from 30 to 40 years of age was of the most critical in CF adults’ lifespan. Furthermore, most exacerbations occurred between 20 and 35 years of age. Conclusions: All factors included in the study signicantly inuenced the survival rate of patients with cystic brosis. In the face of the growing population of CF patients, the research on factors affecting their life expectancy seems to take on greater importance.

0.0005), PA infection (Chi²= 11.48, df= 3, p= 0.009), and BMI (Chi²=31.08, df=4, p<0.000) signi cantly differentiated the probability of survival of CF patients. The shortest life expectancy was observed in patients with a severe type of mutation on both alleles, FEV1% between 40-70%, subjects in whom Pseudomonas culture was extensively drug-resistant or pandrug-resistant, and patients whose BMI was lower than 18.5 kg/m². The period from 30 to 40 years of age was of the most critical in CF adults' lifespan. Furthermore, most exacerbations occurred between 20 and 35 years of age.
Conclusions: All factors included in the study signi cantly in uenced the survival rate of patients with cystic brosis. In the face of the growing population of CF patients, the research on factors affecting their life expectancy seems to take on greater importance.

Background
Cystic brosis (CF) is one of the most common autosomal recessive disease caused by 2102 mutations of the CFTR gene [1]. The type of mutation is recognized as one of the most important factors determining the survival rate. Severe mutations of at least one allele reduce the survival of CF patients and are associated with lung function decrease [2]. It is already known that the main factors contributing to disease exacerbations are lung failure and infection development [3]. One of the most important pathogens in CF adults, causing chronic infection and evolve into antibiotic resistance is Pseudomonas aeruginosa (PA). Previous studies showed that PA, mainly multidrug-resistance culture is associated with morbidity and mortality in CF patients [5]. The decline in lung function is also associated with poor nutritional status [6][7][8][9].
Nowadays, as reported by recent studies, the number of CF adults is growing steadily [10]. The majority of previous studies about CF survival concentrated on groups of younger patients. Therefore, there is an urgent necessity to take a view of biological status and survival in an emerging group of older, adult patients. Moreover, such analysis will show how genetic and biological variables change life expectancy through the life years. A multicenter report on CF patients over forty conducted by Hodson et al. 2008 [11] and showed that signi cant numbers of patients are now surviving to 40 years or more, however, the answer to the question of why some CF patients live longer than others is still unclear. Hence, the study aimed to evaluate the effect of lung function, Pseudomonas aeruginosa infection, nutritional status, and the severity of mutation type on survival rate in CF adult patients. Information about the type of mutation, nutritional status, lung function, and PA infection were collected. Data about the type of mutation of the CFTR gene were obtained from the archives of medical records of the Department of Pulmonology, Allergology, and Respiratory Oncology of the University of Medical Sciences in Poznan. To systematize a host of known mutations, all patients were divided into four groups based on severity, with consideration given to the widely accepted mutation classi cation in the CFTR gene [12][13] (Table 1)   Lung function was determined by a spirometry test using a diagnostic Jaeger MasterScreen system (Erich Jaeger GmbH; Würzburg, Germany). Data about predicted forced expiratory volume in 1 second (FEV1%) were collected from 124 patients. According to the Cystic Fibrosis Trust [14], all subjects were divided into three subgroups based on FEV1%: 1-patients within the norm (FEV1%>70), 2-with moderate pulmonary impairment (FEV1% 70 − 40), and 3-severe pulmonary impairment (FEV1% < 40) ( Table 2).

Methods
The microbiological examination carried out by a microbiological laboratory was performed in all patients. Microbiological data allowed us to classify patients into the following groups ( Table 2): 1-Pseudomonas culture-negative (non-PA,), and 2-Pseudomonas culture-positive (PA). Drug susceptibility was measured using the Eucast v.6.0 method. PA positive patients were divided into 2a -patients in whom all antibiotics used to treat infections caused by bacterial colonization were fully effective (nonmultidrug resistant/non-MDR), 2b -subjects in whom Pseudomonas culture was insensitive (resistant or moderately sensitive) to at least one antibiotic from at least three groups of antibacterial drugs (multidrug-resistant, MDR) and 2c-patients in whom Pseudomonas culture was extensively drug-resistant (XDR) or pandrug-resistant (PDR). The above division was made by the de nitions from the work of Magiorakos et al. [15].
Nutritional status was determined based on BMI (Body Mass Index) calculated by dividing body weight by height squared (kg/m2). To obtain this data anthropometric measurements were taken. The body height was measured without shoes and in underwear, with a GMP anthropometer, with a measurement accuracy of 1 mm. Bodyweight was measured using a medical scale with a measurement accuracy of 100 g. To exclude measurement errors, all measurements were performed by one experienced researcher.
Based on the BMI, a group of CF adults was divided into ve (

