Specifics of cystic fibrosis genetic spectrum in Georgia

Background

Cystic fibrosis is one of the most common life-threatening autosomal recessive diseases caused by a mutation in the cystic fibrosis trans membrane conductance regulator gene (CFTR). The estimated incidence among Caucasians is 1 in 2,000-4,000 births. More than 2000 mutations are identified. The most common mutation in the world is F508del.1, 2, 3

The frequency and spectrum of mutations can differ among countries, populations, and ethnic groups. Knowing the distribution of the CF-causing mutations among each population, especially with mixed ethnic groups, can be beneficial in developing diagnostic tools, disease management, and prenatal diagnosis, also developing a system to screen a panel of the most frequent mutations without sequencing the entire CFTR gene. 4, 5, 6

The total population of Georgia in 2012 -2020 was about 3 716 000. There are three major ethnic groups: Georgians, Azerbaijanis, and Armenians. There is only 1 CF center, located in the capital city, Tbilisi. All diagnosed cases of CF from birth to 18 years are followed up at this center. The neonatal screening program was implemented in 2012 in Tbilisi only and has become nationwide since 2018. 

This study aims to report the distribution of CF-causing mutations in a representative group of Georgian CF patients and assess the clinical course of the disease with different CFTR genotypes.

According to our information, no such study was ever performed.

Materials and Methods

The study cohort comprised 129 patients aged between 25 days and -17,38 years, originating from all parts of Georgia (119 Georgians, 4 Georgian Azerbaijanis, 6 Georgian Armenians) and regularly following up at the Georgian CF center. The diagnosis of CF was based on clinical and laboratory consensus diagnostic criteria. 7, 8 Informed consent was obtained from every patient, their parents, or legal representatives. Blood samples of 91 patients were sent to the Department of Biology and Medical Genetics, University Hospital Motol Prague, Czech Republic. Complete analysis of the CFTR gene, coding region, including analysis in intra-CFTR rearrangements and of adjacent intronic sequences, was performed according to an established methodology. 9, 10, 11, 12, 13 

To assess the genotype-phenotype correlation from 91 consented Georgian CF patients, we collected the following parameters: age at diagnosis and at the last examination visit, sweat chloride test results, weight, height, Body mass index, and clinical information, including the pancreatic sufficiency, lung function, thyroid function, chronic colonization by P. aureginosa and S. Aureus, complications (liver disease, meconium ileus, CF–related diabetes). 

Results

Over the period from 2012 to 2020, a total of 129 patients were registered at the Georgian Cystic Fibrosis Center. Among these, CF was confirmed in 34 children through positive neonatal screening. The age at diagnosis ranged from a minimum of 28 days to a maximum of 143 days, with the mean age for diagnosis being 51 days. The prevalence of CF in Georgia, estimated as 1 in 4000 births, underscores the significance of this genetic disorder within the country. (Data is taken from the National Statistics Office of Georgia).  

For the first time in Georgia, our study analyzed the genetic mutations in 129 CF patients, <18 yerasold from all parts of Georgia. 91 clinically confirmed CF patients, revealing a total of 29 CF-causing mutations. (see the table) The most prevalent mutations in Georgian CF patients were 1677delTA (42.7%), W1282X (11.2%), F508del (6.7%), 3120+1G>A (4.5%), D11OH (3.4%), 3199del6/I148T (2.8%), E92K (2.2%), and G542X (1.7%). Notably, the 1677delTA mutation accounted for 42.7% of detected mutations.

Among the patients, 22 (34.8%) were found to be homozygous for the 1677delTA mutation, while 32 were heterozygous.

Our study identified three novel CF-causing mutations (c708dupT, CFTRdele16_17, and c3170C>G) not previously documented in CF mutation databases.Following mutations were added to the CFTR2 database. The cFTRdele16-17 mutation was found in two patients of Armenian origin from the same family. The patient's clinical course with a combination of CFTRdele16_17 and 2183delAA>G mutations was very severe. A lethal outcome was detected in one case.

Our research provides a foundation for the development of a national mutation panel for Georgia. We propose a basic mutation panel consisting of 14 mutations with a prevalence exceeding 1% in our analyzed data, and an extended panel comprising all 29 mutations identified in Georgian CF patients. These panels will support population-based CF screening and more targeted treatment strategies.

Discussion

Our study presents a significant contribution to the understanding of cystic fibrosis (CF) in Georgia, as it is the first to report on the CFTR mutation distribution in a representative cohort of 91 Georgian CF patients. Our findings reveal several important insights into the genetic and clinical aspects of CF in this population.

Our study highlights that eight mutations with a prevalence greater than 1.5% play a crucial role in Georgian CF patients, among which 1677delTA is the most prevalent. The cDNA name for this mutation is c1545_c1546delTA, protein p.Tyr515X, it is located on the 11th exon. The mutation causes cystic fibrosis in the homozygous state and in combination with other mutations. 7 This mutation is unique in its distribution, being practically absent in the American continent and rare in Northern and Central European countries. In 2011, only 32 patients with such a mutation were recorded in mutation database.

Our research confirms that 1677delTA is the leading mutation in Georgia and neighboring countries, including Turkey, Cyprus, Greece, Bulgaria, the Russian Federation (particularly the northern Caucasian region), Armenia, and Iran.17, 18, 19, 20, 21 The concentration of this mutation is found the most in the southern regions of the Russian Federation, especially Chechnya. 22, 23, 24, 25

Our research confirmed that 1677delTa is the leading mutation for the population of Georgia. A high prevalence of the 1677delta in Georgia and neighboring countries could be associated with the origin of mutation from the Caucasus region.26

Our study not only identified the high prevalence of the 1677delTA mutation but also highlighted its clinical significance. Patients with this mutation exhibited a rather severe disease course, resembling the typical manifestations and complications of CF. However, some distinct characteristics were observed, including a higher frequency of growth retardation and liver damage, as well as a lower frequency of chronic respiratory manifestations and chronic malnutrition. Notably, pancreatic insufficiency was more severe in individuals with the 1677delTA mutation.27, 28 Moreover, CF-associated diabetes, distal intestinal obstruction syndrome, and hemoptysis were exclusively observed in individuals with this mutation. Despite these severe clinical manifestations, the mortality rate was lower compared to patients with the F508del and W1282X mutations.29, 30

We identified three novel mutations (c708dupT, CFTRdele16_17, and c3170C>G) not previously documented in CF mutation databases. These mutations have been added to the CFTR2 database, contributing to the global understanding of CF mutations and their implications.

Conclusion

According to the results, the distribution of CFTR mutations in Georgia CF patients differs from neigbouring counties, regarding the high frequency of mutation c. 1545-1546delTa (1677delTA). Our study documented a higher frequency of pancreatic insufficiency, liver damage, meconial ileus, and intestinal obstruction in patients with this mutation, than the general CF patient population. . The identification of common mutations and the discovery of novel mutations lay the foundation for improved diagnosis and management of CF in the country.

We hope this study can play a role in the future and provide a basis for recommendations on testing, will contribute to the development of a more targeted and comprehensive mutation panel and our own national diagnostic algorithm, and implementing variant-specific CF therapies in the future.

Conflict of Interest

The authors declare no conflict of interest.

Source of Funding

None.