1 Introduction
GNE-myopathy (GNEM) is a rare autosomal recessive muscle disease that belongs to the group of distal myopathies with adult-onset initial involvement of anterior leg compartment and caused by biallelic variants in the GNE gene (Udd, 2009). GNEM was initially reported by Nonaka et al., in 1981, in 3 patients from 2 Japanese families, as a myopathy with a predominant involvement of the distal limb muscles, and with rimmed vacuoles in muscle biopsies (Nonaka et al., 1981). Later, Argov and Yarom reported it in 9 cases from 4 different Iranian-Jewish families in 1984, as rimmed vacuole myopathy sparing the quadriceps (Argov and Yarom, 1984).
In 2001 and 2002, it was reported that GNEM (Nonaka myopathy, distal myopathy with rimmed vacuoles) is associated with the GNE gene, which encoding the bifunctional enzyme, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) (Eisenberg et al., 2001; Nishino et al., 2002). This enzyme is the key of the sialic acid metabolism pathway, which limits the rate of biosynthesis of sialic acid and regulates cell surface sialylation (Hinderlich et al., 1997; 2013; Keppler et al., 1999).
Several large cohorts of patients with GNEM have been previously described, and for some patients, a founder effect has been demonstrated (Argov et al., 2003; Nishino et al., 2005; Lu et al., 2011; Chaouch et al., 2014; Mori-Yoshimura et al., 2014; Chamova et al., 2015; Bhattacharya et al., 2018; Chen et al., 2019; Murtazina et al., 2022). The most frequent variants were identified in Middle Eastern Jewish and Bulgarian Roma populations, with the estimated spread age of the mutations reaching up to 1,500 years (Eisenberg et al., 2001; Argov et al., 2003; Chamova et al., 2015).
Recently, a cohort of Russian patients was published uncovering the genetic and clinical spectrum of GNEM in Russia (Murtazina et al., 2022). The study indicated a novel frequent variant c.169_170delinsTT, p.(Ala57Phe), which is clustered among the neighboring Russian regions of Bashkortostan Republic, Mari El Republic and Tatarstan Republic. This variant accounted for approximately 20% of alleles observed in a cohort consisting of 31 patients from 27 unrelated families. This is especially intriguing, given the different historical origins of ethnic groups from these republics. Tatars and Bashkirs are members of Turkic ethnic group indigenous to Russia, while the Mari people are a part of the Finno-Ugric group (Galieva, 2020).
Here, we present a population-based study of the c.169_170delinsTT variant in the GNE gene among Russian patients with GNEM from the Mari, Tatar and Bashkir populations.
4 Discussion
In our study, we conducted a population-based analysis of the frequent variant c.169_170delinsTT in the GNE gene among Russian patients with GNEM, which was found in 6 families in homozygous state and in 3 families in compound heterozygous state. Notably, in two unrelated patients, the c.169_170delinsTT variant was found in a compound heterozygous state with the another globally frequent c.1853T>C variant, which had previously been identified in one-third of Russian cases (Murtazina et al., 2022). The c.1853T>C variant has an ancient founder effect in the Bulgarian Roma population, likely originating before the Roma migration from Northwest India around 1,500 years ago (Chamova et al., 2015). This variant accounts for 99% of mutant alleles in that population. Despite the genetic homogeneity of their cohort, the clinical course and severity of the disease vary both inter- and intra-familially. Our two patients with compound heterozygous variants did not exhibit any outstanding features. Additionally, the c.1652G>A variant identified in one of our patients had been previously reported in two unrelated mainland Chinese patients (Zhao et al., 2015).
In the current study, the disease first manifested in the second or third decade of life in all cases. For most patients, the initial symptom was gait disturbance due to predominantly distal muscle weakness and foot drop. In patient 2.1, in addition to gait disturbance, there was also weakness of the distal arms muscles. This patient later developed respiratory issues, dysphagia and loss of ambulation, making it the most severe case in our cohort. It is noteworthy that, despite sharing the same c.169_170delinsTT homozygous variant, patients exhibited varying degrees of disease severity, as reflected in their GNEM-FAS scores. Even within the same family, there was considerable variability: patient 1.3 lost an ambulation at age 33, 10 years after disease onset, while the other two his family members (patients 1.1 and 1.2) showed better outcomes at similar or later stages. It has also been previously reported that the phenotype shows significant intra- and inter-family heterogeneity, even among patients with the same homozygous variant (Chamova et al., 2015; Alrohaif et al., 2018; Mitrani-Rosenbaum et al., 2023). Presumably, the genetic background and environmental factors likely have some effect on the severity and the rate of disease progression.
Previously, it was shown that the frequent variant c.169_170delinsTT in the GNE gene is clustered among the neighboring regions of Russia of the Bashkortostan, Mari El, and Tatarstan Republic (Murtazina et al., 2022). In this study, we conducted a more in-depth examination of the history of families living in Tatarstan and discovered that their ancestors actually originate from Mari El. Three new patients with this homozygous variant were also found in the Bashkortostan and Mari El Republics. It is worth noting that, therefore, all patients in our cohort originated from the Republics of Bashkortostan and Mari El (Figure 1).
