Compound heterozygous TMEM67 biallelic variants including a novel frameshift mutation in two Filipino adolescent siblings with Joubert syndrome

Joubert Syndrome (JS) is a congenital cerebellar ataxia typically inherited in an autosomal recessive pattern, although rare X-linked inheritance can occur. It is characterized by hypotonia evolving into ataxia, global developmental delay, oculomotor apraxia, breathing dysregulation, and multiorgan involvement. To date, there are 40 causative genes implicated in JS, all of which encode proteins of the primary cilium. Primary cilia play a crucial role in the normal development and function of many organs, including parts of the brain (cerebellum and brainstem), kidneys, and the retina. This likely explains the multiorgan involvement seen in JS. In this report, we present the first genetically confirmed case of JS in two Filipino adolescent siblings who had early onset ataxia, hepatomegaly, and global developmental delay. A cranial CT scan revealed the Molar Tooth Sign (MTS). Whole Exome Sequencing (WES), performed via buccal swab, showed biallelic pathogenic variants at NM_153704.6:c.2086 C > T (NP_714915.3:p.Leu696Phe) and NM_153704.6:c.431del (NP_714915.3:p.Leu144CysfsTer19) in TMEM67, which are associated with Joubert Syndrome 6 (OMIM:610688) in a compound heterozygous state. The prevalence of NM_153704.6:c.2086 C > T (NP_714915.3:p.Leu696Phe) in TMEM67 variant is very rare (< 0.001%), and the NM_153704.6:c.431del (NP_714915.3:p.Leu144CysfsTer19) has not been recorded. This case contributes valuable information to the expanding knowledge of JS and its related disorders.