Clinical and experimental evidence suggest a link between KIF7 and C5orf42-related ciliopathies through Sonic Hedgehog signaling.

Acrocallosal syndrome (ACLS) is an autosomal recessive neurodevelopmental disorder caused by KIF7 defects and belongs to the heterogeneous group of ciliopathies related to Joubert syndrome (JBTS). While ACLS is characterized by macrocephaly, prominent forehead, depressed nasal bridge, and hypertelorism, facial dysmorphism has not been emphasized in JBTS cohorts with molecular diagnosis. To evaluate the specificity and etiology of ACLS craniofacial features, we performed whole exome or targeted Sanger sequencing in patients with the aforementioned overlapping craniofacial appearance but variable additional ciliopathy features followed by functional studies. We found (likely) pathogenic variants of KIF7 in 5 out of 9 families, including the original ACLS patients, and delineated 1000 to 4000-year-old Swiss founder alleles. Three of the remaining families had (likely) pathogenic variants in the JBTS gene C5orf42, and one patient had a novel de novo frameshift variant in SHH known to cause autosomal dominant holoprosencephaly. In accordance with the patients' craniofacial anomalies, we showed facial midline widening after silencing of C5orf42 in chicken embryos. We further supported the link between KIF7, SHH, and C5orf42 by demonstrating abnormal primary cilia and diminished response to a SHH agonist in fibroblasts of C5orf42-mutated patients, as well as axonal pathfinding errors in C5orf42-silenced chicken embryos similar to those observed after perturbation of Shh signaling. Our findings, therefore, suggest that beside the neurodevelopmental features, macrocephaly and facial widening are likely more general signs of disturbed SHH signaling. Nevertheless, long-term follow-up revealed that C5orf42-mutated patients showed catch-up development and fainting of facial features contrary to KIF7-mutated patients.

[Intellectual disability - a frequent reason for referral to medical genetics]. / Mentale Retardierung - eine häufige Fragestellung in der medizinischen Genetik.

Mental retardation affects about 2-3% of the population and is often associated with comorbidities. So far, more than 450 different medical conditions are known with mental retardation as a sign and it is assumed that there are many more yet to be defined. The diagnosis of the underlying entity allows for a few specific optimization of cognitive function, but usually improves the treatment of comorbidities. Furthermore, the detection of the underlying genetic defect allows the specification of the risk of recurrence and enables prenatal diagnosis for future pregnancies of persons at risk in the family. Recent findings suggest that especially in diseases that are associated with defective synaptic signal transduction may be targeted by specific drugs for improvement of cognitive performance in the near future.

La déficience intellectuelle touche environ 2­3% de la population et est souvent associée à des comorbidités. Jusqu'à présent, plus de 450 maladies sont connues pour être associées à un retard mental et on suspecte encore de nombreuses autres entités jusqu'alors inconnues. Le diagnostic de l'entité sous-jacente permet rarement aujourd'hui d'optimiser spécifiquement les fonctions cognitives, mais permet d'améliorer le traitement des comorbidités. En outre, la détection de l'anomalie génétique sous-jacente permet de connaître le risque de récidive et de réaliser un diagnostic prénatal pour les futures grossesses des personnes à risque dans la famille. Des résultats récents suggèrent que dans un futur proche des thérapies ciblées permettront d'améliorer les performances cognitives chez les patients dont la maladie est associée à un défaut de transmission synaptique.