X-linked intellectual disability

A family with intellectual disability segregating in males through unaffected (or mildly affected) female carriers, or a male proband whose ID gene maps to the X chromosome. X-linked intellectual disability (XLID) accounts for roughly 10-15% of ID in males overall, with more than 150 known genes. The clinical work is two-step: recognize the pedigree pattern (multiple affected males in the maternal line, no male-to-male transmission), then localize to one of a handful of syndromic XLID disorders or a "nonsyndromic" gene.

1. Pedigree first. Multiple affected males connected through clinically unaffected obligate-carrier females → classic X-linked. But sporadic male ID can still be XLID (new mutation in the proband, or undiagnosed carrier mother).
2. Always test FMR1 first in any male with unexplained ID. Fragile X syndrome is the single most common monogenic cause of inherited ID and is missed by CMA and standard sequencing. Order FMR1 CGG repeat sizing on every male ID work-up.
3. CMA before panel. A small but real fraction of XLID is contiguous-gene Xq or Xp deletions, sometimes with additional features the family didn't notice.
4. Phenotype-driven syndromic gene panel or exome once FMR1 and CMA are negative.
5. Don't forget female carriers. Skewed X-inactivation can produce affected females in classically X-linked disorders, including Fragile X (~50% of female full-mutation carriers have measurable cognitive features) and Rett (the typical proband is female).

Always-first: Fragile X syndrome

• Fragile X syndrome (FMR1 CGG > 200, methylated full mutation): the prototypic XLID. Postpubertal phenotype in males: long narrow face, large ears, prominent jaw, macroorchidism, hypermobile joints, autism / anxiety, ID (variable, often moderate). Prepubertal phenotype is much more subtle.
• Female carriers (full mutation): ~50% have measurable cognitive features (typically mild to borderline ID, learning disability, or shyness / social anxiety), driven by X-inactivation pattern.
• FMR1 premutation (CGG 55-200): FXTAS (Fragile X-associated tremor-ataxia syndrome) in older male carriers (intention tremor, ataxia, cognitive decline, characteristic middle cerebellar peduncle MRI sign) and FXPOI (Fragile X-associated primary ovarian insufficiency) in ~20% of female carriers. Premutation carriers are not typically intellectually disabled, but the late-onset complications are clinically and reproductively significant.
• Test: FMR1 CGG repeat sizing (Southern blot + PCR), not on a standard exome.

Syndromic XLID (recognizable phenotypes)

• MECP2 (Rett syndrome in females; MECP2 duplication syndrome in males): Rett: female with normal development to 6-18 months, then regression of purposeful hand use, hand stereotypies, deceleration of head growth, gait apraxia. MECP2 duplication in males: severe ID, recurrent infections, progressive spasticity.
• ARX (lissencephaly + ambiguous genitalia in males, XLAG): also a major cause of X-linked infantile spasms and nonsyndromic XLID with milder ARX variants.
• CDKL5: early-infantile epileptic encephalopathy (within first months of life), profound ID, Rett-like features. Mostly female probands (de novo); X-linked dominant.
• MED12 (Opitz-Kaveggia / FG syndrome, Lujan syndrome): broad forehead, macrocephaly, anal anomalies (FG), or Marfanoid habitus (Lujan); behavioral phenotype.
• IL1RAPL1, OPHN1: cerebellar hypoplasia plus nonsyndromic-appearing XLID.
• SLC9A6 (Christianson syndrome): Angelman-like in males (microcephaly, severe ID, ataxia, epilepsy, happy demeanor); cerebellar atrophy progresses.
• ATRX (alpha-thalassemia X-linked intellectual disability): coarse face, short stature, genital anomalies, profound ID, mild alpha-thalassemia trait (Hb H inclusions on supravital stain). Check Hb electrophoresis if features fit.
• CASK: microcephaly with pontine and cerebellar hypoplasia; mostly female probands.
• HUWE1, GRIA3, SYP, ZDHHC9: additional non-syndromic-appearing XLID genes.
• PHF6 (Borjeson-Forssman-Lehmann syndrome): obesity, hypogonadism, large ears, tapered fingers, gynecomastia, ID.
• RPS6KA3 (Coffin-Lowry syndrome): coarse face, hypertelorism, tapering fingers, short stature, postnatal ID; females mildly affected.

Contiguous-gene Xq / Xp microdeletions

• Detected by CMA. Examples: Xp22.31 (often STS-only, ichthyosis with mild or no ID) vs larger Xp deletions affecting flanking genes including NLGN4X, KAL1.

• 50% recurrence in male offspring of a carrier mother per pregnancy.
• Female obligate carriers vary widely in expression because of X-inactivation skewing; cognitive testing of mothers and sisters is part of the work-up, not an afterthought.
• Reproductive options for confirmed-carrier women: prenatal diagnosis (CVS or amnio), PGT-M, donor egg, adoption, accept-risk. Discussion should happen before the next pregnancy.
• Premutation carriers (FMR1 specifically): counsel about expansion risk (maternal-only) and FXPOI / FXTAS for the carrier and offspring.

1. FMR1 CGG repeat sizing in every male with unexplained ID, regardless of pedigree. Yield is highest here.
2. CMA in parallel (catches Xp / Xq microdeletions, autosomal CNVs).
3. Targeted single-gene testing if a syndromic pattern is recognizable (MECP2, CDKL5, ATRX, etc.).
4. X-linked ID gene panel or exome when first-tier tests are negative and pedigree is suggestive.
5. Carrier testing for at-risk female relatives once the family variant is identified; offer prenatal diagnosis options.
6. EEG, brain MRI, ophthalmology as guided by phenotype.