A Novel Transglutaminase-1 Missense Mutation in a Palestinian Family with Autosomal Recessive Congenital Ichthyosis: A Case Report

This work presents the molecular genetics investigation of a male neonate referred to our genetics laboratory with the diagnosis of classical lamellar ichthyosis (one form of autosomal recessive congenital ichthyosis). The neonate was born as a "collodion-baby" and he is the product of a maternal first cousin marriage. DNA sequencing of the coding exons of transglutiminase-1 (TGM1) gene revealed a novel missense (c.A1621C) mutation in exon 11. The mutation altered codon 541 from ACC into CCC thus changing the amino acid threonine into proline (p.T541P) and was predicted to be pathogenic. The presence of the mutation in both parents in heterozygous form and in the patient in homozygous form was further confirmed by PCR-restriction fragment length polymorphism (PCR-RFLP) designed specifically for the identified mutation. It is concluded that the T541P mutation is the cause of the congenital ichthyosis in the presented case and the parents were advised to undergo a PGD-IVF for embryo selection prior to their next pregnancy.

Here we report for the first time in our area the identification of a novel TGM1 mutation in a Palestinian patient with classic lamellar ichthyosis phenotype.

CASE REPORT
A two-weeks "collodion baby" male born to maternal first cousin consanguineous parents at 39 weeks gestation presented to the Genetics lab at the Islamic University of Gaza for genetics study. The coding region (exons 2 through 14) of TGM1 was amplified from patient genomic DNA by PCR using the primers described previously [8]. Direct sequencing of the patient's PCR products revealed that the patient had a missense p.T541P (c.A1621C) mutation in homozygous form in exon 11 Fig. 1, a novel mutation that has not been reported before. The presence of the mutation in heterozygous form in the parents was also confirmed by direct sequencing.
The identified mutation effect was evaluated in silico using Sorting Intolerant FROM Tolerant (SIFT) (http://sift.jcvi.org/www/SIFT_BLink_ submit.html) [11] and PolyPhen: prediction of functional effect of human nsSNPs; (http://genetics.bwh.harvard.edu/pph) [12] programs. The threonine residue mutated into proline is located in a motif of the catalytic core domain [9] of the enzyme and both programs predicted that the mutation is functionally damaging. This threonine residue was confirmed to be highly conserved across many primate and non-primate species, a sample of which is illustrated in Fig. 2. The c.A1621C mutation thus identified abolishes the cutting site of HphI restriction enzyme therefore, the following PCR primers (TGMF: AGTGACAAGGTGTACTGGCA and TGMR: GAGG TTCCAATTCCCACGTG) were designed to amplify a 238bp fragment encompassing the mutation. HphI digests the normal allele into two fragments (124 and 114 bp fragments) whereas the mutant allele remains uncut. Fig. 3 shows a photo of an agarose gel where the patient and his parents' genotypes are indicated. The parents' genotypes confirm their heterozygosity for the mutation (as illustrated by having 3 fragments).

DISCUSSION
Classic lamellar itchthyosis (ARCI-1) is a rare disorder that is present worldwide [7]. Deleterious mutations in TGM1 gene have been shown to be responsible for most of the cases reported so far [8]. The case presented here is the first genetically characterized one in Gaza-Palestine. The novel mutation identified here is located in one of the exons coding for part of the catalytic core domain of transglutaminase-1 enzyme. Most of the previously reported mutations also affect this domain of the enzyme [13].
The A to C transversion at codon 541 detected in our patient is expected to lead to an amino acid change from threonine to proline.
Several lines of evidence suggest the this mutation is pathogenic: (1) The mutation is present in homozygous form in the patient, in heterozygous form in the proband parents and absent in healthy control subjects Figs. 1 and 3, (2) Amino acid sequence alignment Fig. 2 showed that the mutated threonine residue is highly conserved across distant species i.e., it is important for the enzyme activity and, (3) The altered amino acid is located in a critical region of the enzyme (the catalytic core domain) and in silico analysis using SIFT and PolyPhen programs predicted that the amino acid change is damaging for the enzyme function.
Additionally, threonine and proline have quite different characteristics: threonine is non-cyclic and polar, proline is cyclic and hydrophobic therefore, the two amino acids serve entirely different structural roles during polypeptide folding. While threonine fits into regular secondary structures (particularly β-strand conformation) proline behaves as a structural disruptor in the middle of regular secondary structural elements. Hence it is believed that proline disrupts the proper folding of TGase-1 enzyme and its catalytic function.

CONCLUSION
The ARCI-1 in the present case is attributed to a novel p.T541P mutation (RCV accession: RCV000114750) in the catalytic core domain of the TGase-1 enzyme. This finding expands the mutations spectrum of TGM-1 gene and is important for delivering proper counseling and diagnosis opportunities for the proband's family.

CONSENT
The objective of the study was fully explained to the parents and their written informed consent was taken to carry out the investigation. Moreover, the authors declare that 'written informed consent was obtained from the patient's parents for publication of this case report.

ETHICAL APPROVAL
All authors hereby declare that all experiments have been examined and approved by the appropriate ethics committee and have therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki.

FUNDING
This work was supported by the Palestinians Relief and Development Fund (Interpal)-Gaza Field Office.