A polymalformative syndrome characterized by agenesis of corpus callosum (CC), distal anomalies of limbs, minor craniofacial anomalies and intellectual deficit.
The prevalence is not known but fewer than 50 cases of ACS have been published since the first description in 1979.
In ACS, craniofacial anomalies include macrocephaly with protruding forehead and occiput, hypertelorism, large anterior fontanel, short mandible and nose with anteverted nostrils and broad nasal bridge. Cases of anencephaly were observed, as well as, in other infants, an extra bone within the anterior fontanel, a calvarian defect or a Dandy-Walker malformation (see this term). CC hypoplasia or agenesis is the main distinctive feature of ACS. It may be associated with arachnoidal cysts in about one third of cases and with various other brain abnormalities (medulla oblongata, temporal lobe or pons hypoplasia, micropolygyria and hypoplasia or agenesis of cerebellar vermis). Distal anomalies of limbs include preaxial or postaxial polydactyly or polysyndactyly of toes and/or hands. The large majority of ACS patients have intellectual deficit that is severe in 80% of cases, and substantial psychomotor retardation. Additional malformations have been described occasionally: short philtrum/upper lip, high-arched palate, cleft lip/palate, heart defects, hypospadias and inguinal and umbilical hernias.
Mutations of the kinesin KIF7 (15q26.1) and the transcriptional activator GLI3 (7p14.1) genes are responsible for ACS. Both genes are involved in the ciliary Sonic Hedgehog pathway and their mutation most likely influences the early development of midline structures during embryogenesis.
Diagnosis is based on physical examination and, given the high variability of phenotypes, a consensus on minimal diagnostic criteria has been established, with 3 of the 4 following criteria being necessary to suspect the ACS diagnosis: (1) total or partial absence of the CC, (2) minor craniofacial anomalies, (3) moderate to severe psychomotor retardation with hypotonia and (4) polydactyly.
Differential diagnosis includes Greig cephalopolysyndactyly, oral-facial-digital I and II, Meckel-Gruber, Smith-Lemli-Opitz, Rubinstein-Taybi, cerebrooculofacioskeletal, Aicardi, Neu-Laxova, pseudotrisomy 13, Toriello-Carey, otopalatodigital II and Da Silva syndromes (see these terms).
Antenatal diagnosis is based on ultrasonography examination from the 20th week of gestation and magnetic resonance imaging (MRI) of the fetus.
ACS is an autosomal recessive disease. There is therefore a 25% recurrence risk for a subsequent pregnancy. If the gene mutation in the kinesin/transcriptional activator genes has been identified in an affected sibling, molecular genetic diagnosis can be offered after chorionic villus sampling.
Management and treatment
Surgical intervention may be considered for the polydactyly.
Prognosis depends on the severity of malformations and hypotonia, and on the occurrence of seizures.