TMEM216 is required for the assembly and function of cilia (Lee et al., 2012).

By searching for genes in a region of chromosome 11 linked to Joubert syndrome-2 (JBTS2; 608091), Edvardson et al. (2010) identified TMEM216. The deduced 87-amino acid protein has 2 transmembrane domains.

Valente et al. (2010) identified a 1.4-kb TMEM216 mRNA in fetal tissue. Screening of a human fetal brain cDNA library identified 4 major splice isoforms, the longest and most prevalent predicting a 148-amino acid full-length protein with 4 transmembrane domains. There was also extensive alternative splicing, resulting in transcripts encoding 3 very short proteins of 25, 30, 34 amino acids, respectively. In situ hybridization of human embryonic tissues showed TMEM216 expression in the central nervous system, limb bud, kidney, and cartilage. Specific localization was also found in mouse inner medullary collecting duct cells, human retinal pigment epithelium, and proximal renal tubule, and TMEM216 localized to the base of the primary cilium or adjacent basal body.

Using expression profiling, Lee et al. (2012) showed that TMEM216 was expressed in all tissues examined. In situ hybridization of human embryos at 4 to 8 weeks' gestation revealed ubiquitous TMEM216 expression that increased with time. TMEM216 expression appeared to parallel that of TMEM138, which is closely linked to TMEM216, in all tissues examined. Immunohistochemical analysis of mouse IMCD3 cells and transfected COS-7 cells revealed that Tmem216 localized to basal body and to the Golgi apparatus surrounding the base of cilium and that Tmem138 localized to ciliary axoneme/basal body. The 2 proteins also appeared to localize to different vesicle pools that moved toward the primary cilia over time. Tmem138 vesicles, but not Tmem216 vesicles, colocalized with endogenous Cep290 (610142) in IMCD3 cells.

Malka et al. (2024) analyzed expression of TMEM216 in human tissues in the GTEx database and observed significantly higher levels of TMEM216 transcript in the peripheral retina compared to levels recorded from other tissues in the GTEx database.

Edvardson et al. (2010) determined that the TMEM216 gene contains 3 coding exons. Valente et al. (2010) determined that the TMEM216 gene contains 6 exons with alternative splicing of exon 2.

Lee et al. (2012) determined that the TMEM216 gene contains 5 exons. The 23-kb intergenic region between TMEM138 and TMEM216 contains a functional conserved RFX4 (603958)-binding site.

By genomic sequence analysis, Edvardson et al. (2010) mapped the TMEM216 gene to chromosome 11q13.

By genomic sequence analysis, Lee et al. (2012) mapped the TMEM216 gene to chromosome 11q12.2, 23 kb from the TMEM138 gene. TMEM216 and TMEM138 are in a head-to-tail orientation.

Lee et al. (2012) mapped the mouse Tmem216 gene to a region of chromosome 19 that shares homology of synteny with human chromosome 11q12.2.

By RNA interference (RNAi) of TMEM216, Valente et al. (2010) found that knockdown of TMEM216 prevented ciliogenesis in polarized cells and blocked correct docking of centrosomes at the apical cell surface. This was associated with increased RhoA (165390) signaling and mislocalization of RhoA. Loss of TMEM216 also caused increased phosphorylation and activation of DVL1 (601365). Valente et al. (2010) presented a working model in which DVL1, RhoA, and TMEM216 serve as part of a complex in the pericentrosomal compartment to mediate cellular polarization and centrosomal apical docking. Hyperactivation of Rho in the absence of TMEM216 might be responsible for the docking defect, as it was rescued by a Rho inhibitor. Finally, immunoprecipitation studies showed the TMEM216 interacted with TMEM67 (609884), which is mutant in Meckel syndrome-3 (MKS3; 607361) and other ciliopathies.

Lee et al. (2012) found that RFX4 mediated coordinated expression of TMEM138 and TMEM216 by binding to a regulatory element within their intergenic region. Knockdown of either Tmem138 or Tmem216 in mouse IMCD3 cells resulted in short cilia and a defect in ciliogenesis. Knockdown of Tmem216 disrupted vesicular trafficking of Tmem138 and Cep290, whereas knockdown of Tmem138 had little effect on vesicular movement of Tmem216. Knockdown of Trappc9 (611966), a component of the transport protein particle (TRAPP) II complex, disrupted vesicular tethering of both proteins and reduced ciliogenesis.

Joubert Syndrome 2 and Meckel Syndrome 2

In 13 affected members of 8 Ashkenazi Jewish families with Joubert syndrome-2 (JBTS2; 608091), Edvardson et al. (2010) identified a homozygous mutation in the TMEM216 gene (R73L; 613277.0001). The carrier rate in this ethnic group was determined to be 1 in 92.

In affected members of 14 families with Joubert syndrome-2 and 6 families with Meckel syndrome type 2 (MKS2; 603194), Valente et al. (2010) identified 7 different homozygous mutations in the TMEM216 gene (see, e.g., 613277.0001-613277.0004). Ten families with Joubert syndrome were of Ashkenazi Jewish descent and shared the common founder mutation, R73L, previously identified by Edvardson et al. (2010). Two individuals with Joubert syndrome and polydactyly who had the R73L mutation also demonstrated tongue tumors or multiple oral frenula, respectively, reminiscent of orofaciodigital syndrome type VI (OFD6; 277170).

Retinitis Pigmentosa 98

In 71 patients from 47 families of African or South Asian ancestry with nonsyndromic retinitis pigmentosa (RP98; 620996), who were negative for mutation in known retinal dystrophy-associated genes, Malka et al. (2024) identified homozygosity for 2 different nucleotide substitutions at the same genomic location, noncoding variants located 69 basepairs upstream of the TMEM216 start codon: in the patients of African ancestry, the mutation was a c.-69G-T transversion (613277.0005) and in the patients of South Asian ancestry, the mutation was a c.-69G-A transition (613277.0006). In 2 additional families of African ancestry, compound heterozygosity for the c.-69G-T variant and another mutation was present: in 2 affected brothers from Zimbabwe, the second mutation was a large deletion that included exons 1 to 3 of TMEM216, and in a female patient from the Caribbean, the mutation in trans was a splice site variant (613277.0007). Both recurrent variants were found in the gnomAD database, with enrichment in the African and South Asian populations, respectively. Haplotype analysis in both cohorts revealed a conserved haplotype that strongly suggested a single founder mutation unique to each group. None of the patients (age range at examination, 5-72 years) exhibited systemic features suggestive of generalized ciliopathy. Functional analysis showed significantly reduced expression with the recurrent variants compared to wildtype TMEM216.

Lee et al. (2012) found that the TMEM138 and TMEM216 genes are aligned in a head-to-tail orientation, with a conserved intergenic region, in higher vertebrates only. They determined that the 2 genes came into close association during an ancient chromosomal rearrangement at the amphibian-to-reptile transition about 340 million years ago. Conservation in the intergenic region becomes progressively weaker from human to anolis lizard, and the orthologous genes map to different chromosomes in zebrafish.

Valente et al. (2010) found that knockout of Tmem216 in zebrafish embryos resulted in defects in gastrulation, such as shortened body axis, broad notochords and misshapen somites, similar to defects observed in Tmem67 knockouts. The results indicated an alteration of convergence to the midline and extension along the anteroposterior axis, consistent with a defect in the planar cell polarity pathway.

Lee et al. (2012) found that knockdown of zebrafish Tmem138 or Tmem216 resulted in similar, but distinct, phenotypes. Knockdown of either gene resulted in pericardial effusion, curved or kinked tail, and gastrulation defects. However, only knockdown of Tmem16 resulted in hydrocephalic brains.