Novel Pathological Mutation Screening For A Family Disorder Resembling Aarskog Syndrome

Aarskog syndrome or Aarskog-Scott syndrome (AAS) was referred to by Aarskog as Faciodigitogenital syndrome in1970, and is characterized by short stature, and facial, limb, and genital anomalies. Clinical features of AAS show remarkable inter-familial and intra-familial variability, and it is uncertain which phenotype is necessary for diagnosis. AAS can be inherited as an X-linked disorder caused by FGD1mutations at Xp11.21. FGD1encodes a guimine nucleotide exchange factor (GEF) that can specifically activate Cdc42, a RhoGTPase that controls the organization of the actin cytoskeleton. However, there is also evidence that AAS can be inherited in an autosomal dominant mode, an autosomal recessive mode or an X-linked dominant pattern. Different inheritant modes might occur even in one pedigree. However, the potential causative gene other than FGD1has been not reported as yet.Here we report on a Chinese family in whom9members show signs of AAS. Affected members had manifestations characteristic for AAS such as short stature, hypertelorism, shawl scrotum, the fifth finger clinodactyly, broad hands with interdigital webbing and hyperextensible elbows, but no hyperextension of the proximal interphalangeal joints. They did have several other features less common but still found in AAS such as widow’s peak, highly arched palate, abnormal teeth, retarded bone age, four-finger flexion creases, adducted thumbs, vertebral malformations, pectus excavatum, broad feet with bulbous toes, phimosis, cryptorchidism and mental retardation. Furthermore, the present family showed additional characteristics such as central facial paralysis, trismus, decreased hearing, congenital absent12th ribs and frequent micturition. Male to male inheritance in the pedigree suggested an autosomal dominant or recessive pattern of inheritance. The affected individuals in generation IV came from the two families of consanguineous marriage, and they might carry additional mutation as their clinical phenotypes were obviously more severe than those from generation III.The sequencing of the18exons and promoter of FGD1showed no pathogenic mutation. The linkage analysis also excluded FGD1as the candidate gene. A submicroscopic chromosome imbalance is less likely at Xp11.21(FGD1) and other421positions identified for common known disorders. Whole-exome sequencing was then carried out to screen the novel pathogenic genes, and no compelling candidate gene containing novel variants in all affected individuals were found. When grouping the affected individuals according to their phenotype variation, RAB35was identified as a compelling pathogenic gene, which carried a homozygous missense mutation (chrl2: g.119021012>T, c.463G>A, p.V155I) in the most severely affected proband. His consanguineously mated parents both carried heterogenous mutation, and it presented a recessive mode of inheritance. The155th amino acid is the conserved domain, and phylogenetically conservative in various species such as nematode and10vertebrates including danio rerio. RAB35is also a small GTPase, and the complex with its GEF includes residues4-33and35-177of RAB35. Protein modeling implied a significant conformational change of the mutant RAB35. RAB35is a regulator of actin cytoskeleton, which is associated with the activation of Cdc42. RAB35is also implicated in several crucial pathophysiological processes that might related to AAS, such as bone metabolism, hair development and the function of nervous system.In summary, AAS might be caused by more than one gene, and the mutation in RAB35may underlie a disorder resembling AAS. Further work is required to fully understand the underlying mechanisms of the RAB35mutation induced alterations. The phenotypic diversity of AAS might be, at least partially, due to the presence of the mutation(s) in other pathogenic genes than FGD1. It is could not be completely excluded that novel causative genes might coexist even in those with FGD1mutation. The investigation of these potential causative genes and their interactions, might contribute to knowing the specific phenotype for AAS, and to establishing more specific diagnostic criteria and intervention.