Spectrum of Genes for Non-GJB2-Related Non-Syndromic Hearing Loss in the Russian Population Revealed by a Targeted Deafness Gene Panel.

Hearing loss is one of the most genetically heterogeneous disorders known. Over 120 genes are reportedly associated with non-syndromic hearing loss (NSHL). To date, in Russia, there have been relatively few studies that apply massive parallel sequencing (MPS) methods to elucidate the genetic factors underlying non-GJB2-related hearing loss cases. The current study is intended to provide an understanding of the mutation spectrum in non-GJB2-related hearing loss in a cohort of Russian sensorineural NSHL patients and establish the best diagnostic algorithm. Genetic testing using an MPS panel, which included 33 NSHL and syndromic hearing loss (SHL) genes that might be misdiagnosed as NSHL genes, was completed on 226 sequentially accrued and unrelated patients. As a result, the molecular basis of deafness was found in 21% of the non-GJB2 NSHL cases. The total contribution pathogenic, and likely pathogenic, variants in the genes studied among all hereditary NSHL Russian patients was 12%. STRC pathogenic and likely pathogenic, variants accounted for 30% of diagnoses in GJB2-negative patients, providing the most common diagnosis. The majority of causative mutations in STRC involved large copy number variants (CNVs) (80%). Among the point mutations, the most common were c.11864G>A (p.Trp3955*) in the USH2A gene, c.2171_2174delTTTG (p.Val724Glyfs*6) in the STRC gene, and c.107A>C (p.His36Pro) and c.1001G>T (p.Gly334Val) in the SLC26A4 gene. Pathogenic variants in genes involved in SHL accounted for almost half of the cases with an established molecular genetic diagnosis, which were 10% of the total cohort of patients with non-GJB2-related hearing loss.

Cases report: Mosaic structural variants of the EXT1 gene in previously genetically unconfirmed multiple osteochondromas.

Multiple osteochondromas (MO) is a rare autosomal dominant skeletal disorder characterized by the development of multiple benign tumors known as osteochondromas. The condition is predominantly caused by loss-of-function variants in the EXT1 or EXT2 genes, facilitating relatively precise clinical diagnosis through established diagnostic criteria. Despite this, a notable percentage of MO cases (10%-20%) remains unresolved after sequencing coding regions and copy number analysis of both genes. In our study, we identified mosaic structural variants in two patients who initially yielded negative results on standard genetic analysis for MO. Specifically, mosaic deletions affecting exons 8-11 and exons 2-11 in the EXT1 gene were detected. RNA analysis was performed in one case, while both cases underwent genome sequencing. To date, only six mosaic copy number variations have been reported in association with MO, representing a minority among known variants in both genes. Our report provides a detailed analysis of these findings, highlighting the significance of advanced genetic testing techniques in detecting mosaic variants in the EXT1/2 genes.

Audiological Evidence of Frequent Hereditary Mild, Moderate and Moderate-to-Severe Hearing Loss.

Congenital and early onset bilateral sensorineural hearing loss (SNHL) is mainly caused by mutations in numerous genes. The introduction of universal newborn hearing screening (UNHS) has increased the number of infants with mild, moderate, and moderate-to-severe sensorineural hearing loss (SNHL) detected in the first year of life. We aimed to evaluate the audiological features in patients with mild, moderate, and moderate-to-severe SNHL according to genotype. Audiological and genetic data were analyzed for 251 patients and their relatives with congenital bilateral mild, moderate, and moderate-to-severe SNHL. Hearing loss severity, audiogram profile, interaural symmetry, and dynamics of hearing thresholds were analyzed. In this case, 165 patients had GJB2 gene mutations, 30 patients were identified with STRC mutations, and 16 patients had pathogenic or likely pathogenic USH2A mutations. The presence of at least one GJB2 non-truncating variant in genotype led to less severe hearing impairment. The flat and gently sloping audiogram profiles were mostly revealed in all groups. The follow-up revealed the stability of hearing thresholds. GJB2, STRC, and USH2A pathogenic variants were detected in most patients in our cohort and were congenital in most cases.

Clinical and genetic characterization of three Russian patients with pycnodysostosis due to pathogenic variants in the CTSK gene.

BACKGROUND: Pycnodysostosis (PD, OMIM # 265800) is a rare variant of skeletal dysplasia with an autosomal recessive type of inheritance, characterized by a combination of specific features such as disproportionate nanism, generalized osteosclerosis, and distinct craniofacial dysmorphism. Radiographic features include acro-osteolysis of the distal phalanges in association with sclerosing bone lesions with multiple fractures. The polymorphism of the clinical manifestations of pycnodysostosis and low prevalence of the disorder lead to the difficulties with early. METHODS: The following tests were used for diagnostics: genealogical analysis, clinical examination, neurological examination according to the standard method with an assessment of the psychoemotional sphere, radiological analysis, searching for pathogenic variants in the CTSK gene by the automated Sanger sequencing. RESULTS: We describe first clinical and genetic characteristics of three Russian patients with pycnodysostosis from unrelated families. Two patients have a novel homozygous nucleotide substitution c.746T>A (p. Ile249Asn), and one has a previously described homozygous pathogenic variant c.746T>C (p.Ile249Thr) in the CTSK gene. In all three cases, a transition or transversion was found at nucleotide position 746 in exon 6 of the CTSK gene, leading to two different amino acid substitutions in the polypeptide chain. The obtained results may indicate the presence of a major pathogenic variant in the CTSK gene, leading to the typical manifestation of the disease. CONCLUSION: The data presented in the study enlarge the clinical, radiological, and mutational spectrum of pycnodysostosis. Typical clinical manifestations and the small size of the CTSK gene make the automated Sanger sequencing the optimal method for diagnosis of pycnodysostosis.

Ehlers-Danlos syndrome kyphoscoliotic type 2 caused by mutations in the FKBP14 gene: an analysis of five cases.

Background. This study deals with a rare (orphan) monogenic connective tissue disorder - Ehlers-Danlos syndrome kyphoscoliotic type 2 (EDSKS2). Kyphoscoliotic type 2 Ehlers-Danlos syndrome is an autosomal recessive disorder caused by mutations in the FKBP14 gene (7p14.3), which encodes the FKBP22 protein. According to the 2017 classification, this type is in group seven - collagen spatial structure and cross-linking defects. We present results of clinical examination and molecular genetic analysis for five patients with age varying from two to fifteen years.  Methods. Five patients were examined using clinical and laboratory methods. DNA samples used for the analysis were extracted from whole blood samples using a Wizard® Genomic DNA Purification Kit (Promega, USA) according to the manufacturer's protocol.  Results. The major clinical findings were kyphoscoliosis, early motor development delay, muscular weakness, hypotonia and hearing loss. Molecular genetic analysis detected a homozygous c.362dupC duplication in exon 3 of the FKBP14 gene in all five patients. This mutation is common in various countries. Differential diagnostics were carried out to exclude other Ehlers-Danlos syndrome types and myopathies.  Conclusions. Literature analysis and examination of five EDSKS2 patients demonstrated the involvement of major organs and systems, such as joints, spine, muscles, cardiovascular system, respiratory system, hearing, and vision, into the pathological process. Kidney mobility increases and nephroptosis seems to be secondary caused by muscular weakness. During molecular genetic analysis, to verify EDSKS2 it is recommended to initially search for the c.362dupC duplication, which appears to be common in European countries, including Russia.