## 4 Genetic Diagnosis of VCP-MSP
### Genetic Testing Methods  
There are several testing strategies generally available to clinicians, which broadly speaking can be divided into three categories: (a) targeted single gene testing, (b) multi-gene panel sequencing, and (c) unbiased sequencing (exome or whole genome).  
_Single gene/variant testing_: This is when only _VCP_ is sequenced, or if specific variants in _VCP_ are sequenced as a targeted investigation. Generally, this type of approach is only used for testing of family members in which a diagnosis of VCP-MSP is already established in the family. We do not suggest single gene testing in situations where there is not a known diagnosis of VCP-MSP in the family; this is due to heterogeneity of clinical syndromes associated with VCP-MSP that includes a vast genetic differential diagnosis. Therefore, it is more cost- and time-effective to use parallel sequencing approaches for diagnosis of undifferentiated cases. This also applies to cases in which a diagnosis of VCP-MSP is highly likely (e.g., dominant history with PDB, FTD, and myopathy) because there remains a differential diagnosis with other multisystem proteinopathy genes (as per Section 2 above).  
_Panel sequencing_: This is when multiple genes are tested simultaneously, usually applying short-read parallel sequencing methods. Panels are offered by numerous genetic testing companies with clinical certification. Since _VCP_ mutations are associated with so many phenotypes, it is included in panels to investigate genetic causes of myopathy, dementia, and ALS. However, the clinician should be aware that _VCP_ may not be included in panels from all companies. Some panels for skeletal abnormalities, spastic paraplegia, and neuropathies do not routinely include _VCP_. Since the content of sequencing panels is adjusted over time, panels will hopefully be more inclusive in the future; thus, in patients without an established genetic diagnosis, re-evaluating that _VCP_ was indeed on the initial panel may be warranted, leading to repeat panel testing. In general, for undifferentiated cases, panel sequencing will usually be the most readily available test and the most suitable investigation for diagnosing patients with VCP-MSP or other genetic conditions in the differential diagnosis.  
_Unbiased sequencing_: Genomic approaches that sequence all coding regions (exome sequencing) or the entire genome are typically reserved for use by medical genetic specialists or in research settings. As long as the read depth and coverage are sufficient, _VCP_ will always be sequenced in cases receiving this type of testing. However, because all other genes are also sequenced, this can result in challenges in the interpretation if multiple potentially relevant variants are identified. This also has a high likelihood of identifying incidental genetic findings that have special genetic counselling implications. There is evidence indicating a higher diagnostic yield for unbiased sequencing approaches compared with panel sequencing for genetic diseases, and this may also be the case for VCP-related disorders which can produce unexpected phenotypes.  
Our recommendation is that in most situations, multi-gene panel sequencing will be the best option for testing of undifferentiated cases, whereas single gene testing should be reserved for cases with a known family history of VCP-MSP. Broad genetic testing (such as panel or exome/genome) also has the advantage of being able to identify dual (or multiple) genetic diagnoses, which are believed to occur in -5% of patients with Mendelian genetic disorders [119]. We may eventually become aware of non-Mendelian gene modifier effects or interactions [120], for which broader sequencing approaches will also be necessary.  
We recommend that _VCP_ should be included in genetic sequencing panels for a broad range of phenotypes, including myopathy, skeletal disorders, dementia, ALS, spastic paraplegia, and neuropathy, to aid in the identification of a definitive diagnosis for as many patients as possible.