New Biomarker Discovery for FSHD Muscular Dystrophy Could Improve Patient Monitoring

Researchers have identified a new circulating biomarker for facioscapulohumeral muscular dystrophy (FSHD) that could improve disease monitoring in patients. The protein KHDC1L has been identified as a circulating biomarker for DUX4 activity during FSHD, potentially allowing doctors to track disease progression through simple blood tests rather than invasive muscle biopsies. This discovery, detailed in a recent study published in Human Molecular Genetics, emerged from collaboration between researchers at the Tapscott lab and scientists at Avidity Biosciences.

FSHD is an inherited disorder characterized by progressive muscle degeneration, initially involving the face, shoulder, and upper arms and progressing to affect all skeletal muscle groups. The disease is caused by inappropriate expression of the transcription factor DUX4 in skeletal muscle. During normal embryonic development, DUX4 is expressed for a very short and specific time and targets multiple genes for transcription, but in adulthood, DUX4 and its targets are silenced. In individuals with FSHD, muscle biopsies reveal that DUX4 is aberrantly expressed and its genetic targets are reactivated, and their abundance correlates with disease progression.

The research team began their work by using DUX4-expressing myoblasts to identify DUX4 gene targets that become secreted proteins. Using mass spectrometry, they found several secreted proteins from DUX4-expressing cells including KHDC1L. This protein was particularly interesting to researchers because its gene promoter has a DUX4 binding site, KHDC1L mRNA is present in FSHD but absent in healthy muscle biopsies, and it is not expressed by other adult tissues.

The biological function of KHDC1L is unknown, and few tools existed to study the gene. The group developed monoclonal antibodies to detect KHDC1L protein, confirming that KHDC1L protein is released from DUX4-expressing myoblasts. Further work confirmed that one of their antibody clones was highly specific for KHDC1L, giving researchers a new tool to study this protein.

The collaboration with scientists at Avidity Biosciences focused on developing assays for detecting KHDC1L in FSHD plasma. This proved especially challenging because reagents to detect KHDC1L did not exist prior to this work. They attempted mass spectrometry-based approaches, but due to reproducibility issues, the team moved on to other techniques.

Researchers then tried using an electrochemiluminescence immunoassay to detect protein from plasma samples. They used the highly specific antibody to capture KHDC1L from patient plasma and another antibody clone to detect how much KHDC1L protein bound to the capture antibody. KHDC1L abundance in FSHD plasma trended higher than that in healthy volunteers, but ultimately the group felt that they needed a more sensitive assay to detect KHDC1L.

Treatments for FSHD could come in the form of silencing DUX4 expression. Previous research has shown that DUX4 RNA expression and the downstream DUX4 targets can be silenced using an antisense siRNA, and several drug companies are developing therapies to suppress DUX4 expression. To determine whether these drugs suppress DUX4 in clinical trials, having a way to monitor DUX4 activity throughout the entire body would be useful. Muscle biopsies can be useful, but these procedures are invasive and only give insights about disease progression in a single tissue. Identifying a marker for FSHD and DUX4 activity that circulates in the blood would solve both of these issues: Doctors would have information on how FSHD is progressing through the whole body, and patients would only need to have their blood drawn.