The proband’s parents denied inbreeding. The proband reported 4 years of progressive hearing loss with no history of otitis media, ototoxic drug use, or noise exposure. The proband was 168 cm in height and hyperopic, with broad thumbs and toes, difficulty bending little finger, and limited range of motion in the neck. Eardru ms on both sides were intact. The Rinne and 512 Hz tests using a tuning fork were negative, the Weber test was biased to the side with severe deafness, the Schwabach test showed bone conduction extension, and the Gelle test was negative, suggesting stapes plate fixation. Pure-tone audiometry indicated bilateral conductive deafness. No obvious abnormalities were found in acoustic conductivity tests or on ear computed tomography (Fig 1). The clinical diagnosis was stapes ankylosis.
Six family members had conductive hearing loss, stapes rigidity, hyperopia, and other phenotypic traits, such as broad thumbs and/or toes, difficulty bending little finger, decreased extremity and neck (range of motion) ROM. The proband had conductive hearing loss, hyperopia, broad thumbs and difficulty bending little finger, decreased extremity and neck ROM. The proband’s first sister had no obvious problems. The proband's second sister had the same symptoms as the proband except difficulty bending little finger. The proband’s third sister had the same symptoms as the proband except hyperopia. The proband's father showed the same symptoms as the proband, and her mother was congenitally deaf and mute. One of the proband's uncles, who had died in an accident, had hyperopia and conductive deafness. Another uncle was normal. The proband's grandmother had the same symptoms as the proband except decreased extremity and neck ROM. The patients in the family were tall except the proband's grandmother, and their walking function was normal. There was no other family history. Autosomal dominant genetic characteristics noted in this family are shown in Fig 2.
The sequencing results were compared to the Human Gene Mutation Database (HG19), Locus Specific Mutation Database (HTTP://www.hgvs.org/dblist/glsdb.htm), dbSNP (v144), US National Center for Bioinformatics, SPIDEX Database, and other databases. The DNA sequencing analysis indicated that the NOG heterozygous mutation, namely, c.532T>C, p.C178R (code no. 532 nucleotide variation for cytosine by thymine, NM_005450.6:c.532T>C), was present in the proband’s father and grandmother. The proband’s two sisters also carried the mutation. To verify that this was the pathogenic mutation that caused stapes sclerosis, we screened 50 individuals with normal hearing from the Chinese population for NOG gene mutations. None of them had the mutation. Therefore, the c.532T>C, p.C178R mutation does not belong to the polymorphic site and is considered pathogenic (Fig 3).
We searched the Chinese literature and PubMed database to find relevant literature on NOG gene mutations in a Chinese population. The results showed that nearly 40 mutations in the NOG gene have been identified, most of which are missense mutations located in the evolutionarily conserved region. Within this region is a functionally critical domain, suggesting a correlation between the region and the NOG protein. The c.532T>C, p.C178R mutation identified in this family has not been previously reported. The only clinical symptoms that were similar to those reported in our pedigree were those of Brown et al, who showed stapes ankylosis accompanied by broad thumb,hyperopia,et al. In addition, the clinical characteristics of the affected individuals in this lineage are highly heterogeneous.
The variant analysis: the mutation causes the amino acid residue of Noggin protei n at C-terminal 178 to change from cysteine to arginine. The amino acid residue is located in this highly conserved region of cystine – Knot B. SIFT; Polyphen2; REVE L online software predicts the function of the mutant protein, suggesting that changes in the amino acid residues are harmful. The mutation was not included in the normal population database: 1000 genomes, gnomAD and ExAC databases. This mutation achieves genotype-phenotypic co-isolation in this family.
Nog mutation site and clinical phenotype analysis：we searched literature database and PubMED data Library to find out the relevant literature of nog gene mutation.Tab le 1 lists the nog mutation sites reported so far Clinical phenotype [12-19]. The results showed that the mutation site of nog was varied, the clinical features also vary. The clinical characteristics of the affected individuals in this family are highly heterogeneous.