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Background  The c.429_452dup24 of the  ARX  gene is a rare genetic anomaly, leading to X-Linked Intellectual Disability without brain malformation. While in certain cases c.429_452dup24 has been associated with specific clinical patterns such as Partington syndrome, the consequence of this mutation has been also often classified as “non-specific Intellectual Disability”. The present work aims at a more precise description of the clinical features linked to the c.429_452dup24 mutation.    Methods  We clinically reviewed all affected patients identified in France over a five-year period, i.e. 27 patients from 12 different families. Detailed cognitive, behavioural, and motor evaluation, as well as standardized videotaped assessments of oro-lingual and gestural praxis, were performed. In a sub-group of 13  ARX  patients, kinematic and MRI studies were further accomplished to better characterize the motor impairment prevalent in the  ARX  patients group. To ensure that data were specific to the  ARX  gene mutation and did not result from low-cognitive functioning per se, a group of 27 age- and IQ-matched Down syndrome patients served as control.    Results  Neuropsychological and motor assessment indicated that the c.429_452dup24 mutation constitutes a recognizable clinical syndrome:  ARX  patients exhibiting Intellectual Disability, without primary motor impairment, but with a very specific upper limb distal motor apraxia associated with a pathognomonic hand-grip. Patients affected with the so-called Partington syndrome, which involves major hand dystonia and orolingual apraxia, exhibit the most severe symptoms of the disorder. The particular “reach and grip” impairment which was observed in all  ARX  patients, but not in Down syndrome patients, was further characterized by the kinematic data: (i) loss of preference for the index finger when gripping an object, (ii) major impairment of fourth finger deftness, and (iii) a lack of pronation movements. This lack of distal movement coordination exhibited by  ARX  patients is associated with the loss of independent digital dexterity and is similar to the distortion of individual finger movements and posture observed in Limb Kinetic Apraxia.    Conclusion  These findings suggest that the  ARX  c.429_452dup24 mutation may be a developmental model for Limb Kinetic Apraxia.

The c.429_452 duplication of the ARX gene: a unique developmental-model of limb kinetic apraxia