Saturday, December 14, 2019

TBCD and TBCE mutations

Inspired by a colleague's patient

Miyake N, Fukai R, Ohba C, Chihara T, Miura M, Shimizu H, Kakita A, Imagawa E, Shiina M, Ogata K, Okuno-Yuguchi J, Fueki N, Ogiso Y, Suzumura H, Watabe Y, Imataka G, Leong HY, Fattal-Valevski A, Kramer U, Miyatake S, Kato M, Okamoto N, Sato Y, Mitsuhashi S, Nishino I, Kaneko N, Nishiyama A, Tamura T, Mizuguchi T, Nakashima M, Tanaka F, Saitsu H, Matsumoto N. Biallelic TBCD Mutations Cause Early-Onset Neurodegenerative Encephalopathy. Am J Hum Genet. 2016 Oct 6;99(4):950-961.

We describe four families with affected siblings showing unique clinical features: early-onset (before 1 year of age) progressive diffuse brain atrophy with regression, postnatal microcephaly, postnatal growth retardation, muscle weakness/atrophy, and respiratory failure. By whole-exome sequencing, we identified biallelic TBCD mutations in eight affected individuals from the four families. TBCD encodes TBCD (tubulin folding co-factor D), which is one of five tubulin-specific chaperones playing a pivotal role in microtubule assembly in all cells. A total of seven mutations were found: five missense mutations, one nonsense, and one splice site mutation resulting in a frameshift. In vitro cell experiments revealed the impaired binding between most mutant TBCD proteins and ARL2, TBCE, and β-tubulin. The in vivo experiments using olfactory projection neurons in Drosophila melanogaster indicated that the TBCD mutations caused loss of function. The wide range of clinical severity seen in this neurodegenerative encephalopathy may result from the residual function of mutant TBCD proteins. Furthermore, the autopsied brain from one deceased individual showed characteristic neurodegenerative findings: cactus and somatic sprout formations in the residual Purkinje cells in the cerebellum, which are also seen in some diseases associated with mitochondrial impairment. Defects of microtubule formation caused by TBCD mutations may underlie the pathomechanism of this neurodegenerative encephalopathy.

Sferra A, Baillat G, Rizza T, Barresi S, Flex E, Tasca G, D'Amico A, Bellacchio E, Ciolfi A, Caputo V, Cecchetti S, Torella A, Zanni G, Diodato D, Piermarini E, Niceta M, Coppola A, Tedeschi E, Martinelli D, Dionisi-Vici C, Nigro V, Dallapiccola B, Compagnucci C, Tartaglia M, Haase G, Bertini E. TBCE Mutations Cause Early-Onset Progressive Encephalopathy with Distal Spinal
Muscular Atrophy. Am J Hum Genet. 2016 Oct 6;99(4):974-983.

Tubulinopathies constitute a family of neurodevelopmental/neurodegenerative disorders caused by mutations in several genes encoding tubulin isoforms. Loss-of-function mutations in TBCE, encoding one of the five tubulin-specific chaperones involved in tubulin folding and polymerization, cause two rare neurodevelopmental syndromes, hypoparathyroidism-retardation-dysmorphism and Kenny-Caffey syndrome. Although a missense mutation in Tbce has been associated with progressive distal motor neuronopathy in the pmn/pmn mice, no similar degenerative phenotype has been recognized in humans. We report on the identification of an early-onset and progressive neurodegenerative encephalopathy with distal spinal muscular atrophy resembling the phenotype of pmn/pmn mice and caused by biallelic TBCE mutations, with the c.464T>A (p.Ile155Asn) change occurring at the heterozygous/homozygous state in six affected subjects from four unrelated families originated from the same geographical area in Southern Italy. Western blot analysis of patient fibroblasts documented a reduced amount of TBCE, suggestive of rapid degradation of the mutant protein, similarly to what was observed in pmn/pmn fibroblasts. The impact of TBCE mutations on microtubule polymerization was determined using biochemical fractionation and analyzing the nucleation and growth of microtubules at the centrosome and extracentrosomal sites after treatment with nocodazole. Primary fibroblasts obtained from affected subjects displayed a reduced level of polymerized α-tubulin, similarly to tail fibroblasts of pmn/pmn mice. Moreover, markedly delayed microtubule re-polymerization and abnormal mitotic spindles with disorganized microtubule arrangement were also documented. Although loss of function of TBCE has been documented to impact multiple developmental processes, the present findings provide evidence that hypomorphic TBCE mutations primarily drive neurodegeneration.

Ajarmeh SA, Al Tamimi EM. Sanjad-Sakati syndrome with macrocytic anemia and failure to thrive: a case from South Jordan. J Pediatr Endocrinol Metab. 2018 Apr 25;31(5):581-584.

Backgorund: Sanjad-Sakati syndrome (SSS) is a rare autosomal recessive disease caused by a deletion mutation (155-166del) in exon 3 of the TBCE gene on chromosome 1q42-43. The syndrome is characterized by primary hypoparathyroidism, typical dysmorphic features and severe growth retardation.

We encountered a 2-year-old boy with hypocalcemia, failure to thrive and macrocytic anemia. The patient had the characteristic features of SSS and genetic testing confirmed that he was homozygous for the TBCE mutation. Although malabsorption was initially considered the cause of his symptoms, the results did not confirm that diagnosis. Our patient had cow milk protein allergy and folic acid deficiency, which has not been described in previous SSS cases. It was difficult to treat the patient's hyperphosphatemia and we ultimately selected sevelamer treatment, which was tolerated well and improved his hypocalcemia.

SSS should be considered in the differential diagnosis of any infant with hypocalcemia, dysmorphism and failure to thrive.

Touati A, Nouri S, Halleb Y, Kmiha S, Mathlouthi J, Tej A, Mahdhaoui N, Ben Ahmed A, Saad A, Bensignor C, H'mida Ben Brahim D. Additional Tunisian patients with Sanjad-Sakati syndrome: A review toward a consensus on diagnostic criteria. Arch Pediatr. 2019 Feb;26(2):102-107.

Sanjad-Sakati syndrome (SSS; OMIM 241410) is a rare autosomal recessive disorder found almost exclusively in people of Arab origin. It is characterized by congenital hypoparathyroidism, severe prenatal and postnatal growth retardation, and distinct facial dysmorphism. The molecular pathology of this syndrome was shown to be due to a mutation in the tubulin-specific chaperone E (TBCE) gene in chromosomal area 1q42-q43. We aimed to detect and confirm the common mutation responsible for SSS in Tunisian patients and review the literature in order to create a set of clinical diagnostic criteria that might provide appropriate indications for molecular testing.

Three Tunisian patients with clinical feature of SSS were examined via direct Sanger sequencing of exon 3 of the TBCE gene.

Mutation analysis of the TBCE gene revealed the common 12-bp (155-166del) deletion in three new patients, thus raising the number of reported SSS patients to 73. Reviewing the literature, we suggest a scoring system that assigns one point each for major criteria and one half point for minor criteria.

SSS is an autosomal recessive disorder found in the Middle Eastern population with an estimated incidence of 1 per 40,000-100,000 live births in Saudi Arabia. Reviewing the literature on both its clinical and biochemical characteristics, we suggest for the first time, based on defined major and minor SSS criteria, a clinical scoring system for the diagnosis of SSS. On the one hand, an established scoring system will provide appropriate indications for molecular testing and, on the other hand, reviewed data on SSS will help delineate the phenotype and draw a distinction between differential diagnoses.

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