Additional findings of tibial dysplasia in a male with orofaciodigital syndrome type XVI

We describe the case of a male patient with orofaciodigital (OFD) syndrome type XVI with a homozygous variant of TMEM107 (p.Phe106del) and the additional findings of tibial dysplasia, which is a pivotal finding of OFD syndrome type IV. His family history included two fetuses with anencephaly with or without cleft lip/palate and polydactyly with no genetic information. Careful attention should be given to the interpretation of this rare pattern.

(−2.9 SD), and his humeral length was 38 mm. These measurements are equivalent to those typically seen at 24 gestational weeks. At 32 gestational weeks, fetal magnetic resonance imaging (MRI) detected polydactyly and limb shortening. At 41 gestational weeks, his delivery was uneventful. He weighed 3074 g (−0.6 SD) and measured 48.5 cm (−1.0 SD) in length. His occipitofrontal circumference (OFC) was 35.0 cm (0.8 SD). Echocardiography and abdominal echoes revealed no abnormalities. At 5 months old, he was referred to our clinic for failure to thrive and developmental delay. He had a broad forehead, hypertelorism, short nose, anteverted nares, broad nasal bridge, broad nasal tip, thin upper lip, hyperplastic oral frenula, high-arched palate, lobulated tongue, oral lingual nodules, tongue hamartomas, puffy cheeks, micrognathia, bilateral postaxial polydactyly of the hands, bilateral mesoaxial polydactyly of the feet, short limbs, left inguinal hernia, buried penis, and migrating testis ( Fig. 1a-h). Occasionally, sudden temporary hyperpnea occurred. He also had strabismus. Radiographic imaging at 48 days showed the 6 th finger, including two proximal phalanxes and one distal phalanx (Fig. 1i), Y-shaped 5 th metacarpal (Fig. 1i) and broad 5 th metacarpal (Fig. 1j), polydactyly of feet without fused bone (Fig. 1k, l), shortly bowed tibia and mildly flared femoral metaphysis (Fig. 1m, n). These findings showed improvement at 14 months (Fig. 1o). MRI showed cerebellar vermis hypoplasia and a molar tooth sign (Fig. 1p, q). Additionally, an echocardiogram, abdominal echogram, and auditory brainstem response test revealed no apparent abnormalities. At 2 years old, his weight was 8219 g (−2.9 SD), length was 70.8 cm (−4.9 SD), and the OFC was 48.5 cm (0.1 SD). Eventually, he underwent bilateral orchidopexy, Potts' operation for left inguinal herniation, and resection of two tongue hamartomas and ectopic upper/lower labial zonules. His ability to eat was improved after the surgery.
After his parents provided written informed consent, we conducted trio-based whole-exome sequencing using the Sure Select Human All Exon V6 kit (Agilent Technologies, Santa Clara, CA) and HiSeq2500 (Illumina, San Diego, CA) and in silico analyses, as described previously 7 . After performing filtering in the in silico analyses, we identified a homozygous deletion in exon 4 of TMEM107, NM_032354: c.316_318del (p.Phe106del). This mutation had been previously described and was indicated to cause disruptions in cilia formation and length 3,4 . Located in the third transmembrane domain of TMEM107, Phe106 is highly conserved from fungi to humans, emphasizing that the residue is probably functionally important. The patient's parents had a heterozygous variant of p.Arg275Gln within TMEM107. However, this variant was not found in the Japanese Genome Database of Human Genetic Variation (http://www.hgvd. genome.med.kyoto-u.ac.jp) or the Single Nucleotide Polymorphism Database (http://www.ncbi.nlm.gov/SNP). Instead, it was found in the Genome Aggregation Database (https:// gnomad.broadinstitute.org). Its frequency is extremely low (0.00002122) [allele count: 6/282768, east Asian: 0/19952, European (non-Finnish): 6/129126)], with zero homozygotes reported. The ACMG-AMP guidelines have classified this variant as pathogenic (PVS1, PS3, PM3, PM4, PP3, and PP4). This study was performed in accordance with the standards of the Ethics Committee of the Ryukyus Graduate School of Medicine (Okinawa, Japan).
The patient's final diagnosis was OFD syndrome type XVI based on clinical and molecular findings, as he did not have the metacarpal abnormalities with central polydactyly that are seen in OFD syndrome type VI. The clinical characteristics of the previously reported cases of TMEM107 gene variants are summarized in Table 1 [3][4][5][6]8,9 . Although case 1 had insufficient clinical findings of MKS, all five cases, including the siblings, exhibited various phenotypes involving the oral and central nervous systems. Both cases 4 and 5 had the same gene variants but different findings. In case 5, we observed tibial dysplasia and mildly flared femoral metaphysis, which were not previously reported in OFD syndrome type XVI. Tibial dysplasia is a pivotal feature of OFD syndrome type IV 10,11 . In Table 1, five out of six patients had molar tooth signs, which are important clinical data for OFD syndrome type VI 12 .
Bruel et al. (2017) suggested a novel classification of OFD syndrome in which the molar tooth sign is an associated clinical feature of OFD syndrome type VI that is caused by genetic variants in TMEM107, TMEM216, TMEM231, TMEM138, C5orf42, and KIAA0753. Tibial dysplasia is an associated clinical feature of OFD syndrome type IV that is caused by variants in TCTN3 5 . Case 5 had both tibial dysplasia and molar tooth signs, revealing an overlap between OFD syndrome type IV and OFD syndrome type VI. We think that OFD syndrome type XVI is difficult to classify because the clinical manifestation is extremely diverse, and the number of cases is not yet sufficient for classification. For the time being, it seems reasonable to consider it as a spectrum caused by the TMEM107 gene.
Mouse embryos with complete TMEM107 knockout manifested an embryonic lethal type, exhibiting a broad spectrum of craniofacial defects 13,14 . In our case, the relationship between TMEM107 and anencephaly could not be investigated because the biological samples of the affected fetuses with anencephaly were not preserved. To the best of our knowledge, there have been no reports of anencephaly in OFD syndromes. These repeated cases of affected fetuses with anencephaly is a very rare pattern, and great care should be taken in interpreting this phenomenon.

HGV DATABASE
The relevant data from this Data Report are hosted at the Human Genome Variation Database at https://doi.org/10.6084/m9. figshare.hgv.3137.