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Purpose: Lanosterol synthase (LSS) gene was initially described in families with extensive congenital cataracts. Recently, a study has highlighted LSS associated with hypotrichosis simplex. We expanded the phenotypic spectrum of LSS to a recessive neuroectodermal syndrome formerly named alopecia with mental retardation (APMR) syndrome. It is a rare autosomal recessive condition characterized by hypotrichosis and intellectual disability (ID) or developmental delay (DD), frequently associated with earlyonset epilepsy and other dermatological features. Methods: Through a multicenter international collaborative study, we identified LSS pathogenic variants in APMR individuals either by exome sequencing or LSS Sanger sequencing. Splicing defects were assessed by transcript analysis and minigene assay. Results: We reported ten APMR individuals from six unrelated families with biallelic variants in LSS. We additionally identified one affected individual with a single rare variant in LSS and an allelic imbalance suggesting a second event. Among the identified variants, two were truncating, seven were missense, and two were splicing variants. Quantification of cholesterol and its precursors did not reveal noticeable imbalance. Conclusion: In the cholesterol biosynthesis pathway, lanosterol synthase leads to the cyclization of (S)-2,3-oxidosqualene into lanosterol. Our data suggest LSS as a major gene causing a rare recessive neuroectodermal syndrome.

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The microphthalmia-associated transcription factor (MITF) is a basic helix-loop-helix leucine zipper transcription factor, which regulates melanocyte development and the biosynthetic melanin pathway. A notable relationship has been described between non-truncating mutations of its basic domain and Tietz syndrome, which is characterized by albinoid-like hypopigmentation of the skin and hair, rather than the patchy depigmentation seen in Waardenburg syndrome, and severe hearing loss. Twelve patients with new or recurrent non-truncating mutations of the MITF basic domain from six families were enrolled in this study. We observed a wide range of phenotypes and some unexpected features. All the patients had blue irides and pigmentation abnormalities that ranged from diffuse hypopigmentation to Waardenburg-like patches. In addition, they showed congenital complete hearing loss, diffuse hypopigmentation of the skin, freckling and ocular abnormalities, more frequently than patients with MITF mutations outside the basic domain. In conclusion, the non-truncating mutations of the basic domain do not always lead to Tietz syndrome but rather to a large range of phenotypes. Sun-exposed freckles are interestingly observed more frequently in Asian populations. This variability argues for the possible interaction with modifier loci.