Joubert syndrome (JS) is a genetically heterogeneous primary ciliopathy characterized by a pathognomonic cerebellar and brainstem malformation, the “molar tooth sign,” and variable organ involvement. Over 40 causative genes have been identified to date, explaining up to 94% of cases. To date, gene‐phenotype correlates have been delineated only for a handful of genes, directly translating into improved counseling and clinical care. For instance, JS individuals harboring pathogenic variants in TMEM67 have a significantly higher risk of liver fibrosis, while pathogenic variants in NPHP1, RPGRIP1L, and TMEM237 are frequently associated to JS with renal involvement, requiring a closer monitoring of liver parameters, or renal functioning. On the other hand, individuals with causal variants in the CEP290 or AHI1 need a closer surveillance for retinal dystrophy and, in case of CEP290, also for chronic kidney disease. These examples highlight how an accurate description of the range of clinical symptoms associated with defects in each causative gene, including the rare ones, would better address prognosis and help guiding a personalized management. This review proposes to address this issue by assessing the available literature, to confirm known, as well as to propose rare gene‐phenotype correlates in JS.

BackgroundJoubert syndrome (JS) is a recessively inherited ciliopathy characterised by congenital ocular motor apraxia (COMA), developmental delay (DD), intellectual disability, ataxia, multiorgan involvement, and a unique cerebellar and brainstem malformation. Over 40 JS-associated genes are known with a diagnostic yield of 60%–75%.In 2018, we reported homozygous hypomorphic missense variants of the SUFU gene in two families with mild JS. Recently, heterozygous truncating SUFU variants were identified in families with dominantly inherited COMA, occasionally associated with mild DD and subtle cerebellar anomalies.MethodsWe reanalysed next generation sequencing (NGS) data in two cohorts comprising 1097 probands referred for genetic testing of JS genes.ResultsHeterozygous truncating and splice-site SUFU variants were detected in 22 patients from 17 families (1.5%) with strong male prevalence (86%), and in 8 asymptomatic parents. Patients presented with COMA, hypotonia, ataxia and mild DD, and only a third manifested intellectual disability of variable severity. Brain MRI showed consistent findings characterised by vermis hypoplasia, superior cerebellar dysplasia and subtle-to-mild abnormalities of the superior cerebellar peduncles. The same pattern was observed in two out of three tested asymptomatic parents.ConclusionHeterozygous truncating or splice-site SUFU variants cause a novel neurodevelopmental syndrome encompassing COMA and mild JS, which likely represent overlapping entities. Variants can arise de novo or be inherited from a healthy parent, representing the first cause of JS with dominant inheritance and reduced penetrance. Awareness of this condition will increase the diagnostic yield of JS genetic testing, and allow appropriate counselling about prognosis, medical monitoring and recurrence risk.

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ObjectiveTo estimate the prevalence of Joubert syndrome (JS) in Italy applying standards of descriptive epidemiology and to provide a molecular characterization of the described patient cohort.MethodsWe enrolled all patients with a neuroradiologically confirmed diagnosis of JS who resided in Italy in 2018 and calculated age and sex prevalence, assuming a Poisson distribution. We also investigated the correlation between proband chronological age and age at diagnosis and performed next-generation sequencing (NGS) analysis on probands' DNA when available.ResultsWe identified 284 patients with JS: the overall, female- and male-specific population-based prevalence rates were 0.47 (95% confidence interval [CI] 0.41–0.53), 0.41 (95% CI 0.32–0.49), and 0.53 (95% CI 0.45–0.61) per 100,000 population, respectively. When we considered only patients in the age range from 0 to 19 years, the corresponding population-based prevalence rates rose to 1.7 (95% CI 1.49–1.97), 1.62 (95% CI 1.31–1.99), and 1.80 (95% CI 1.49–2.18) per 100,000 population. NGS analysis allowed identifying the genetic cause in 131 of 219 screened probands. Age at diagnosis was available for 223 probands, with a mean of 6.67 ± 8.10 years, and showed a statistically significant linear relationship with chronological age (r2 = 0.79; p < 0.001).ConclusionsWe estimated for the first time the age and sex prevalence of JS in Italy and investigated the patients’ genetic profile. The obtained population-based prevalence rate was ≈10 times higher than that available in literature for children population.

Trisomy 21 causes Down syndrome (DS), the most common human genetic disorder and the leading genetic cause of intellectual disability. The alteration of one‐carbon metabolism was described as the possible metabolic cause of the intellectual disability development in subjects with DS. One of the biochemical pathways involved in the one‐carbon group transfer is the folate cycle. The cytotoxic drug methotrexate (MTX) is a folic acid (FA) analogue which inhibits the activity of dihydrofolate reductase enzyme involved in the one‐carbon metabolic cycle. Trisomy 21 cells are more sensitive to the MTX effect than euploid cells, and in 1986 Jérôme Lejeune and Coll. demonstrated that MTX was twice as toxic in trisomy 21 lymphocytes than in control cells. In the present work, the rescue effect on MTX toxicity mediated by FA and some of its derivatives, tetrahydrofolate (THF), 5‐formyl‐THF, and 5‐methyl‐THF, in both normal and trisomy 21 skin fibroblast cells, was evaluated. A statistically significant rescue effect was obtained by 5‐formyl‐THF, 5‐methyl‐THF, and their combination, administered together with MTX. In conclusion, trisomy 21 fibroblast cell lines showed a good response to the rescue effects of 5‐formyl‐THF and 5‐methyl‐THF on the MTX toxicity almost as normal cell lines.