Testing With Intent in Mosaic Conditions: A Case-Based Review.

Area of Science:

• Genetics
• Developmental Biology
• Medical Genetics

Background:

• Mosaicism, defined as the presence of two distinct cell lineages within an organism, arises from post-zygotic mutations or chromosomal errors.
• This phenomenon can occur early in development, impacting various tissues in a pattern-specific manner.

Observation:

• Genetic testing may yield negative results from standard tissue samples if the variant is present in other, untested tissues.
• Mosaic conditions often present with variable expressivity and reduced severity compared to germline mutations.
• Two cases, Pallister-Killian syndrome and tuberous sclerosis complex, illustrate diagnostic challenges due to mosaicism.

Findings:

• The timing and mechanisms of mosaicism influence the tissue distribution of genetic variants.
• Tissue-dependent variant patterns can lead to prolonged diagnostic odysseys.
• Identifying affected germ layers aids in selecting appropriate tissues for diagnostic testing.

Implications:

• Targeted genetic testing strategies informed by the understanding of mosaicism mechanisms can shorten the diagnostic timeline.
• Accurate identification of mosaic variants improves the assessment of recurrence risks for offspring.
• Enhanced genetic counseling and more precise patient management are facilitated by recognizing and characterizing mosaic conditions.