The Cathepsin Key

Papillon-Lefèvre syndrome (PLS) is an inherited disorder that occurs in about one to four in every million people. The disease, which occurs equally in both genders and more often with parental consanguinity, is characterized by palmoplantar keratoderma (an abnormal thickening of the skin on the hands and feet) and early, severe periodontitis that causes loss of both primary and permanent teeth. Other features of the disease may include intellectual disability, intracranial calcifications, recurrent skin infections, hyperhidrosis, and liver or cerebral abscesses. Although the palmoplantar keratosis may be visible at birth or shortly thereafter, it’s most common for all of the symptoms to develop in parallel sometime between the ages of six months and four years – often along with teething – and become apparent by the time the child is five years old. But what gives rise to this unusual disease? Researchers were able to clearly establish its genetic etiology by sequencing the CTSC gene (1)(2)(3).

Pathological protein

CTSC encodes the protein cathepsin C (CatC; see Figure 1a). It’s also known as dipeptidyl peptidase I, a lysosomal cysteine exopeptidase belonging to the papain superfamily. As its alternate name suggests, CatC cleaves two residues from the N-termini of proteins and peptides (4), giving it an important role in the activation of immune system enzymes. The 46 kb gene is located on chromosome 11q14, and so far, 75 different PLS-causing mutations (50 percent missense, 25 percent nonsense, 23 percent frameshift, and 2 percent other) have been reported.

Most diagnoses of PLS nowadays are based on clinical signs and confirmed by genetic testing. But the sequencing needed to confirm PLS has several drawbacks: high costs relative to the low socioeconomic status of patients from countries with frequent intrafamilial marriages; the uncertain interpretation of rare benign mutations; and the lack of an appropriate platform for DNA preparation and sequence analysis. Not only that, but assays at the functional protein level identify pathological, clinically relevant CTSC mutations – as opposed to the mutations of low or no impact that may be spotted in DNA. On the other hand, pathological mutations are not restricted to the coding region, and therefore not limited to the protein sequence. They can affect transcription, splicing, tetramerization or downstream CatC maturation.

Timely treatment

It’s vital to diagnose PLS patients as early as possible; early treatment helps patients avoid or slow the progression of periodontitis, which can preserve their teeth (otherwise typically lost by age 17) and improve their quality of life. But with so many different mutations in the CTSC gene – and so many possible flaws in the protein –is it possible to devise a simple, low-cost screening method?

My colleagues and I hypothesized that active CatC is constitutively excreted, and can therefore easily be traced in the urine of normal subjects. If true, its absence in PLS patients’ urine would give us a reliable test that could be used early and easily with very little expense (especially when compared to a genetic screen). To test our hypothesis, we developed a method of CatC detection that was able to spot the protein in 100 percent of urine samples from about 80 healthy controls, regardless of age or sex. Through a combination of kinetic analysis and immunochemical detection, we ascertained that all of the samples contained both proteolytically active CatC and its precursor. Next, we obtained urine samples from 31 patients with a PLS phenotype; of those, 29 contained neither proteolytically active CatC nor its antigen, confirming the patients’ PLS diagnoses (see Figure 1b). In the remaining two samples, we detected CatC and followed up with a genetic analysis that revealed no loss-of-function mutation in CTSC – indicating that those patients actually have a PLS-like condition that is not PLS (5).

From testing to treating

Our work is the first step toward a simple, easy-to-manage test for PLS. In my experience, urine collection is much easier and less painful for children than venipuncture for blood sampling. And it’s a test that can be used on all members of an at-risk family, including the very youngest children; we have established clean catch collection techniques for babies – including newborns – and there are dedicated urine collectors for them. The test can save patients and healthcare systems money, too. The cost of genetic analysis ranges from €300–600 in Europe, depending on the country. In the United States, patients’ insurance providers may not cover the test, or may decide it isn’t medically necessary. It’s in situations like these, where genetic testing may present a financial hardship or be altogether impossible, that the urinary PLS test might address an unmet need.

We plan to develop a strip that can indicate the presence of both active CatC and its antigen. Such a test should only take a few minutes, and any laboratory technician should be able to administer it. Ultimately, we hope that a test based on the absence of urinary CatC activity will facilitate phenotype-genotype correlation in PLS and overlapping syndromes – and allow us to provide early diagnosis, treatment and symptom prevention to the patients who need it most.