Otosclerosis or Congenital Stapes Ankylosis? The Diagnostic Role of Genetic Analysis

Is the Conductive Hearing Loss in NOG-Related Symphalangism Spectrum Disorder Congenital?

We describe a dominant Japanese patient with progressive conductive hearing loss who was diagnosed with NOG-related symphalangism spectrum disorder (NOG-SSD), a spectrum of congenital stapes fixation syndromes caused by NOG mutations. Based on the clinical features, including proximal symphalangism, conductive hearing loss, hyper-opia, and short, broad middle, and distal phalanges of the thumbs, his family was diagnosed with stapes ankylosis with broad thumbs and toes syndrome (SABTT). Genetic analysis revealed a heterozygous substitution in the NOG gene, c.645C>A, p.C215* in affected family individuals. He had normal hearing on auditory brainstem response (ABR) testing at ages 9 months and 1 and 2 years. He was followed up to evaluate the hearing level because of his family history of hearing loss caused by SABTT. Follow-up pure tone average testing revealed the development of progressive conductive hearing loss. Stapes surgery was performed, and his post-operative hearing threshold improved to normal in both ears. According to hearing test results, the stapes ankylosis in our SABTT patient seemed to be incomplete at birth and progressive in early childhood. The ABR results in our patient indicated the possibility that newborn hearing screening may not detect conductive hearing loss in patients with NOG-SSD. Hence, children with a family history and/or known congenital joint abnormality should undergo periodic hearing tests due to possible progressive hearing loss. Because of high success rates of stapes surgeries in cases of SABTT, early surgical interventions would help minimise the negative effect of hearing loss during school age. Identification of the nature of conductive hearing loss due to progressive stapes ankylosis allows for better genetic counselling and proper intervention in NOG-SSD patients.

A novel nonsense mutation in the NOG gene causes familial NOG-related symphalangism spectrum disorder

The human noggin (NOG) gene is responsible for a broad spectrum of clinical manifestations of NOG-related symphalangism spectrum disorder (NOG-SSD), which include proximal symphalangism, multiple synostoses, stapes ankylosis with broad thumbs (SABTT), tarsal–carpal coalition syndrome, and brachydactyly type B2. Some of these disorders exhibit phenotypes associated with congenital stapes ankylosis. In the present study, we describe a Japanese pedigree with dactylosymphysis and conductive hearing loss due to congenital stapes ankylosis. The range of motion in her elbow joint was also restricted. The family showed multiple clinical features and was diagnosed with SABTT. Sanger sequencing analysis of the NOG gene in the family members revealed a novel heterozygous nonsense mutation (c.397A>T; p.K133*). In the family, the prevalence of dactylosymphysis and hyperopia was 100% while that of stapes ankylosis was less than 100%. Stapes surgery using a CO₂ laser led to a significant improvement of the conductive hearing loss. This novel mutation expands our understanding of NOG-SSD from clinical and genetic perspectives.

Novel NOG mutation in Japanese patients with stapes ankylosis with broad thumbs and toes

Human noggin (NOG) gene mutation causes multiple bony disorders showing up as stapes ankylosis with broad thumbs and toes (SABTT), proximal symphalangism (SYM1), multiple synostoses syndrome 1 (SYNS1), tarsal-carpal coalition syndrome (TCC) and brachydactyly type B2 (BDB2). These phenotypes are defined as NOG-related syndromes with the same mutation. Some of these syndromes feature stapes ankylosis as one of the several bony symptoms. Here, we report a Japanese family with conductive hearing loss due to congenital stapes ankylosis. This family showed multiple features and was diagnosed with SABTT. We performed analysis of the NOG in the family by direct sequence analysis, and found a novel NOG mutation: c.682 T> G (p.C228G). Our results and a review of previous cases with NOG protein conformation suggest that this mutated NOG protein lead to a change in antagonist activity in BMPs and/or a haploinsufficiency that likely impaired finger 2 structure.

Mutations in the NOG gene are commonly found in congenital stapes ankylosis with symphalangism, but not in otosclerosis

Human noggin (NOG) is a responsible gene for multiple synostosis syndrome (SYNS1) and proximal symphalangism (SYM1), two conditions that are recently known to be within a wider range of clinical manifestations of stapes ankylosis with symphalangism. This study was performed to determine the range of phenotype caused by NOG mutations, using Japanese patients with various phenotypes including sporadic inherited SYM1, dominantly inherited SYM1, stapes ankylosis with broad thumb and toes (Teunissen and Cremer syndrome). In addition, 33 patients with typical otosclerosis (without symphalangism) were studied. Direct sequencing analysis disclosed three novel mutations of the NOG gene in three SYM1 families. None of the otosclerosis patients without symphalangism had NOG mutations, indicating that NOG mutations may be restrictively found within patients with various skeletal abnormalities. These results together with the literature review indicated that there are no clear genotype–phenotype correlations for NOG mutations. With regard to surgical outcome, most of the patients in these three families with NOG mutations showed remarkable air–bone gap recovery after stapes surgery. Molecular genetic testing is useful to differentiate syndromic stapes ankylosis from otosclerosis, and even mild skeletal anomalies can be a diagnostic indicator of NOG-associated disease.

A comprehensive review of reported heritable noggin-associated syndromes and proposed clinical utility of one broadly inclusive diagnostic term: NOG-related-symphalangism spectrum disorder (NOG-SSD)

The NOG gene encodes noggin, a secreted polypeptide that is important for regulating multiple signaling pathways during human development, particularly in cartilage and bone. The hallmark of NOG-related syndromes is proximal symphalangism, defined by abnormal fusion of the proximal interphalangeal joints of the hands and feet. Many additional features secondary to NOG mutations are commonly but inconsistently observed, including a characteristic facies with a hemicylindrical nose, congenital conductive hearing loss due to stapes fixation, and hyperopia. The variable clinical presentations led to the designation of five different autosomal dominant syndromes, all subsequently found to have resulted from NOG mutations. These include (1) proximal symphalangism; (2) multiple synostoses syndrome 1; (3) stapes ankylosis with broad thumbs and toes; (4) tarsal-carpal coalition syndrome; and (5) brachydactyly type B2. Herein, we review the phenotypic features associated with mutations in the NOG gene, demonstrating the overlapping characteristics of these syndromes. Due to the variable phenotypic spectrum within families and among families with the same mutation, we propose a unifying term, NOG-related symphalangism spectrum disorder (NOG-SSD), to aid in the clinical recognition and evaluation of all affected individuals with these phenotypes. These NOG gene variants are available in a new locus-specific database (https://NOG.lovd.nl).