Results
Survival analysis using the Kaplan-Meier method has shown that the survival of CF patients remains at the level of 100% up to 20 years of age; afterward, it began to decline gradually (Fig. 1). The decline in the probability curve shows that almost 27% of patients will not exceed 30 years of life, and almost 48% of patients will not exceed 40 years of life.
In our study group, all patients who lived to the age of 40 are still alive (N = 7). At the time of writing this article, three subjects were 41 years old, and four were: 42, 43, 49, and 51 years old.
The probability of survival was different depending on the severity of mutation type (Chi²=12.73, df = 3, p = 0.005). The average life expectancy of patients with I-III/I-III mutations was the lowest. The survival curve (Fig. 2) in this group decreases after the age of 20, and the probability of death before the age of 35 was almost 60%. In this group, there were no people over 40 years of age. Subjects with I-III/IV-V or I-III/u mutations lived longer than in group I-III/I-III. The survival curve in this group began to decline after 23 years, and the probability of death before reaching 32 was over 40%. It is seen that the curve stabilized after 32 years of age. Besides, it should be emphasized that there is one person in this group who has reached the age of 49. Among patients with I-IV/I-IV mutations, the decline in the survival curve was small, and the curve stabilized after 25 years of age. However, it can be seen that 40 years of age is a critical moment because only three patients still live who are over this age. In the group with u/u mutations, there are no dead people, their survival is stable. In this group, two people were over 40 and one was over 50 years old (Fig. 2).
The factor that signi cantly differentiated patients in terms of life expectancy was also lung function (Chi²=15.20, df = 2, p = 0.0005). Those CF patients whose FEV1%>70% live the longest, there are no deceased people in this group, but there are people over 40 and 50 years old. The survival curve of CF adults with FEV1% between 40-70% was steeper. The chance that these patients would live to be 40 years old was almost 50%. Approximately 20% will die before the age of 34 and another 28% of patients before reaching 40 years of life. The lowest life expectancy applies to patients with FEV1%<40. The probability of survival in this group began to decrease after reaching the age of 20. The survival curve was very steep up to the critical point of 26 years of age. In this group, the probability of death before the age of 30 was almost 50%, and only about 35% of patients have a chance to live to be 40 years old (Fig. 3).
Kaplan Meier survival analysis showed also that PA signi cantly in uenced the length of the patient's life (Chi²=11.48, df = 3, p = 0.009). The longest average life expectancy is indicated among non-PA and non-MDR patients (Fig. 4). The survival curve was comparatively stable in both groups. The probability of exceeding 40 years of age in these patients was approximately 80%. The average life expectancy was lower in MDR patients. The probability of survival was decreasing after 22 years of life. The probability of exceeding 30 years of age was less than 60%. The survival of XDR and PDR patients was the most detrimental. The probability of survival was decreasing steadily from 20 to 40 years of age. Further, nutritional status assessed by BMI (Fig. 5)  The probability of survival among CF adults is a result of both genetic and environmental factors shaping the course of the disease. In our CF group survival curve started to decrease after twenty and turned to be stable after forty years of age. Most of the deaths occurred in the period between 30-40 years of age. The fundamental factor contributing to average life expectancy was the type of mutation. Since the early nineties of twentieth-century mutation causing CF has been grouped in ve classes based on molecular mechanisms of CFTR chloride channel dysfunction [16], which re ects the disease phenotype. Our research indicated the difference in life expectancy between groups of CF patients distinguished based on the severity of the mutation. Patients with a mild mutation on one allele or both alleles lived longer than subjects from the group with severe types of mutation on both alleles. Similar results have been shown by McKone et al. [17], who found that the mortality rate is lower among patients with IV and V mutation class comparing to II class (F508del homozygotes). Currently, there are proceeding molecular studies concerning cell and gene therapy, posing a milestone in CF treatment. New strategies of therapy allow the transfer of the wild-type CFTR gene to airway cells [18]. Apart from genetic treatment, an important role is played by the early determination of the CFTR gene mutation type and the rapid implementation of modern methods of treating patients. Neonatal screening allows for early diagnosis and therefore contributes to better growth and pulmonary outcomes [19]. A patient who already undergoes proper medical care at the rst stage of the disease, using knowledge about the effects of a given mutation, has a chance for a much longer life compared to statistics carried out in previous decades. Our study showed, however, that independently of other factors, a strong in uence of the genetic factor on the rate of disease progression is still observed. The most critical period in CF adults with a severe type of mutation is between 20 to 30 years of life. Similarly, for I-III/IV-V patients' crucial stage of the disease occurs between 25 to 31 years of life. Accordingly, to this result, the most in uential impact of CFTR mutation is observed from 20 to about 31 years on the lives of CF people.
