X-linked spinal muscular atrophy (SMAX2) caused by de novo c.1731C>T substitution in the UBA1 gene.

Infantile X-linked spinal muscular atrophy (SMAX2) is a rare form of spinal muscular atrophy manifesting as severe hypotonia, areflexia, arthrogryposis, facial weakness and cryptorchidism, and frequently accompanied by bone fractures. We present a male patient with SMAX2 who presented with typical symptoms at birth, preceded by reduced fetal movements in the second and third trimesters of pregnancy. Clinical examination revealed a myopathic face with a characteristic tent-shaped open mouth, tongue fibrillations, profound muscle weakness, areflexia, multiple contractures, mild skeletal abnormalities and cryptorchidism. In the first days of the patient's life, fractures of the right femur and right humerus were found; however, calcium-phosphate metabolism and densitometric examination were normal. Molecular analysis revealed a de novo c.1731C>T substitution in the UBA1 gene, which was localized in exon 15, the specific hot spot for mutation.