Myogenic Arthrogryposis Multiplex Congenita type 3 (AMC-3), is a rare congenital condition characterized by severe hypotonia, club feet, and multiple joint contractures often affecting both arms and legs which start prior to birth. We report a full-term neonate born to first-degree cousins from fourth-generation consanguineous families, who had with antenatal history of reduced fetal movements. At birth, he was noticed to have bilateral club feet, arthrogryposis, severe hypotonia, and absent deep tendon reflexes. The patient developed difficulty in breathing probably attributed to his generalized severe hypotonia, necessitating mechanical ventilation. His creatinine-phospho-kinase, electromyogram, and brain magnetic resonance imaging were normal. Whole-exome sequencing (WES) was requested for the genetic diagnosis of the case. WES identified a novel homozygous variant c.23415-3799C > G p. in the synaptic nuclear envelope protein1 [SYNE1] gene. Seven out of 20 bioinformatic in silico programs predicted a pathogenic effect for this variant. Segregation analysis of the variant in the parents and siblings revealed that both parents and one sibling were heterozygous for the same mutation which proved the variant significance and its autosomal recessive pattern of inheritance. AMC3 should be suspected in patients with decreased fetal movements, severe hypotonia, absent deep tendon reflexes, and arthrogryposis. SYNE1 gene mutations can be the underlying genetic defect and molecular genetic testing can prove the diagnosis.

Encoding the slow skeletal muscle isoform of myosin binding protein-C, MYBPC1 is associated with autosomal dominant and recessive forms of arthrogryposis. The authors describe a novel association for MYBPC1 in four patients from three independent families with skeletal muscle weakness, myogenic tremors, and hypotonia with gradual clinical improvement. The patients carried one of two de novo heterozygous variants in MYBPC1, with the p.Leu263Arg variant seen in three individuals and the p.Leu259Pro variant in one individual. Both variants are absent from controls, well conserved across vertebrate species, predicted to be damaging, and located in the M-motif. Protein modeling studies suggested that the p.Leu263Arg variant affects the stability of the M-motif, whereas the p.Leu259Pro variant alters its structure. In vitro biochemical and kinetic studies demonstrated that the p.Leu263Arg variant results in decreased binding of the M-motif to myosin, which likely impairs the formation of actomyosin cross-bridges during muscle contraction. Collectively, our data substantiate that damaging variants in MYBPC1 are associated with a new form of an early-onset myopathy with tremor, which is a defining and consistent characteristic in all affected individuals, with no contractures. Recognition of this expanded myopathic phenotype can enable identification of individuals with MYBPC1 variants without arthrogryposis.

Lethal fetal akinesia deformation sequence (FADS) describes a clinically and genetically heterogeneous phenotype that includes fetal akinesia, intrauterine growth retardation, arthrogryposis and developmental anomalies. Affected babies die as a result of pulmonary hypoplasia. We aimed to identify the underlying genetic cause of this disorder in a family in which there were three affected individuals from two sibships. Autosomal-recessive inheritance was suggested by a family history of consanguinity and by recurrence of the phenotype between the two sibships. We performed exome sequencing of the affected individuals and their unaffected mother, followed by autozygosity mapping and variant filtering to identify the causative gene. Five autozygous regions were identified, spanning 31.7 Mb of genomic sequence and including 211 genes. Using standard variant filtering criteria, we excluded all variants as being the likely pathogenic cause, apart from a single novel nonsense mutation, c.188C>A p.(Ser63*) (NM_002478.4), in MYOD1. This gene encodes an extensively studied transcription factor involved in muscle development, which has nonetheless not hitherto been associated with a hereditary human disease phenotype. We provide the first description of a human phenotype that appears to result from MYOD1 mutation. The presentation with FADS is consistent with a large body of data demonstrating that in the mouse, MyoD is a major controller of precursor cell commitment to the myogenic differentiation programme.