CF evolves differently being in uenced by various factors, e.g. biological factors, socio-economic status, health care, or lifestyle [20][21][22]. Priority in CF treatment is proper lung function and prevention of undernutrition. These factors are the basis for maintaining a stable patient. The results of this and previous studies [22] con rm the impact of lung function and nutritional status on the survival of CF adults. Severe and moderate pulmonary impairment has a major impact on average life expectancy.
Critical moments concern exclusively adults with FEV1%<70%. Our results are consistent with other authors [23][24]. The signi cant impact of lung function as a predictor of mortality has been emphasized by many researchers [25][26]. Henry et al. [27] after performing stepwise regression, showed that lung function described by FEV1% has the main effect on patient's survival. According to Belkin et al. [28], FEV1%<30% increases the risk of death among both CF adult and pediatric patients. Similarly, our analysis showed that FEV1%<40% is critical for patients' mortality, unable them to live into 40 years. Furthermore, in a group of patients with severe pulmonary impairment, the majority of subjects die before 30 years old. Effective pulmonary disease therapy can, therefore, form the basis for preventing exacerbations and contribute to better survival of CF patients. Our probability of survival analysis highlights the problem of serious pulmonary exacerbations occurring between 20 and 40 years old. After exceeding 40 years of life the survival curve is mostly stable. A study of Simmonds [23] comparing CF adult patients aged ≥ 40 with those, who died before 30 years of age showed also, that patients without respiratory disease and with higher BMI were more likely to live into 40 years of age.
Of crucial importance for the course of lung disease are pathogen infections. Among CF adults, PA is the most frequently occurring pathogen, concerning approximately 80% of the population [29]. Our studies showed that the presence of PA decreases average life expectancy. Previous studies conducted among children con rm the adverse effect of PA on survival, lung function, and weight percentiles [30]. The negative impact of this pathogen remains in adulthood, with a tendency to exacerbate. Courtney et al. [26] showed that PA infection signi cantly affects CF adults' mortality. Their studies revealed that the majority of patients who died (98%) had a chronic infection of this pathogen. We showed that the most critical point in survival is when PA become fully or partly resistant to antibiotic treatment. Lechtzin et al. [29] obtained similar results, con rming that multiple-antibiotic-resistant PA is associated with FEV1% reduction and pulmonary disease.
Moreover, exacerbations in lung function may severely affect nutritional status in CF people. Our study showed very explicit differences in the probability of survival with nutritional status. A signi cant number of patients died before turning 30 in a group of patients with a BMI < 18.5. Undernourished CF adults lived no longer than 34 years old. Undernutrition in CF is severely affected by pancreas insu ciency, fat malabsorption, micronutrient de ciencies, and increased energy expenditure [31]. Furthermore, energy losses in the course of lung disease exceed dietary intake. In turn, poor nutritional status may be a subsequent considerable factor in the CF adult lifespan. This is con rmed by previous studies [21][22][23][24][25].
Research by Yen et al. [9] showed that higher body weight in childhood is associated with fewer pulmonary exacerbations and better survival through the age of 18 years. In turn, according to Courtney et al. [26], BMI is not a reliable predictor of mortality but has a signi cant impact on pulmonary disease. However, studies conducted by Sharma et al. [32] showed that body wasting has a signi cant impact on the probability of survival in CF adult patients regardless of lung function.

Conclusions
In summary, we can assume that genetic factors pose a major impact on the average life expectancy in CF adults. However, if patients can step into adulthood despite severe CFTR mutation, the in uence of other factors increases. The shortest average life expectancy is indicated in people exhibiting moderate and severe lung impairment, antibiotic resistance, PA infection, and consequently poor nutrition. Severe mutation type, FEV < 40%, BMI < 18.5, and XDR or PDR PA infection precluded CF adults from exceeding 40 years of age. These factors are the most alarming in the course of illness, contributing to disease exacerbations. In comparison, among patients with lung function within the norm and BMI above 25, no one has died. CF adults above 40 remained in groups of an unknown or mild type of mutation, with normal or moderate lung function, with no PA infection, or with strains susceptible to antibiotic treatment.
Our study allowed us to determine that period from 30 to 40 years of age is of the most critical in CF adults' life span. Furthermore, most exacerbations occur between 20 and 35 years of age. However, the distinguished period is wide and unspeci ed and there are still not enough studies among adult CF people. That is why more research should be done to precise the impact of different factors on survival in an emerging group of adult CF people.