Case Report: Recurrent Variant c.298 TA in CCN6 Gene Found in Progressive Pseudorheumatoid Dysplasia Patients From Patni Community of Gujarat: A Report of Three Cases

Abstract

Biallelic mutations in the CCN6 gene are known to cause a rare genetic disorder-progressive pseudorheumatoid dysplasia (PPD). PPD is characterized by distinct joint deformities of interphalangeal joints, stiffness, gait disturbance, abnormal posture, and absence of inflammation, resulting in significant morbidity. The largest case series of PPD from India suggests c.233G>A and c.1010G>A to be the most common mutations in the CCN6 gene, although the distribution of these variants among endogamous communities in India has not been carried out. We here report three cases of PPD from three independent families belonging to the Patni community of Gujarat, a community known to practice endogamy. All three cases had short stature, gait disturbance, scoliosis, and interphalangeal joint deformities. Analysis by whole-exome sequencing in the first case showed the presence of a previously known, homozygous, missense variant c.298T>A (p.Cys100Ser) in exon 3 of the CCN6 gene in all cases. Due to all three families belonging to the same community, analysis by Sanger sequencing in the remaining two cases for the variant mentioned earlier showed both cases to be of homozygous mutant genotype. Unaffected family members, i.e., parents and siblings, were either heterozygous carriers or wildtype for the said variant. The present case series is the first report of a recurrent variant occurring across multiple PPD-affected individuals from unrelated families belonging to the same community from India.

Keywords: CCN6; Gujarat; Patni; progressive pseudorheumatoid dysplasia; recurrent variant.

FIGURE 1

Pedigree chart of (A) case 1, (B) case 2, and (C) case 3. Affected individuals are shaded in gray strips, and carriers are depicted with a dot.

FIGURE 2

Clinical photographs and radiographs of proband's (A–F) case 1, (G–H) case 1’s mother, (J–L) case 1’s father, (M–O) case 2, and (P) Case 3. Case 1 is observed with (A) chest deformity and enlarged elbow joints; (B) scoliosis; (C) deformities of interphalangeal joints; (D) knocked knees; (E) radiograph anterior-posterior view, showing osteopenia; (F) anterior beaking of vertebrae. (G–I) Clinical picture of mother (carrier) of case 1; (J–L) clinical picture of father (carrier) of case 1. Case 2 is observed with (M) scoliosis and (N,O)clinodactyly with deformities of interphalangeal joints. Case 3 is observed with (P) mild scoliosis.

FIGURE 3

Sanger sequencing chromatograms of all three proband's and their respective first-degree relatives (A) case 1, (B) case 2, and (C) case 3. Red and black arrows show the position of c.298T>A and c.296A>T variants, respectively.