A novel truncating mutation in MYH3 causes spondylocarpotarsal synostosis syndrome with basilar invagination

Bi-allelic variants in MYH3 cause recessively-inherited arthrogryposis

Arthrogryposis is a clinical feature defined by congenital joint contractures in two or more different body areas which occurs in between 1/3000 and 1/5000 live births. Variants in multiple genes have been associated with distal arthrogryposis syndromes. Heterozygous variants in MYH3 have been identified to cause the dominantly-inherited distal arthrogryposis conditions, Freeman-Sheldon syndrome, Sheldon-Hall syndrome, and multiple pterygium syndrome. In contrast, MYH3 variants underlie both dominantly and recessively inherited Contractures, Pterygia, and Spondylocarpotarsal Fusion syndromes (CPSFS) which are characterized by extensive bony abnormalities in addition to congenital contractures. Here we report two affected sibs with distal arthrogryposis born to unaffected, distantly related parents. Sequencing revealed that both sibs were homozygous for two ultra-rare MYH3 variants, c.3445G>A (p.Glu1149Lys) and c.4760T>C (p.Leu1587Pro). Sequencing and deletion/duplication analysis of 169 other arthrogryposis genes yielded no other compelling candidate variants. This is the first report of biallelic variants in MYH3 being implicated in a distal arthrogryposis phenotype without the additional features of CPSFS. Thus, akin to CPSFS, both dominant and recessively inherited distal arthrogryposis can be caused by variants in MYH3.

Novel FLNB Variants in Seven Argentinian Cases with Spondylocarpotarsal Synostosis Syndrome

Spondylocarpotarsal synostosis syndrome (SCT) is a very rare skeletal dysplasia characterized by vertebral, carpal, and tarsal fusion; growth retardation; and mild dysmorphic facial features. Variants in FLNB, MYH3, and RFLNA have been implicated in this dysplasia. We report the clinical and radiological follow-up of seven SCT pediatric cases associated with biallelic FLNB variants, from four Argentinian families. The seven cases share previously described facial characteristics: round facies, large eyes, and wide based nose; all of them had variable height deficit, in one case noted early in life. Other findings included clinodactyly, joint limitation without bone fusion, neurosensorial hearing loss, and ophthalmological compromise. All cases presented with spinal fusion with variable severity and location, carpal bones coalition, and also delay in carpal ossification. The heterozygous carrier parents had normal height values to -2.5 score standard deviation, without skeletal defects detected. Three different FLNB variants, one nonsense and two frameshift, were detected, all of which were predicted to result in a truncated protein or are degraded by nonsense mediated decay. All cases had at least one copy of the nonsense variant, c.1128C> G; p. (Tyr376*), suggesting the presence of a common ancestor.

Expanding the mutation and phenotype spectrum of MYH3-associated skeletal disorders

Pathogenic variants in MYH3 cause distal arthrogryposis type 2A and type 2B3 as well as contractures, pterygia and spondylocarpotarsal fusion syndromes types 1A and 1B. These disorders are ultra-rare and their natural course and phenotypic variability are not well described. In this study, we summarize the clinical features and genetic findings of 17 patients from 10 unrelated families with vertebral malformations caused by dominant or recessive pathogenic variants in MYH3. Twelve novel pathogenic variants in MYH3 (NM_002470.4) were identified: three of them were de novo or inherited in autosomal dominant way and nine were inherited in autosomal recessive way. The patients had vertebral segmentation anomalies accompanied with variable joint contractures, short stature and dysmorphic facial features. There was a significant phenotypic overlap between dominant and recessive MYH3-associated conditions regarding the degree of short stature as well as the number of vertebral fusions. All monoallelic variants caused significantly decreased SMAD3 phosphorylation, which is consistent with the previously proposed pathogenic mechanism of impaired canonical TGF-β signaling. Most of the biallelic variants were predicted to be protein-truncating, while one missense variant c.4244T>G,p.(Leu1415Arg), which was inherited in an autosomal recessive way, was found to alter the phosphorylation level of p38, suggesting an inhibition of the non-canonical pathway of TGF-β signaling. In conclusion, the identification of 12 novel pathogenic variants and overlapping phenotypes in 17 affected individuals from 10 unrelated families expands the mutation and phenotype spectrum of MYH3-associated skeletal disorders. We show that disturbances of canonical or non-canonical TGF-β signaling pathways are involved in pathogenesis of MYH3-associated skeletal fusion (MASF) syndrome.