After analyzing the carrier status of the c.169_170delinsTT variant in the GNE gene among unrelated healthy individuals from three different ethnic groups, Bashkirs, Tatars, and Maris, we identified this variant exclusively in the Mari population, with an allele frequency of 0.003788. The estimated incidence rate of the disease in the Mari population is 1.43480 per 100,000. In our cohort, seven patients identified themselves as Mari, which currently reside in Mari El, Bashkortostan and Tatar Republic, four as Tatars, despite the fact that they currently reside in Bashkortostan Republic, and one as Bashkir. Given the predominantly Mari origin of our patients, along with historical data on the migration of the Mari people to Bashkortostan Republic, and absent this variant in the Russian population database RUSeq, we can hypothesize that c.169_170delinsTT variant has a Mari origin (Barbitoff et al., 2024).
Since the proportion of closely related marriages among the Mari population is low (Mamedova et al., 1997), such a high estimated frequency of GNEM in the studied population suggests a founder effect in the distribution of the c.169_170delinsTT variant in the indigenous population. To test this hypothesis, haplotype study in patients with homozygous c.169_170delinsTT variant was performed. We found that the estimated age of the mutation spread was 160.46 years (95%CI: 45.55–244.14). In patients with GNEM, more than 10 different variants exhibiting a founder effect have been identified worldwide, with the estimated spread age of the variant reaching approximately 700 to 1,500 years (Eisenberg et al., 2001; Argov et al., 2003; Chamova et al., 2015; Attri et al., 2021).
In the pedigrees of many of our patients, there is a connection to Mari peoples, including two Tatar patients from Bashkortostan. Additionally, it has been discovered that there are distinct groups of formerly Mari people living in the Republic of Bashkortostan who have been assimilated and now identify themselves as Bashkirs. Based on these data, we hypothesized that a portion of the Mari people have relatively recently migrated from Mari El Republic to neighboring regions and settled there.
It is important to note that while the Republic of Tatarstan borders the Republic of Mari El, Bashkortostan is located farther away (Figure 1). Therefore, the presence of Mari people in Bashkortostan cannot be solely attributed to the geographical proximity of these regions. The Bashkirs are a Turkic people, indigenous to the Southern and partially Middle Urals, Bashkortostan. The Bashkirs became part of the Russian state in the 16th century. Over the next four centuries, the Islamization and adoption of the Tatar language by various peoples facilitated the integration of Tatar and Bashkir cultural elements. This period saw the consolidation of the Bashkirs and Tatars with the Mari, leading to an ethnogenetic unity among these diverse groups (Narody Baškortostana: istoriko-ètnografičeskie očerki, 2002).
According to the 2020 All-Russian Population Census, there are 423,803 Mari people living in Russia. Of these, 246,560 reside in the Mari El Republic, 84,988 in the Republic of Bashkortostan, and only 15,666 in the Republic of Tatarstan. The Mari are a Finno-Ugric ethnic group primarily residing in the Middle Volga region, with the majority located in the Mari El Republic. Compact groups of Mari also inhabit other regions, including the Republics of Bashkortostan and Tatarstan. The Mari began settling these territories in the 16th and 17th centuries. The 16th-century military conflict with the Kazan Khanate accelerated their migration to the Bashkir Urals. The peak of Mari migration to Bashkortostan occurred between the late 17th and early 18th centuries. This migration was driven by several factors: the peasant war of 1,670–1,671, increased taxation, and forced Christianization in the Middle Volga region. Additionally, the agricultural conditions in Bashkortostan resembled those in the Volga region, and the local Muslim population exhibited a more tolerant attitude toward the pagan Mari people. These factors laid the groundwork for significant cultural integration of the Mari people with other ethnic groups in the Republic of Bashkortostan. Over time, the ethnic identity of Finno-Ugric immigrants increasingly shifted toward a Bashkir identity, largely due to intermarriage with the Bashkirs (Rachmatullin, 1988; Akmanov, 1991; Narody Baškortostana: istoriko-ètnografičeskie očerki, 2002; Sadikov, 2016; 2023; Galieva, 2020). We highlight this fact because at least one Bashkir family in our cohort reported having Mari ancestry. Compared to the 18th century, the Mari population has grown sevenfold (Jurkov and Russland, 1998). This significant growth over three centuries, along with limited migration, may have led to the preservation of certain genotypes.
Forced Christianization continued actively in these territories throughout the 18th century (Sadikov, 2016; 2023). Although mass migrations of the Mari people to Bashkortostan were not reported in the 19th century, we assume that migration processes continued on a smaller scale due to ongoing negative political and social events.
We identified a homozygous c.169_170delinsTT variant in two unrelated families from the Volzhsky district, one family from the neighboring Medvedevsky district Mari El Republic, and two Bashkir families from the Sharansky and Blagovarsky districts Bashkortostan Republic. These districts in Bashkortostan Republic are historically inhabited by former Mari people (Narody Baškortostana: istoriko-ètnografičeskie očerki, 2002). These findings, in conjunction with the historical context, suggest a founder effect within the Mari population, likely originating in the Volzhsky district. The c.169_170delinsTT variant may have subsequently spread to nearby territories and, through migration processes, reached the more distant Bashkortostan.
The limitation of our study included the small number of patients with the c.169_170delinsTT homozygous variant who underwent WES or WGS to estimate the predicted age of the mutation. Additionally, we had a limited number of samples from healthy Mari populations, which restricted our ability to precisely estimate the allele frequency of this variant.